Front Page Titles (by Subject) § 5.: Apology for Metaphysical Absurdities—Sensation without a Sentient, a Consequence of the Theory of Ideas—Consequences of this Strange Opinion - Selections from the Scottish Philosophy of Common Sense
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§ 5.: Apology for Metaphysical Absurdities—Sensation without a Sentient, a Consequence of the Theory of Ideas—Consequences of this Strange Opinion - Thomas Reid, Selections from the Scottish Philosophy of Common Sense 
Selections from the Scottish Philosophy of Common Sense, edited, with an introduction by G.A. Johnston (Chicago: Open Court, 1915).
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Apology for Metaphysical Absurdities—Sensation without a Sentient, a Consequence of the Theory of Ideas—Consequences of this Strange Opinion
Having considered the relation which the sensation of smelling bears to the remembrance and imagination of it, I proceed to consider what relation it bears to a mind, or sentient principle. It is certain, no man can conceive or believe smelling to exist of itself, without a mind, or something that has the power of smelling, of which it is called a sensation, an operation, or feeling. Yet, if any man should demand a proof that sensation cannot be without a mind or sentient being, I confess that I can give none; and that to pretend to prove it, seems to me almost as absurd as to deny it.
This might have been said without any apology before the Treatise of Human Nature appeared in the world. For till that time, no man, as far as I know, ever thought either of calling in question that principle, or of giving a reason for his belief of it. Whether thinking beings were of an ethereal or igneous nature, whether material or immaterial, was variously disputed; but that thinking is an operation of some kind of being or other, was always taken for granted, as a principle that could not possibly admit of doubt.
However, since the author above mentioned, who is undoubtedly one of the most acute metaphysicians that this or any other age hath produced, hath treated it as a vulgar prejudice, and maintained that the mind is only a succession of ideas and impressions without any subject; his opinion, however contrary to the common apprehensions of mankind, deserves respect. I beg therefore, once for all, that no offence may be taken at charging this or other metaphysical notions with absurdity, or with being contrary to the common sense of mankind. No disparagement is meant to the understandings of the authors or maintainers of such opinions. Indeed, they commonly proceed, not from defect of understanding, but from an excess of refinement; the reasoning that leads to them often gives new light to the subject, and shews real genius and deep penetration in the author; and the premises do more than atone for the conclusion.
If there are certain principles, as I think there are, which the constitution of our nature leads us to believe, and of which we 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; and what is manifestly contrary to them, is what we call absurd.
Indeed, if it is true, and to be received as a principle of philosophy, that sensation and thought may be without a thinking being, it must be acknowledged to be the most wonderful discovery that this or any other age hath produced. The received doctrine of ideas is the principle from which it is deduced, and of which indeed it seems to be a just and natural consequence. And it is probable, that it would not have been so late a discovery, but that it is so shocking and repugnant to the common apprehensions of mankind, that it required an uncommon degree of philosophical intrepidity to usher it into the world. It is a fundamental principle of the ideal system, that every object of thought must be an impression or an idea—that is, a faint copy of some preceding impression. This is a principle so commonly received, that the author above mentioned, although his whole system is built upon it, never offers the least proof of it. It is upon this principle, as a fixed point, that he erects his metaphysical engines, to overturn heaven and earth, body and spirit. And, indeed, in my apprehension, it is altogether sufficient for the purpose. For, if impressions and ideas are the only objects of thought, then heaven and earth, and body and spirit, and everything you please, must signify only impressions and ideas, or they must be words without any meaning. It seems, therefore, that this notion, however strange, is closely connected with the received doctrine of ideas, and we must either admit the conclusion, or call in question the premises.
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 everything 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 ideas and impressions as the sole existences in the universe. What if, at last, 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?
However, hitherto these philosophers acknowledge the existence of impressions and ideas; they acknowledge certain laws of attraction, or rules of precedence, according to which, ideas and impressions range themselves in various forms, and succeed one another: but that they should belong to a mind, as its proper goods and chattels, this they have found to be a vulgar error. These ideas are as free and independent as the birds of the air, or as Epicurus’s atoms when they pursued their journey in the vast inane. Shall we conceive them like the films of things in the Epicurean system?
Or do they rather resemble Aristotle’s intelligible species, after they are shot forth from the object, and before they have yet struck upon the passive intellect? But why should we seek to compare them with anything, since there is nothing in nature but themselves? They make up the whole furniture of the universe; starting into existence, or out of it, without any cause; combining into parcels, which the vulgar call minds; and succeeding one another by fixed laws, without time, place, or author of those laws.
Yet, after all, these self-existent and independent ideas look pitifully naked and destitute, when left thus alone in the universe, and seem, upon the whole, to be in a worse condition than they were before. 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—a discovery big with consequences which cannot easily be traced by those deluded mortals who think and reason in the common track. We were always apt to imagine, that thought supposed a thinker, and love a lover, and treason a traitor: but this, it seems, was all a mistake; and it is found out that there may be treason without a traitor, and love without a lover, laws without a legislator, and punishment without a sufferer, succession without time, and motion without anything moved, or space in which it may move: or if, in these cases, ideas are the lover, the sufferer, the traitor, it were to be wished that the author of this discovery had farther condescended to acquaint us whether ideas can converse together, and be under obligations of duty or gratitude to each other; whether they can make promises and enter into leagues and covenants, and fulfil or break them, and be punished for the breach. If one set of ideas makes a covenant, another breaks it, and a third is punished for it, there is reason to think that justice is no natural virtue in this system.
It seemed very natural to think that the Treatiseof 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.
After all, this curious system appears not to be fitted to the present state of human nature. How far it may suit some choice spirits, who are refined from the dregs of common sense, I cannot say. It is acknowledged, I think, that even these can enter into this system only in their most speculative hours, when they soar so high in pursuit of those self-existent ideas as to lose sight of all other things. 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 an 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.