Thomas Reid: The Father of Common Sense Philosophy

The stakes of philosophy are often high. How we answer the problems of knowledge, conduct, and governance governs our way of life for those that live according to their own philosophy. Modern philosophy has some original defect which originated with Des Cartes (1596-1650) starting it off with absolute skepticism, to which he could only find a way out using the golden thread of reason. With this original defect we get the possibility of an inauthentic life where we have reason to not believe in our own sense. From this point, Des Cartes had to fend off the evil genius that could be manipulating him every step of the way. However, since his system of philosophy was built upon ideas and not perceptions, his senses could be deceived about the existence of the external world, since how he came to know the world was always by way of “ideas” impressed upon the mind. Aristotle used the example of a wax stamp to illustrate how the senses gathered information. The external world acts as a stamp, and our mind is the wax. So the stamp impresses an image onto our mind, and those are how we see the world. These ideas and impressions gathered in the mind are not directly seen but always mediated, so we can never be certain we are seeing the world out there. To use the brain in the vat analogy, we could just be that, with electrical signals being sent to our brain. But would we ever choose such a life? Certainly the hero would never choose such a life because it’s an inauthentic form of life, which would mean the hero’s actions would never better the lives of others since it would be a mere illusion of electrical signals to the brain. Thomas Reid unravels the idea theory, that all we see are ideas in the mind, sees the thread of it weaving it’s way through modern philosophy, in Locke, Malebranche, Berkeley and is startled by the conclusions of which it ultimately comes to in the great skepticism of David Hume.

Thomas Reid (1710-1796) was born in Scotland during the Scottish Enlightenment. His father was a minister of the Scottish church called Kirk. Thomas studied its theology at Marischal College University of Aberdeen at age 12, and was parish minister (1737-51) at New Machar. Afterwards he went to his Alma mater back at Aberdeen University (1751-64), where he was a founder of the philosophical society called the “Wise Club” in 1758, which discussed Locke, Berkeley and Hume often. In 1764 Reid wrote An Inquiry into the Human Mind. Shortly after he wrote the book, when Adam Smith left his moral philosophy seat at Glasgow University, Reid took his position. He taught there at Glasgow until 1780. When he retired he focused on his publications of his two major works Essays on the Intellectual Powers of Man (1785) and Essays on the Active Powers of Man in 1788). He passed away in 1790.

To understand Reid you must understand the context of the history of Modern Philosophy which he was brought up in and actively debated. In Reid’s Inquiry into the Human Mind, he discusses how Des Cartes began it with doubt:

[Descartes] wholes system concerning matter and spirit is built upon one axiom, expressed in one word, cogito. Upon the foundation of conscious thought, with ideas for his materials, he builds his system of human understanding, and attempts to account for all its phaenomena: and having, as he imagined, from his consciousness, proved the existence of matter; upon the existence of matter, and of a certain quantity of motion originally impressed upon it, he builds his system of the material world, and attempts to account for all its phaenomena. (pg. 211)

Reid then goes on to relate how Locke and Hume’s theory was built upon Des Cartes. Locke used Isaac Newton’s theory of Gravitation for his theory of mind. The association of ideas acted as gravity that moved simple, indivisible ideas towards each other to compound them and make complex ideas. Hume wrote that what attracted these ideas together was through the law of resemblance, the law of contiguity, and the law of cause and effect, which was contiguity through time. Here is how Reid saw Modern Philosophy’s failing:

But to come to the system of Des Cartes, concerning the human understanding; it was built, as we have observed, upon the consciousness as its sole foundation, and with ideas as its materials; and all his followers have built upon the same foundation and with the same materials. They acknowledge that nature hath given us various simple ideas: These are analogous to the matter of Des Cartes physical system. They acknowledge likewise a natural power by which ideas are compounded, disjoined, and associated, compared: This is analogous to the original quantity of motion in Des Cartes’s physical system. From these principles they attempt to explain the phaenomena of the human understanding, just as in the physical system the phaenomena of nature were to be explained by matter and motion. It must indeed be acknowledged, that there is a great simplicity in this system as well as in the other. There is such a similitude between the two, as may be expected between children of the same father: but as the one has been found to be the child of Des Cartes, and not of Nature, there is ground to think that the other is so likewise.

That the natural issue of this system is scepticism with regards to every thing except the existence of our ideas, and of their necessary relations which appear upon comparing them, is evident: for ideas being the only object of thought, and having no existence but when we are conscious of them, it necessarily follows, that there is no object of our thought which can have a continued and permanent existence. (pg. 212)

This could very well be the brain in the vat existence. Where an evil genius is playing with us, manipulating our neurons with electrical signals. The electrical signals amounting to ideas that are put into our brains. For Des Cartes, Locke, Berkeley, and Hume, all we can know are these “ideas”. This idea theory is just what Reid wants to lay to rest, and scrap modern philosophy. He wants to lay the foundation for common sense, which ought to be on equal grounding as the cogito.

In the next quote Reid will talk of original perception. Original perception gives us primary qualities of bodies such as hardness, softness, roughness, smoothness, figure, motion, and extension. We see the world as a composite, not atomized into indivisible elements, but as complex wholes.

If the original perceptions and notions of the mind were to make their appearance single and unmixed, as we first received them from the hand of nature, one accustomed to reflection would have less difficulty in tracing them; but before we are capable of reflection, they are so mixed, compounded, and decompounded, by habits, associations, and abstractions, that it is hard to know what they were originally. The mind may in this respect be compared to an apothecary or a chemist, whose materials indeed are furnished by nature; but for the purposes of his art, he mixes, dissolves, evaporates, and sublimes them, till they put on a quite different appearance; so that it is very difficult to know what they were at first, and much more to bring them back to their original and natural form. (pg. 14)

What Reid is saying is that what don’t see simple ideas of just hard, just rectangle, just cold, just the smell of pine, no we sense a the complexity of a wooden desk in front of us with all of its mixed aspects as once. It’s reason that separates it mixed aspects. The theory of simple ideas that compound to make complex ideas like gravity collects matter in motion is a false analogy. We see the world as it is, richly variegated, never simple! Furthermore, it can’t be as mechanistic as the Newtonian mind theory. Reid believes there must be common principles given in our constitution that makes sense of the external world around us.

It must therefore require great caution, and great application of mind, for a man that is grown up in all the prejudices of education, fashion, and philosophy, to unravel his notions and opinions, till he finds out the simple and original principles of his constitutions, of which no account can be given but the will of our Maker. This may be truly called an analysis of the human faculties; and till this is performed, it is in vain we expect any just system of the mind; that is, an enumeration of the original powers and laws of our constitution, and an explication from them of the various phaenomena of nature. (Pg. 15)

Reid is saying that philosophy of mind is limited until we can enumerate the powers and laws of our very constitution, of which we ought to call common sense.

Des Cartes finding nothing established in this art of philosophy, in order to lay the foundation of it deep, resolved not to believe his own existence till he should be able to give good reason for it… [B]ut if he could indeed have effected his purpose, and really become diffident of his existence, his case would have been deplorable, and without any remedy from reason or philosophy. A man that disbelieves his own existence, is surely as unfit to be reasoned with, as a man that believes he is made of glass. (pg. 16)

Reid wants us to recognize that Descartes may have started with absolute skepticism, however, not even he could be a complete skeptic or else his position would have been deplorable. He chose at will to believe in his cogito, yet never in common sense, which establishes the very reliable basis with which we can get to the market and back.

In this unequal contest betwixt Common Sense and Philosophy, the latter will always come off both with dishonor and loss; nor can she ever thrive till this rivalship is dropt, these encroachments given up, and a cordial friendship restored: for, in reality, Common Sense holds nothing of Philosophy, nor needs her aid. But, on the other hand, Philosophy… has no other root but the principles of Common Sense; it grows out of them, and draws its nourishments from them: severed from this root, its honours wither, its sap is dried up, it dies and rots. (pg. 19)

Therefore, if it’s between common sense or philosophical theorizing, it better be philosophical theorizing that takes a back seat, because common sense is foundational and has proven its worth through experience our entire lives. In this next passage he continues, once again, on the follies of modern philosophy’s idea theory later given to us by Berkeley and Hume:

The present age, I apprehend, has not produced two more acute or more practiced in this part of philosophy, than the Bishop of Cloyne (George Berkeley), and the author of the Treatise of Human Nature (David Hume). The first was no friend to skepticism, but had that warm concern for religious and moral principles which became his order: yet the result of his inquiry was, a serious conviction, that there is no such thing as a material world; nothing in nature but spirits and ideas; and that the belief of material substances, and of abstract ideas, are the chief causes of all our errors in philosophy, and of all infidelity and heresy in religion. His arguments are founded upon principles which were formerly laid down by Des Cartes, Malebranche, and Locke, and which have been very generally received. … The second (David Hume) proceeds upon the same principles, but carries them to their full length; and as the Bishop undid the whole material world, this author upon the same grounds, undoes the world of spirits, and leaves nothing in nature but ideas and impressions, without any subject on which they may be impressed. (pg. 19-20)

Berkeley destroyed the material world when he collapsed primary qualities with secondary qualities, and exclaimed all that exists is but what is in the mind. But Hume took empiricism, based on ideas and impressions, to its fated conclusion, that neither exists. For Mr. Hume, there wasn’t even a self at the center stage. that all we can know are a bundle of perceptions that move at an unconceivable rate. This left Modern philosophers with major limitations towards what they could know. Hume’s great skepticism psychologized the world, leaving metaphysics barred from access to further knowledge.

Reid recounts the legend of the ultimate sceptic named Pyrrho who had great friends that took good care of him as he walked into the streets. However, one night his cooks made the evening meal and he disagreed with it so much he chased them out of his house with a frying pan. So even Pyrrho drew the line of skepticism, his being the ancient cuisine. Reid next discusses how skepticism towards the senses began with Des Cartes and continued:

But what if these profound disquisitions into the first principles of human nature, do naturally and necessarily plunge a man into this abyss of skepticism? May we not reasonably judge so from what hath happened? Des Cartes no sooner began to dig in this mine, than skepticism was ready to break in upon him. He did what he could to shut it out. Malebranche and Lock, who dug deeper, found the difficulty of keeping out his enemy still to increase: but they labored honestly in the design. Then Berkeley, who carried on the work, disparaging of securing all, bethought himself of an expedient: By giving up the material world, which he thought might be spared without loss, and even with advantage, he hoped by an impregnable partition to secure the world of spirits. But, alas! The Treatise of Human Nature wantonly sapped the foundation of this partition, and drowned all in one universal deluge.

These facts, which are undeniable, do indeed give reason to apprehend, that Des Cartes’s system of the human understanding, which I shall beg leave to call the ideal system, [we might want to call it the “idea system” for easier understanding] and which, with some improvements made by later writers, is now generally received, hath some original defect: that this skepticism is inlaid in it, and reared along with it; and, therefore, that we must lay it open to the foundation, and examine the materials, before we can expect to raise any solid and useful fabric of knowledge on this subject. (pg. 23)

The original defect being how ideas are the mediation between our minds and the world. Ideas mediate how we see the world, so we can never see the world directly, it’s always represented, it’s always mediated. This idea system is what Reid wants to topple. He wants us to believe that we can directly perceive the world out there, that we don’t see ideas, we see objects outside our mind. Instead of seeing impressions and ideas, Reid wants us to believe that the external world makes impressions on our senses, not our minds, and we directly perceive what the senses pick up, leaving no room for Des Cartes deceptive, evil genius to manipulate the mind and what we see. He talks of the idea system next:

Ideas seem to have something in their nature unfriendly to other existences. They were first introduced into philosophy, in the humble character of images or representatives of things; and in this character they seemed not only to be inoffensive, but to serve admirably well for explaining the operations of the human understanding. But since men began to reason clearly and distinctly about them, they have by degrees supplanted their constituents, and undermined the existence of every thing but themselves. First, they discarded all secondary qualities of bodies; and it was found out by their means, that fire is not hot, nor snow cold, nor honey sweet; and in a word, that heat and cold, sound, colour, taste, and smell, are nothing but ideas or impressions. Bishop Berkeley advanced them a step higher, and found out, by just reasoning, from the same principles, that extension, solidity, space, figure, and body, are ideas, and that there is nothing in nature but ideas and spirits. But the triumph of ideas was completed by the Treatise of Human Nature, which discards spirits also, and leaves having nothing else to contend with, they should fall foul of one another, and leave no existence in nature at all? This would surely bring philosophy into danger; for what should we have left to talk or to dispute about? (pg. 33-34)

Des Cartes, Malebranche, and Locke, as they made much use of ideas, treated them handsomely, and provided them in decent accommodation; lodging them either in the pineal gland, or in the pure intellect, or even in the divine mind. They moreover clothed them with a commission, and made them representatives of things, which gave them some dignity and character. But the Treatise of Human Nature, though no less indebted to them, seems to have made but a bad return, by bestowing upon them this independent existence; since thereby they are turned out of house and home, and set adrift in the world, without friend or connection, without a rag to cover their nakedness; and who knows but the whole system of ideas may perish by the indiscreet zeal of their friends to exalt them? … However this may be, it is certainly a most amazing discovery, that thought and ideas may be without any thinking being. … It seemed very natural to think, that the Treatise of Human Nature required an author, and a very ingenious one too; but now we learn, that it is only a set of ideas which came together, and arranged themselves by certain associations and attractions (pg. 35)

So on the Humeian account, philosophy is in a deplorable situation, and the self is “Nothing but a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement.” The parade formation of a bundle of perceptions and ideas in rapid motion are what make the self, perceptions having citizenship in the mind as being ideas themselves. These theories proposed by modern philosophers, Reid believes, are not founded on the same principles that Francis Bacon and Isaac Newton used to discover facts about reality. His devotion to the common sense of Bacon and Newton, and his devotion towards God shine through in this next passage:

Conjectures and theories are the creatures of men, and will always be found very unlike creatures of God. If we would know the works of God, we must consult themselves with attention and humility, without daring to add any thing of ours to what they declare. A just interpretation of nature is the only sound and orthodox philosophy: whatever we add of our own, is apocryphal, and of no authority. (pg. 12)

Wise men now agree, or ought to agree in this, that there is but one way to the knowledge of nature’s works; the way of observation and experiment. (pg. 11)

Reid is talking about how we falsely use analogies like Locke, Berkeley, and Hume did when using the Newtonian theory of mind. Also, the conjectures of Descartes, which led us into this entire mess, by doubting common sense principles, using only one axiom, the cogito, which he erected his entire philosophy from. However, even with their skepticism, these philosophers had to leave their theoretical arm chairs and go out into the world where common sense aided them every step along the way.

If there are certain principles, as I think there are, which the constitution of our nature leads us to believe, and which are under a necessity to take for granted in the common concerns of life, without being able to give a reason for them; these are what we call the principles of common sense. (pg. 33)

So common sense is what is universally and pragmatically represented in nature. It’s the foundation of understanding from the lowly caterpillar to man that Nature suited us out with.

But when they condescend to mingle again with the human race, and to converse with a friend, a companion, or a fellow-citizen, the ideal system vanishes; common sense, like an irresistible torrent, carries them along; and, in spite of all their reasoning and philosophy, they believe their own existence, and the existence of other things.
Indeed, it is happy they do so; for if they should carry their closet-belief into the world, the rest of mankind would consider them as diseased, and send them to the infirmary. Therefore, as Plato required certain previous qualifications of those who entered his school, I think it would be prudent for the doctors of this ideal philosophy to do the same, and to refuse admittance to every man who is so weak, as to imagine that he ought to have the same belief in solitude and in company, or that his principles ought to have any influence upon his practice: for this philosophy is like a hobby-horse, which a man in bad health may ride in his closet, without hurting his reputation; but if he should take him abroad with him to church, or to the exchange, or to the play-house, his heir would immediately call a jury, and seize his estate. (pg. 35-36)

Reid here is asking that the philosopher live according to his philosophy, and if this were so, most of the skeptics would be out by sun down. In the next passage Reid talks about how even the sense of smell gives rise automatically to the belief in a faculty of smell, and how also this faculty gives rise to the idea that there is a mind present. These are common sense beliefs.

It is incumbent upon those who think that these are no natural principles, to show, in the first place, how we can otherwise get the notion of mind and its faculties; and then to show, how we come to deceive ourselves into the opinion that sensation cannot be without a sentient being. (pg. 37)

What is smell in the rose? It is a quality or virtue of the rose, or of something proceeding from it, which we perceive by the sense of smelling; and this is all we know of the matter. But what is smelling? It is an act of the mind, but is never imagined to be a quality of the mind. Again, the sensation of smelling is conceived to infer necessarily a mind or sentient being; but smell in the rose infers no such thing. … From what hath been said, we may learn, that the smell of a rose signifies to things. First, A sensation, which can have no existence but when it is perceived, and can only be in a sentient being or mind. Secondly, It signifies some power, quality, or virtue, in the rose, or in effluvia proceeding from it, which hath permanent existence, independent of the mind, and which, by the constitution of nature, produces the sensation in us. By the original constitution of our nature, we are both led to believe, that there is a permanent cause of the sensation, and prompted to seek after it; and experience determines us to place it in the rose. (pg. 42-43)

The sense of smell produces a secondary quality, which is unlike the primary qualities of motion, figure, extension, roughness, and smoothness. Secondary qualities are of color, heat and cold, and smell. However, Reid wants to make a firm stance that there is a permanent quality in the rose that is causing the secondary quality of smell to arise. He doesn’t know how it happens, except that it is the external world that impinges itself on our sensory organs, not the mind, causing the smell. On primary and secondary qualities, Reid writes:

[T]here appears to me to be a real foundation for the distinction; and it is this: That our senses give us a direct and distinct notion of the primary qualities, and inform us what they are in themselves: But of the secondary qualities, our senses give us only a relative and obscure notion. They inform us only, that they are qualities that affect us in a certain manner, that is, senses leave us in the dark. (Essays on the Intellectual Powers od Man, pg. 201).
Reid then turns from the sense of smell to hearing in his next chapter.

One of the noblest purposes of sound undoubtedly is language; without which mankind would hardly be able to attain any degree of improvement above the brutes. Language is commonly considered as purely an invention of men, who by nature are no less mute than the brutes, but having superior degree of invention and reason, have been able to control artificial signs of their thoughts and purposes, and to establish them by common consent. (pg. 50)
[I] think it is demonstrable, that if mankind had not a natural language, they could never have invented an artificial one by their reason and ingenuity. For all artificial language supposes some compact or agreement to affix a certain meaning to certain signs; therefore there must be compacts or agreements before the use of artificial signs; but there can be no compact or agreement without signs, nor without language; and therefore there must be a natural language before any artificial language can be invented: Which was to be demonstrated. (pg. 51)

The elements of this natural language of mankind, or the signs that are naturally expressive of our thoughts, may, I think, be reduced to these three kinds: modulations of the voice, gestures, and features. By means of these, two savages who have no common artificial language, can converse together. (pg. 51-52)

Man, by nature, is a social animal inclined towards the company of others, Aristotle wrote. This is part of what it is to be essentially man. Reid is making a remarkable observance with his natural and artificial language statement that Wittgenstein will later suggest in a similar manner. That without the natural language of body postures and grimaces there could not be an artificial language of words which our books are made of. Words is what sets human beings from the rest of the animal kingdom, it’s what we do. The more expressive our language the more natural it is. We now move to the next chapter on touch.

[B]y touch we perceive not one quality only, but many, and those of very different kinds. The chief of them are heat and cold, hardness and softness, roughness and smoothness, figure, solidity, motion, and extension. (pg. 54)

The simplest man that hath common sense, does not imagine the sensation of heat, or any thing that resembles that sensation, to be in the fire. He only imagines, that there is something in the fire, which makes him and other sentient beings feel heat. Yet as the name of heat, in common language, more frequently and more properly signifies this unknown something in the fire, than the sensation occasioned by it, he justly laughs at the philosopher, who denies that there is any heat in the fire, and thinks that he speaks contrary to common sense. (pg. 55)
Signs are the cause and the thing signified are the effect. We jump from one to the other without the use of reason. It’s conception and belief is immediate. So here, fire is the sign and heat the thing instantaneously signified.
When parts of a body adhere so firmly, that it cannot easily be made to change its figure, we call it hard; when its parts are easily displaced, we call it soft. This is the notion which all mankind have of hardness and softness: they are neither sensations, nor like any sensation; they were real qualities before they were perceived by touch, and continue to be so when they are not perceived. … There is, no doubt, a sensation by which we perceive a body to be hard or soft. This sensation of hardness may easily be had, by pressing one’s hand against the table, and attending to the feeling that ensues, setting aside, as much as possible, all thought of the table and its qualities, or of any external thing. But it is one thing to have the sensation, and another to attend to it, and make it a distinct object of reflection. The first is very easy; the last, in most cases, extremely difficult.

We are so accustomed to use the sensation as a sign, and to pass immediately to the hardness signified, that, as far as appears, it was never made an object of thought, either by the vulgar or by philosophers; nor has it a name in any language. There is no sensation more distinct, or more frequent; yet it is never attended to, but passes through the mind instantaneously, and serves only to introduce the quality in bodies, which by law of our constitution, it suggests. (pg. 55-56)

This is an example of Reid’s direct realism. The sign makes an impression on our sense organs and we are directly made aware of them by how they signify them. Next, is hardness a concept in our head, or is it felt in the object? Reid has us imagining ourselves running into a stone pillar:

The attention of the mind is here entirely turned towards the painful feeling; and, to speak in the common language of mankind, he feels nothing in the stone, but feels a violent pain in his head. It is quite otherwise when he leans his head gently against the pillar; for then he will tell you that he feels nothing in his head, but feels hardness in the stone. (pg. 56)
The pain in head stays when he takes his head away from the pillar for a short while. However, hardness is the concept signified. It is immediate and goes away as soon as your hand leaves it. You feel it in the stone, where as the pain you feel in your head.

Hardness of bodies is a thing that we conceive as distinctly, and believe as firmly, as any thing in nature. We have no way of coming at this conception and belief, but by means of a certain sensation of touch. (pg. 57)
Hardness is a conception, the thing signified. Hardness is divorced and distinct from the sensation of pain.
[S]upposing we have got the conception of hardness, how come we by the belief of it? Is it self-evident, from comparing the ideas that such a sensation could not be felt, unless such a quality of bodies existed? No. Can it be proved by probable or certain arguments? No, it cannot. Have we got this belief then by tradition, by education, or by experience? No, it is not got in any of these ways. Shall we then throw off this belief as having no foundation in reason? Alas! It is not in our power; it triumphs over reason, and laughs at all the arguments of a philosopher. Even the author of the Treatise of Human Nature, though he saw no reason for this belief, but many against it, could hardly conquer it in his speculative and solitary moments; at other times he fairly yielded to it, and confesses that he ground himself under a necessity to do so.
What shall we say then of this conception, and this belief, which are so unaccountable and untraceable? I see nothing left, but to conclude, that by n original principle of our constitution, a certain sensation of touch both suggests to the mind the conception of hardness, and creates the belief of it: or, in other words, that this sensation is a natural sign of hardness. (pg. 58)
Reid’s theory of perception is threefold when it comes to the sign to the thing signified: conception, belief, immediacy, none of which include reason.
It is now proper to observe, that there are different orders of natural signs, and to point out the different classes into which they may be distinguished, that we may more distinctly conceive the relation between our sensations and the things they suggest, and what we mean by calling sensations sighs of external things.
The first class of natural signs comprehends those whose connection with the thing signified is established by nature, but discovered only by experience. The whole of genuine philosophy consists in discovering such connections, and reducing them to general rules., The great Lord Verulam had a perfect comprehension of this, when he called it an interpretation of nature. No man ever more distinctly understood, or happily expressed the nature and foundation of the philosophic art. What is all we know of mechanics, astronomy, and optics, but connections established by nature, and discovered by experience or observation, and consequences deduced from them? All the knowledge we have in agriculture, gardening, chemistry, and medicine, is built upon the same foundation. (pg. 59)

What we commonly call natural causes might with more propriety, be called natural signs, and what we call effects, the things signified. The causes have no proper efficiency or causality, as far as we know; and all we can certainly affirm, is, that nature hath established a constant conjunction between them and the things called their effects. (pg. 59)

The second class is that where in the connection between the sign and thing signified, is not only established by nature, but discovered to us by a natural principle, without reasoning or experience. Of this kind are the natural signs of human thoughts, purposes, and desires, which have been already mentioned as the natural language of mankind. An infant may be put into a fright by an angry countenance, and soothed again by smiles and blandishments. A child that has a good musical ear, may be put to sleep or to dance, may be made merry or sorrowful, by the modulation of musical sounds. The principles of all the fine arts, and of what we call a fine taste, may be resolved into connection of this kind. A fine taste may be improved by reasoning and experience; but if the first principles of it were not planted in our minds by nature, it could never be acquired. Nay we have already made it appear, that a great part of this knowledge which we have by nature, is lost by the disuse or natural signs, and the substitution of artificial in their place. (pg. 60)

Principles of fine arts are prewired and then cultivated by natural signs—diminished by use of artificial signs.

A third class of natural signs comprehends those which, though we never before had any notion or conception of the things signified, do once give us a conception, and create a belief of it. I shewed formerly, that our sensations suggest to us a sentient being or mind to which they belong: a being which hath a permanent existence, although the sensations are transient and of short duration: a being which is still the same while its sensations and other operations are varied ten thousand ways: a being which hath the same relation to all that infinite variety of thoughts, purposes, actions, affections, enjoyments, and sufferings, which we are conscious of, or can remember. The conception of a mind is neither an idea of sensation nor of reflection; for it is neither like any of our sensations, nor like anything we are conscious of. The first conception of it, as well as the belief of it, and of the common relation it bears to all that we are conscious of, or remember, is suggested to every thinking being we do not know how.

The notion of hardness in bodies, as well as the belief of it, are got in a similar manner; being, by an original principle of our nature, annexed to that sensation which we have when we feel a hard body. And so naturally and necessarily does the sensation convey the notion and belief of hardness, that hitherto they have been confounded by the most acute inquirers in to the principles of human nature, although they appear, upon accurate reflection, not only to be different things, but as unlike as pain is to the point of a sword.
It may be observed, that as the first class of natural signs I have mentioned, is the foundation of true philosophy [that of Newton and Bacon’s empiricism and logic], and the second, the foundation of the fine arts, or of taste; so the last is the foundation of common sense; a part of human nature which hath never been explained. … It appears as evident that this connection between our sensations and the conception and belief of external existences cannot be produced by habit, experience, education, or any principle of human nature that hath been admitted by philosophers. At the same time, it is a fact, that such sensations are invariably connected with the conception and belief of external existences. Hence, by all rules of just reasoning, we must conclude, that his connection is the effect of our constitution, and ought to be considered as an original principle of human nature, till we find some more general principle into which it may be resolved. (pg. 60-61)

When I grasp a ball in my hand, I perceive it at once hard, figured, and extended. The feeling is very simple, and hath not the least resemblance to any quality of body. Yet it suggests to us three primary qualities perfectly distinct from one another, as well as from the sensation which indicates them. When I move my hand along the table, the feeling is so simple, that I find it difficult to distinguish it into things of different natures; yet it immediately suggests hardness, smoothness, extension, and motion, things of very different natures, and all of them as distinctly understood as the feeling which suggests them.
We are commonly told by philosophers [such as Locke] that we get the idea of extension by feeling along the extremities of a body, as if there was no manner of difficulty in the matter. I have sought, with great pains, I confess, to find out how this idea can be got by feeling, but I have sought in vain. Yet it is one of the clearest and most distinct notions we have; nor is there anything whatsoever, about which the human understanding can carry on so many long and demonstrative trains of reasoning.

The notion of extension is so familiar to us from infancy, and so constantly obtruded by every thing we see and feel, that we are apt to think it obvious how it comes into the mind; but upon a narrow examination we shall find it utterly inexplicable. It is true we have feelings of touch, which every moment present extension to the mind; but how they come to do so, is the question; for those feelings do no more resemble extension, than they resemble justice or courage: nor can the existence of extended things be inferred from those feelings by any rules of reasoning: so that the feelings we have by touch, can neither explain how we get the notion, nor how we come by the belief of extended things. (pg. 63)

What hath imposed upon philosophers in this matter, is, that the feelings of touch, which suggest primary qualities, have no names, nor are they ever reflected upon. They pass through the mind instantaneously, and serve only to introduce the notion and belief of external things, which by our constitution are connected with them. They are natural signs, and the mind immediately passes to the thing signified, without making the least reflection upon the sign, or observing that there was any such thing. Hence it hath always been taken for granted, that the idea of extension, figure, and motion, are ideas of sensation, which enter into the mind by the sense of touch, in the same manner as the sensations of sound and smell do by the ear and nose. The sensations of touch are so connected by our constitution, with the notions of extension, figure, and motion, that philosophers have mistaken the one for the other, and never have been able to discern that they were not only distinct things, but altogether unlike. (pg. 63-64)
There, extension, figure, motion are not sensations of touch in our mind but directly sensed.
Let a man press his hand against the table: he feels it hard. But what is the meaning of this? The meaning undoubtly is, that he hath a certain feeling of touch, from which he concludes, without any reasoning, or comparing ideas, that there is something external really existing, whose parts stick so firmly together, that they cannot be displaced without considerable force.
There is here a feeling, and a conclusion drawn from it, or someway suggested buy it. In order to compare these, we must view them separately, and then consider by what tie they are connected, and wherein they resemble one another. The hardness of the table is the conclusion. Let a man attend distinctly to this medium by which we are led to that conclusion, and he will perceive them to be as unlike as any two things in nature. The one is a sensation of the mind, which can have no existence but in a sentient being; nor can it exist one moment longer than it is felt; the other is in the table, and we conclude without any difficulty, that it was in the table before it was felt, and continues after the feeling is over. The one implies no kind of extension, nor parts, nor cohesion; the other implies all these. Both indeed admit of degrees, and the feeling, beyond a certain degree, is a species of pain; but adamantine hardness does not imply the least pain.
And as the feeling hath no similitude to hardness, so neither can our reason perceive the least tie or connection between them; nor will the logician ever be able to show a reason why we should conclude hardness from this feeling, rather than softness, or any other quality whatsoever. But in reality all mankind are led by their constitution to conclude hardness from this feeling. (pg. 64)
It’s a principle of common sense to go from the sensation to the conception of hardness. Our constitution immediately leads us to believe in an external world.
[T]he wisdom of philosophy is set in opposition to the common sense of mankind. The first pretends to demonstrate a priori, that there can be no such thing as a material world; that sun, moon, stars, and earth, vegetable, and animal bodies, are, and can be nothing else, but sensations in the mind, or images of those sensations in the memory and imagination; that, like pain, and joy, they can have no existence when they are not thought of. The last [common sense] can conceive no other wise of this opinion, than as a kind of metaphysical lunacy; and concludes, that too much learning is apt to make men mad; and that the man who seriously entertains this belief, though in other respects he may be a very good man, as a man may be in who believes he is made of glass yet surely he hath a soft place in his understanding, and hath been hurt by much thinking. (pg. 67-68)
Common sense and reason have both one author; that Almighty author, in all whose other works we observe a consistency, uniformity, and beauty, which charm and delight the understanding… (pg. 68)
To what purpose is it for philosophy to decide against common sense in this or any other matter? The belief of a material world is older, and of more authority, than any principles of philosophy. (pg. 68)
[I]f Reason should stomach and fret ever so much of this yoke (the belief that there is an external world), she cannot throw it off; if she will not be the servant of Common Sense, she must be her slave. (pg. 68-69)
In order to therefore to reconcile reason to common sense in this matter, I beg to leave to offer to the consideration of philosophers these two observations. First, that in all this debate about the existence of a material world, it hath been taken for granted on both sides, that this same material world, if any such there be, must be the express image of our sensations; that we can have no conception of any material thing which is not like some sensation in our minds; and particularly, that the sensation of touch are images of extension, hardness, figure, and motion. Every argument brought against the existence of a material world, either by the Bishop of Cloyne, or by the author of the Treatise of Human Nature, is supposeth this. If this is true, their arguments are conclusive and unanswerable; but, on the other hand, if it is not true, there is no shadow of argument left. Have those philosophers then given any solid proof of this hypothesis, upon which the whole weight of so strange a system rests? No. They have not so much as attempted to do it. But, because ancient and modern philosophers have agreed in this opinion, they have taken it for granted. But let us, as becomes philosophers, lay aside authority; we need not surely consult Aristotle or Locke, to know whether a pain be like the point of a sword. I have as clear a conception of extension, hardness, and motion, as I have of the point of a sword; and, with some pains and practices, I can form as clear a notion of the other sensations of touch, as I have of pain. When I do so, and compare them together, it appears to me clear as day-light, that the former are not of kin to the latter, nor resemble them in any one feature. They are as unlike, yea as certainly and manifestly unlike, as pain is to the point of a sword. It may be true, that those sensations first introduced the material world to our acquaintance; it may be true, that it seldom or never appears without their company; but, for all that, they are as unlike as the passion of anger is to those features of the countenance which attend it. (Pg. 69)
So that, in the sentence those philosophers have passed against the material world, there is an error personae. Their proof [mediational ideas] touches not matter, or any of its qualities; but strikes directly against an idol of their own imagination, a material world made of ideas and sensations, which never had nor can have existence.
Secondly, the very existence of our conceptions of extension, figure, and motion, since they are neither ideas of sensation nor reflection, overturns the whole ideal system, by which the material world hath been tried and condemned: so that there hath been likewise in this sentence an error juris. (pg. 69-70)
The conception of extension, motion, and the other attributes of matter, cannot be the effect of error or prejudice; it must be the work of nature. And the power or faculty, by which we acquire those conceptions, must be something different from any power of the human mind that hath been explained, since it is neither sensation nor reflection.
This I would therefore humbly propose as an experimentum crucis, by which the ideal system must stand or fall; and it brings the matter to a short issue: Extension, figure, motion, may, any one, or all of them, be taken for the subject of this experiment. Either they are ideas of sensation, or they are not. If any one of them can be shown to be an idea of sensation, or to have the least resemblance to any sensation, I lay my hand upon my mouth, and give up all pretence to reconcile reason to common sense in this matter, and must suffer the ideal skepticism in triumph. But if, on the other hand, they are not ideas of sensation, nor like to any sensation, then the ideal system is a rope of sand, and all the labored arguments of the skeptical philosophy against a material world, and against the existence of every thing but impressions and ideas, proceed upon a false hypothesis. (pg. 70)

That our thoughts and sensations must have a subject, which we call ourself, is not therefore an opinion got by reasoning, but a natural principle. That our sensations of touch indicate something external, extended, figured, hard or soft, is not a deduction of reason, but a natural principle. The belief of it, and the very conception of it, are equally parts of our constitution. (pg. 72)
The concept of extension is innate. This is Reid’s nativism that is an a priori intuition.
How a sensation should instantly make us conceive and believe the existence of an external thing altogether unlike to it, I do not pretend to know; and when I say that the one suggests the other, I mean not to explain the manner of their connection, but to express a fact, which every one may be conscious of; namely, that, by a law of our nature, such a conception and belief constantly and immediately follow the sensation. (Pg. 75)
The theory of ideas, like the trojan horse, had a specious appearance both of innocence and beauty; but if those philosophers had known that it carried in its belly death and destruction to all science and common sense, they would not have broken down their walls to give it admittance.
That we have clear and distinct conceptions of extension, figure, motion, and other attributes of body, which are neither sensations, nor have sensations, is a fact of which we may be as certain, as that we have sensations. And that all mankind have a fixed belief of an external material world, a belief which is nether got by reasoning nor education, and a belief which we cannot shake off, even when we seem to have strong arguments against it, and no shadow of argument for it, is likewise a fact, these facts are phenomena of human nature, from which we may justly argue against any hypothesis, however generally received. But to argue from a hypothesis against facts, is contrary to the rules of true philosophy. (pg. 75-76)
Chapter. VI
Of Seeing
I conclude then, that colour is not a sensation, but a secondary quality of bodies, in the sense we have already explained; that it is a certain power or virtue in bodies… Colour differs from other secondary qualities in this, that whereas the name of the quality is sometimes given to the sensation which indicates it, and is occasioned by it, we have, as far as I can judge, give the name of colour to the sensation, but to the quality only. (pg. 87)
[Regarding] colour, we may infer two things. The first is that one of the most remarkable paradoxes of modern philosophy, which have been universally esteemed as a great discovery, is, in reality, when examined to the bottom, nothing else but an abuse of words. The paradox I mean is, The colour is not a quality of bodies, but only an idea in the mind. We have shown, that the word colour, as used by the vulgar, cannot signify an idea in the mind, but a permanent quality of body, to which the common use of this word exactly agrees. Can any stronger proof be desired, that this quality is that to which the vulgar give the name of colour? (pg. 87-88)
When they explained and established the distinction between the appearance which colour makes to the eye, and the modification of the coloured body, which, by the laws of Nature, causes the appearance; the question was, Whether to give the name of colour to the cause, or to the effect? By giving it, as they have done, to the effect, they set philosophy apparently in opposition to common sense, and expose it to the ridicule of the vulgar. But had they given the name of colour to the cause, as they ought to have done, they must have affirmed, with the vulgar, that colour is a quality of bodies; and that there is neither colour nor any thing like it in the mind. (pg. 90)
There is no phaenomenon in nature more unaccountable, than the intercourse that is carried on between the mind and the external world: there is no phaenomenon which philosophical spirits have shown greater avidity to pry into, and to resolve. It is agreed by all, that this intercourse is carried on by means of the senses: and this satisfies the vulgar curiosity, but not the philosophic. Philosophers must have some systems, some hypothesis, that shews the manner in which our senses make us acquainted with external things. All the fertility of human invention seems to have produced only one hypothesis for this purpose, which therefore hath been universally received; and that is, that the mind, like a mirror, receives the images of things from without, by means of the sense; so that there use must be to convey these images to the mind. …The necessary and allowed consequence of this hypothesis is, That no material thing, nor any quality of material things, can be conceived by us, or made an object of thought until its image is conveyed to the mind by means of the senses (pg. 91)
The idea system here traces the outline among all theories in modern philosophy. They are copy, mediational, representational theories that originated with Des Cartes. Which ultimately leads to solipsism of the external world.
The first-born of this union, and perhaps the most harmless, was, That the secondary qualities of body were mere sensations of the mind. To pass by Malebranche’s notion of seeing all things in the ideas of the divine mind, as a foreigner never naturalized in this island; the next was Berkeley’s system, That extension, and figure, and hardness, and motion; that land, and sea, and houses, and our own bodies, as well as those of our wives children, and friends, are nothing but ideas of the mind and that there is nothing existing in nature, but minds and ideas. (pg.94)
Here, Reid tells us the implication of the idea system, that it strips us of the things we are willing to die for, or more importantly, live for. That everything, even loved ones, are reduced to ideas in the mind, which results to mere figments of the imagination. Next, in Reid’s later book titled, Essays on the Intellectual Powers of man, he distinguishes between primary and secondary qualities.
[T]here appears to me to be a real foundation for the distinction; and it is this: That our senses give us a direct and distinct notion of the primary qualities, and inform us what they are in themselves: But of the secondary qualities, our senses give us only a relative and obscure notion. They inform us only, that they are qualities that affect us un a certain manner, that is, produce in us a certain sensation; but as to what they are in themselves, our senses leave us in the dark (Essays on the Intellectual Powers of Man, pg. 201)
Our conception of a primary quality, a triangle for example, is direct. However, the concept of redness of a body isn’t direct but relative. Something external to us in the body causes us to see it as red, but it remains a mystery to Reid why that is.
When I see an object, the appearance which the colour of it makes, may be called the sensation, which suggests to me some external thing as its cause; but it suggests likewise the individual direction and position of this cause with regard to the eye. I know it is precisely in such a direction, and in no other. At the same time, I am not conscious of any thing that can be called sensation, but the sensation of colour. The position of the coloured thing is no sensation, but it is by the laws of my constitution presented to the mind along with the colour. The position of the coloured thing is no sensation, but it is by the laws of my constitution presented to the mind alone with the colour, without any additional sensations. (pg. 99)
We have reason to believe, that the rays of light make some impression upon the retina; but we are not conscious of this impression; nor have anatomists or philosophers been able to discover the nature and effects of it; whether it produces a vibration in the nerve, or the motion of some subtile fluid contained in the nerve, or something different from either, to which we cannot give a name. Whatever it is, we shall call it the material impressions; remembering carefully, that it is not an impression upon the mind, but upon the body; and that it is no sensation, nor can resemble sensation, any more than firure or motion can resemble thought. Now, this material impression made upon a particular point of the retina, by the laws of our constitution, suggests two things to the mind, namely, the colour, and the position of some external object. (pg. 100)
There wasn’t a clear understanding of color vision in Reid’s day. Today we know the first response to light is biochemical, and there are cones in the fovea, which resides in the retina of the eyes. Cones pick up wavelengths of three shades of color, blue, red, and green. Rods, in contrast to cones, reside outside the fovea, in periphery vision, only picks up the light that impresses on them, so everything to them appears in black and white.

Let us suppose, therefore, since it plainly appears to be possible, that our eyes had been so framed, as to suggest one of them without the other. Let us suppose, therefore, since it plainly appears to be possible, that our eyes had been so framed, as to suggest to us the position of the object, without suggesting colour, or any other quality: What is the consequence of this supposition? It is evidently this, that the person endued with such an eye, would perceive the visible figure of bodies, without having any sensation or impression made upon his mind. The figure he perceives is altogether external; and therefore cannot be called an impression upon the mind, without the grossest abuse of language. If it should be said, that it is impossible to perceive a figure, unless there be some impression of it upon the mind; I beg leave not to admit the impossibility of this without some proof.; and I can find none. Neither can I conceive what is meant by an impression of figure upon the mind. I can conceive an impression of figure upon wax, or upon any body that is fit to receive it; but an impression of it upon the mind, is to me quite unintelligible; and although I form the most distinct conception of the figure, I cannot, upon the strictest examination, find any impression of it upon my mind. (pg. 100-101)
We see out there, external to ourselves, visible figures, Reid is saying. The impression made is upon the sense organs and not the mind. An impression upon the mind is an unintelligible thing to Reid, and one that we can’t ever understand or witness.
The Geometry of the Visibles section is the lynch pin of Reid’s vision theory against mediation. In the patronage of Bacon and Newton, he gives us an experiment for us to think about: If the image of a right angle triangle is projected on the spherical retina of the eye, one should see a curved triangle. However, you see a right angle triangle instead. There is a fitness between our physiology and mind that goes from the sign to the thing signified. He wants to argue that this is not mediated or copied, but direct and accurate. (pg. 103-108)
Philosophers conceive, that the impression made on the retina by the rays of light, is communicated to the optic nerve, and by the optic nerve conveyed to some part of the brain, by them called the sensorium; and that the impression thus conveyed to the sensorium is immediately perceived by the mind, which is supposed to reside there. But we know something of the seat of the soul: and we are so far from perceiving immediately what is transacted in the brain, that of all parts of the human body we know least about it. (pg. 120)
Here, Reid admits of the limitations we know of the connection between the mind and brain. The gap between the two have been persistent in philosophy of mind because ideas, sensations, and feelings seem so different than tables, automobiles, and apples. Some have tried to bridge the divide by way of philosophical theories called dual aspect theory, or panpsychism, which claim that there is just one substance with two aspects to it, and that there is an aspect to matter that we can’t see with the scientific eye, for that can only see the extrinsic nature of reality, where subjective consciousness beings to see the intrinsic nature of reality.

Of Seeing
Perceptions in General
Thus, I feel a pain; I see a tree: the first denoteth a sensation, the last a perception. The grammatical analysis of both expressions is the same: for both consist of an active verb and an object. But, if we attend to the things signified by these expressions, we shall find, that in the first, the distinction between the act and the object is not real but grammatical; in the second, the distinction is not only grammatical but real.
The form of the expression, I feel pain, I might seem to imply, that the feeling is something distinct from the pain felt; yet, in reality, there is no distinction. As thinking a thought is an expression which could signify no more than thinking, so feeling a pain signifies no more than being pained. What we have said of pain is applicable to even other mere sensation. (pg. 167-168)
Reid wants to make a distinction between sensations and perceptions. Sensations are objectless, where a percept has an object—the tree for instance. “Feel” is grammatically the same thing as “pain”. Sensations are in mind; perceptions are outside of mind and have an object.
The sceptic asks me, Why do you believe the existence of the external object which you perceive? This belief, Sir, is none of my manufacture; it came from the mind of Nature; it bears her image and superscription; and, if it is not right, the fault is not mine: I even took it upon trust, and without suspicion. Reason, says the sceptic, is the only judge of truth, and you ought to throw off every opinion and every belief that is not grounded in reason. Why, Sir, should I believe the faculty of reason more than that of perception; they came both from the same shop, and were made by the same artist; and if he puts one piece of false ware into my hands, what should hinger him from putting another? (pg. 168-169)
Why would Nature fit us out with two faculties, common sense and reason, one to deceive us, and another, that being reason, to uncover the deception? Not trusting common sense, which is a difficult thing to do in daily affairs, results buries us in skepticism that all we might be are brains in vats with electrical signals being sent to the brain to create whatever illusion driven into it. Reid is trying to break the brain in the vat existence, or that some evil genius is deceiving us. This skepticism towards the senses started as far back as Plato’s Allegory of the Cave. It’s a bias philosophers have had against common sense which philosophers have tried to reason their way out of ever since, but the mines of reason have been dug deeper and deeper and only lead us into further caves of ignorance and skepticism.
If Nature intended to deceive me, and impose upon me by false appearances, and I, by my great cunning and profound logic, have discovered the imposture; prudence would dictate to me in this case, even to put up this indignity done me, as quality as I could, and not to call her an imposter to her face, lest she should be even with me in another way… But what is the consequence? I resolve not to believe my senses. I breaky my nose against a post that comes in my way; I step into a dirty kennel; and, after twenty such wise and rational actions, I am taken up and clapt into a mad-house. (pg. 169-170)
Reid’s theory of perception, as said before, is threefold:-conception, belief, and immediacy, none of which are given by reason but direct. The value we must place on perception is akin to a mathematical axiom that we must take for granted or else reason will ebb and flow at the value of perception until we find ourselves cornered into solipsism the likes of which modern philosophy has found herself in.
There must be some action or impression upon the organ of sense, either by the immediate application of the object, or by the medium that goes between them. Thirdly, The nerves which go from the brain to the organ, must receive some impression by means of that which was made up the organ; and, probably, by means of the nerves, some impression must be made upon the brain. Fourthly, The impression made upon the organ, nerves, and brain, is followed by a sensation. And, last of all, This sensation is followed by the perception of the subject. (pg. 174)
This is Reid’s direct realism train succession: impression on sense organ, sensation nerve fibers send signal to brain, which then makes an impression on the brain, which is followed by the sensation, which then results in the perception. However, how they communicate to each other is largely left unknown, which Reid himself states in next:
If we can only acquire the habit of attending to our sensations, we may know them perfectly. How are the sensations of the mind produced by impressions upon the body? Of this we are absolutely ignorant, having no means of knowing how the body acts upon the mind, or the mind upon the body. When we consider the nature and attributes of both, they seem to be so different, as so unlike, that we can find no handle by which the one may lay hold of the other. There is a deep and dark gulf between them, which our understanding cannot pass; and the manner of their correspondence and intercourse is absolutely unknown. (pg. 176)
Here Reid questions how the mind and body interact with each other. Reid seems to take a position that they are different, but they could very well be different sides of the same coin, as in panpsychism and dual aspect theory.
The perceptions we have, might have been immediately connected with the impressions upon our organs without any intervention of the sensations. This last seems really to be the case in one instance, to wit, in our perception of the visible figure of bodies. (pg. 176)

The appearance of things to the eye, always corresponds to the fixed laws of nature; therefore, if we speak properly, there is no fallacy of the senses. (pg. 189)
The signs in original perception are sensations, of which nature hath given us a great variety, suited to the variety of the things signified by them. Nature hath established a real connection between the signs and the things signified; and nature hath also taught us the interpretation of the signs; so that, previous experience, the sign suggests the thing signified, and creates the belief of it.
The signs in natural language are features of the faces, gestures of the body, and adulations of the voice; the variety of which is suited to the variety of the things signified by them. Nature hath established a real connection between these signs, and the thoughts and dispositions of the mind which are signified by them; and nature hath taught us the interpretation of these signs; so that, previous to experience, the sign suggests the thing signified, and creates the belief of it. … The signs in the natural language of the human countenance and behavior, as well as the signs in our original perceptions, have the same signification in all climates and in all nations; and the skill of interpreting then is not acquired, but innate. (pg. 190-191)
An original principles of the human constitution is the propensity to tell the truth and to believe what others say, which Reid calls the principle of credulity. There is a fitness between our words and our beliefs that nature has created, but that we diminish as we get older, as we trust less others that have proven liars. Next, Reid talks of the shortcomings of Humeian causation called constant conjunction.
It is undeniable, and indeed is acknowledged by all, that when we have found two things to have been constantly conjoined in the course of nature, the appearance of one of them is immediately followed by the conception and belief of the other. The former becomes a natural sign of the latter; and the knowledge of their constant conjunction in time past, whether got by experience or otherwise, is sufficient to make us rely with assurance upon the continuance of that conjunction. … Thus, if a certain degree of cold freezes water to-day, and has been known to do so in all time past, we have no doubt but the same degree of cold will freeze water to-morrow, or a year hence. That this is a truth which all men believe as soon as they understand it, I readily admit; but the question is, Whence does its evidence arise? Not from comparing ideas, surely. For when I compare the idea of cold with that of water hardened into a transparent solid body, I can perceive no effect of the other: no man can give a shadow of reason why nature hath True; experience informs us that they have been conjoined in time past: but no man ever had any experience of what is future: and this is the very question to be resolved. How we come to believe the future will be like the past? … But though this consideration, when we come to the use of reason, may confirm our belief of the continuance of the present course of nature, it is certain that it did not give rise to this belief; for children and idiots have this belief as soon as they know that fire will burn them. It must therefore be the effect of instinct, not of reason.
The wise Author of our nature intended, that a great and necessary part of our knowledge should be derived from experience, before we are capable of reasoning, and he hath provided means perfectly adequate to this intention. (pg. 195-96)
[H]e hath implanted in human minds an original principle by which we believe and expect the continuance of the course of nature, and the countenance of those connections which we have observed in time past. It is by this general principle of our nature, that when two things have been found connected in time past, the appearance of the one produces the belief of the other. (pg.197)
Reid challenges Hume’s Constant conjunction starting on page 199. No two events in human history have been as reliable as day and night, but no one believes that one caused the other. Instead, we have active powers within ourselves from a young age when we can darken the world by closing our eyes, do you see? By our own agentic powers of our causing things to happen in the world, we extend those same powers to the natural world, and that’s how we first come to understand causation in the external world, by initially relating it to our own active powers.
All our knowledge of nature, beyond our original perceptions, is got by experience, and consists in the interpretation of natural signs. The constancy of nature’s laws connects the sign with the thing signified, and, by the natural principle just now explained, we rely upon the continuance of the connections which experience hath discovered; and thus the appearance of the sign, is followed by the belief of the thing signified.

Upon this principle of our constitution, not only acquired perception, but all inductive reasoning, and all our reasoning from analogy, is grounded: and therefore, for want of another name, we shall beg leave to call it the inductive principle. It is from the force of this principle, that we immediately assent to that axiom upon which all our knowledge of nature is built, That effects of the same kind must have the same cause. For effects and causers, in the operations of nature, mean nothing but signs, and the things signified by them. We perceive no proper causality of efficiency in any natural cause; but only a connection established by the course of nature between it and what is called its effect. Antecedently to all reasoning, we have, by our constitution, an anticipation, that there is a fixed and steady course of nature; and we have an eager desire to discover this course of nature. … If any reader should imagine that the inductive principle may be resolved into what philosophers usually call the association of ideas, let him observe, that, by this principle, natural signs are not associated with the idea only but the belief of the things signified. Now this can with no propriety be called an association of ideas, unless ideas and belief be one and the same thing. (pg. 198-199)

No perception itself could generate a belief. The belief is therefore instinctive.
Take away the light of this inductive principle, and Experience is as blind as a mole: she may indeed feel what is present, and what immediately touches her; but she sees nothing that is either before or behind, upon the right hand or upon the left, future or past. (pg. 200)

So what are we to say of Reid’s account of the follies of Modern Philosophy? He was Hume’s most successful critic, but he was overshadowed by the accomplishments of Immanuel Kant. Nevertheless, his critique of the idea theory that traces the outlines of modern philosophy is critical to understand Modern Philosophy original defect, and he also criticized Locke’s Newtonian theory of mind stating it was a false analogy popularized by later philosophers. The stakes of philosophy are often high for serious people who live according to their own philosophy. Most of the sceptics would be out by sundown if they had to do so, and Modern Philosophy, with its idea system has dire consequences for what we can know. Hume, the great skeptic, is the consequence of this idea theory, which brought it to it’s fated conclusion and shows we really can’t know much about anything. For Hume we could be living the life of a brain in the vat existence, which no one would wantonly choose if they had the choice. But if ideas always mediate our knowledge there is no other conclusion, and Des Cartes evil genius could be playing on our heart strings when constructing a false world where our actions would bear no consequences. However a hero would never choose such a life, because heroes set out to make real differences in the world.
I view it as a pragmatic decision when ultimately deciding which to live by. Reid gives good reasons to follow common sense. But when faced with indeterminable metaphysical problems we can’t curl up and lay paralyzed, so we must choose and ask ourselves, “what’s in our highest virtue to believe in?” I choose Reid’s view for good reason and for good consequence. Hume and Reid had a correspondence before Reid released An Inquiry into the Human Mind in 1764. Hume’s first response was that clerics should stick to theology, but Hume was then pressed to read the Inquiry. Hume thought it was well reasoned, but he didn’t understand the Section of the Geometry of the Visibles. which was the lynch pin of Reid’s experimental philosophy, that was so influenced by Bacon and Newton. Reid replies to Hume with Enlightenment sensibilities. Reid writes:
Your Friendly Adversaries Drs Campbel and Gerard as well as Dr Gregory return their complements to you respectfully. A little philosophical Society here of which all the three are members, is much indebted to you for its Entertainment. Your Company would, although we are all good Christians, be more acceptable than that of Saint Athanasius. And since we cannot have you upon the bench, you are brought oftener than any other man, to the bar, accused and defended with great Zeal but without bitterness. If you write no more in morals politicks or metaphysicks, I am afraid we shall be at a loss for Subjects.