Notes on The Principles of Common Sense by Thomas Reid
June 27, 2026
by William P. Meyers

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An Inquiry into the Human Mind on the Principles of Common Sense.

"Common sense" is a tricky term. One person's view of common sense can be another person's view of idiocy. Thomas Reid wrote his Inquiry not to argue about whether it is better to save or to spend, but in opposition to a philosophic trend that was popular in his time. His time was 1710 to 1797, his plase was Scotland (part of Great Britain). The leaders in the tend he opposed are far better known to modern students of philosophy: Rene Descartes, John Locke, Bishop Geoerge Berkeley, and David Hume.

According to Reid, in An Inquiry into the Human Mind on the Principles of Common Sense, the problem with these philosophers is that they are so skeptical in their reasoning that they deny the reality of what ordinary people, people with common sense but no philosophical training, call reality. In fact they all argued with each other, and each made contributions in subjects other than the human mind's place in reality. I would note that each of these men were proponents of the science and reasoning. So to some extent, in his critique, Reid over-simplified the view of his opponents. In particular he identified them with Berkeleys assertion (or deduction) that the outside world, that we know through our senses, consists of ideas, which are created by God.

In reading Reid it is well to keep in mind the state of science in his era. Many advancements were made in science and industry in the 1700s, but biological knowledge lagged behind the much more simple field of physics. Charles Darwin had not been born and there was minimal understanding of how nerve cells actually worked, though the gross anatomy of the nervous system was well known.

[Caveat: these notes are what I found interesting; others might disagree; readers are welcome to read the entire book, available in reprints, for themselves.]

"I know it is said, that in a delirium, or in dreaming, men are apt to mistake one for the other. But does it follow from this, that men who are neither dreaming, nor in a delirium, cannot distinguish them? I cannot tell; neither can I tell how a man knows that he exists; but if any man seriously doubts whether he is in a delirium, I think it highly probable that he is, and that it is time to seek for a cure, which I am persuaded he will not find in the whole system of logic." [Chapter 2, Section 5, paragraph 2]

"This occasions an ambiguity in many words, which having the same causes in all languages, is common to all, and is apt to be overlooked even by philosophers." [Chapter 2, Section 9, paragraph 4]

"There is no doubt a sensation by which we perceive a body to be hard or soft. . . "

"We are so accustomed to use the sensation asa sign, and so pass immediately to the hardness signified, that, as far as appears, it was never made an object of thought. . ."

"Thus, when one is learning a language, he attends to the sounds; but when he is the master of it, he attends only to the sense of what he would express." [Chapter 3, Section 2]

"Thus, the wisdom of [Bishop Berkeley's] philosophy is set in opposition to the common sense of mankind. The first pretend 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." [Chapter 5, section 7, paragraph 3]

"To such a sceptic I have nothing to say; but of the semi-sceptics, I should beg to know, why they believe in the existence of their impressions and ideas. The true reason I take to be, because they cannot help it;and the same reason will lead them to believe many other things." [Chapter 5, Section 7, paragraph 17]

"If you ask a man that is no philosopher, what colour is? or what makes one body appear white, another scarlet? he cannot tell. He leaves that inquiry to philosophers, and can embrace any hypothesis abut it, except that of our modern philosophers, who affirm, that colour is not in the body, but only in the mind." [Chapter 6, section 5, paragraph 4]

"It is evident, therefore, that the pictures upon the retina are, by the laws of nature, a mean of vision; but in what way they accomplish their end, we are totally ignorant. Philosophers conceive, thatthe impression on the retina by the rays of light, is communicated to theoptic nerve and by the optic nerve conveyed to some part of the brain, by them called the sensorium; and that the impression thus converyed to the sensorium is immediately perceived by the mind, which is supposed to reside there. But we know nothing 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." [Chapter 6, Section 7, paragraph 4]. I would note that scientists now know much more about the retina, optic nerve, nerve cells, and vision processing in the brain, but that the basic philosophic problem remains unsolved.

"When one speaks to us in a language that is familiar, we hear certain sounds, and this is all the effect that his discourse has upon us by nature: but by custom we understand the meanings of these sounds; and therefore we fix our attention, not upon the sounds, but upon the things signified by them. In like manner, we see only the visible appearance of objects by nature; but we learn by custom to interpret these appearances, and to understand their meaning. And when this visual language is learned and becomes familiar, we attend only to the things signified; and cannot, without great difficulty, attend to the signs by which they are presented." [Chapter 6, Section 9, near its end]

"My belief is carried along by perception, as irresistibly as my body by the earth. And the greatest skeptic will find himself to be in the same position. He may struggle hard to disbelieve the information of the senses, as a man does to swim against a torrent; but ah! it is in vain. It is in vain that he strains every nerve and wrestles with nature, and with every object that strikes upon his senses. For after all, when his strength is spend in the fruitless attempt, he will be carried down the torrent with the common herd of believers." [Chapter 6, Secton 20, paragraph 7]

I, William Meyers, note futher that this principle of common sense also works against the Buddhist doctrine proposing that the world is an illusion. While Ludwig Wittgenstein does not mention Reid, much of the combing through the mistake philosophers make through misuse of language seem to echo the writings in this book.

Think of this essay as an appetizer. If you are interested in philosophy, I believe that Thomas Reid is one of the philosophers worthy of study. On the other hand, in our universities, he is sadly neglected.