Descarte’s argument for sensations is set forth most forcibly in his Principles of Philosophy, Part IV. I shall quote in full two paragraphs from the translation by Haldane and Ross.*

CXCVII. That mind is of such a nature that from the motion of the body alone the various sensations can be excited in it.

It may, in the next place, be (easily) proved that our mind is of such a nature that the motions which are in the body are alone sufficient to cause it to have all sorts of thoughts, which do not give us any image of any of the motions which give rise to them; and specially that there may be excited in it those confused thoughts called feelings or sensations [jertSHi, sive sensationes]. For (first of all) we observe that words, whether uttered by the voice or merely written, excite in our minds al! sorts of thoughts and emotions. On the same paper, with the same pen and ink, by moving the point of the pen ever so little over the paper in a certain way, we can trace letters which bring to the minds of our readers thoughts of battles, tempests or furies, and the emotions of indignation and sadness; while if the pen be moved in another way, hardly different, thoughts may be given of quite a different kind, viz. those of quietude, peace, pleasantness, and the quite opposite passions of love and joy. Someone will perhaps reply that writing and speech do not immediately excite any passions in the mind, or imaginations of things different from the letters and sounds, but simply so to speak various acts of the understanding; and from these the mind, making them the occasion, then forms for itself the imaginations of a variety of things. But what shall we say of the sensations of what is painful and pleasurable? If a sword moved towards our body cuts it, from this alone pain resuits which is certainly not less different from the local movement of the sword or of the part of the body which is cut, than are color or sound or smell or taste. And therefore, as we see clearly that the sensation of pain is easily excited in us from the fact alone that certain parts of our body are locally disturbed by the contact with certain other bodies, we may conclude that our mind is of such a nature that certain local motions can excite in it all the affections belonging to all the other senses.

CXCVIII. That there is nothing known of external objects by the senses but their figure, magnitude or motion.

Besides this, we observe in the nerves no difference which may cause us to judge that some convey to the brain from the organs of the external sense any one thing rather than another, nor again that anything is conveyed there excepting the local motion of the nerves themselves. And we see that this local motion excites in us not alone the sensations of pleasure or pain, but also the sensations of sound and light. For if we receive a blow in the eye hard enough to cause the vibration to reach the retina, we see myriads of sparks which are yet not outside our eye; and when we place our finger on our ear to stop it, we hear a murmuring sound whose cause cannot be attributed to anything but the agitation of the air which is shut up within it. Finally we can likewise frequently observe that heat and the other sensible qualities, inasmuch as they are in objects, and also the forms of these bodies which are purely material, such as e.g. the forms of fire, are produced in them by the motions of certain other bodies, and that these again also produce other motions in other bodies. And we can very well conceive how the movement of one body can be caused by that of another, and diversified by the size, figure, and situation of its parts, but we can in nowise understand how these same things (viz. size, figure, and motion) can produce something entirely different in nature from themselves, such as are those substantial forms and real qualities which many suppose to exist in bodies; nor likewise can we understand how these forms or qualities possess the force adequate to cause motion in other bodies. But since we know that our mind is of such a nature that the diverse motions of body suffice to produce in it all the diverse sensations that it has, and as we see by experience that some of the sensations are really caused by such motions, though we do not find anything but these movements to pass through the organs of the external senses to the brain, we may conclude that we in no way likewise apprehend that in external objects like light, color, smell, taste, sound, heat, cold, and the other tactile qualities, or what we call their substantial forms, there is anything but the various dispositions of these objects which have the power of moving our nerves in various ways.

I shall never forget my first reading of these lines. They struck me with the same freshness and poignancy with which I now fancy they must have struck Descartes’s contemporaries. There was about them, too, a classic ring, a splendid economy of utterance and a compelling directness of argument. The total impact was overwhelming; I was completely convinced. And I still remember the trenchant summary which I devised for myself or purloined from some forgotten source: “Our sensations bear no more resemblance to their outer causes than a feeling of pain bears to the sword that inflicts a wound.”

I still know of no bolder statement of the argument. Subsequent repetitions and the burgeoning of natural science have resulted in various refinements. But they were refinements only, adding not a major stroke to the classic sketch of Descartes.

The argument itself is familiar to all. The brain, it holds in effect, is the seat of consciousness, the central switchboard of the nervous system where the myriad nerve fibers converge and to which they transmit the disturbances set up in them at their sensory endings. Here in the brain these disturbances are deciphered and collated—by a mechanism not yet fully understood. The disturbances themselves arrive in the form of neural or electro-chemical impulses—”motions” Descartes calls them—all generically identical and all conveying identically the same “message.” In spite of this sameness, however, these messages on arriving at the central exchange there “cause” (excite, arouse, stimulate) in the mind or consciousness an enormous variety of “sensations” or “impressions” (sensa, sense data, etc.), the familiar colors, sounds, odors, flavors, tactile qualities, aches, pains, itches, pleasures, and so on. These elementary sensations are all on precisely the same footing; they are all subjective constituents of mind or consciousness; and some of them are the primitive materials from which, and out of which, mind then proceeds to perform its characteristic operations of perceiving and experiencing.

The differences between this argument and Galileo’s argument are apparent. Descartes is not arguing for the distinction between primary and secondary qualities; he is arguing from this distinction as a premise. Nor is he arguing for the mathematical constitution of the world, for its possessing only primary qualities; he is arguing from this view as a premise.

Their conclusions were correspondingly different. Galileo’s conclusion is the distinction between primary and secondary qualities and the primacy of the former. Descartes’s conclusion is the utter subjectivity of the secondary qualities, i.e., their status as purely mental events (sensations), and their occurrence as incorporeal effects of corporeal causes. Descartes has gone far beyond Galileo; he has removed the secondary qualities from the sense organs, from the sentient body, from the world itself. And in thus lodging them in a mind outside the natural world, in thus “subjectivizing” them, he has transformed them from secondary qualities into subjective sensations. This transformation was wrought by Descartes alone.

But this is not all that Descartes wrought. He also brought it about that the theory of sensation emerged in the guise of a “theory of ideas,” as it was currently called, a theory which was even farther from Galileo’s mind than the sensation theory. The theory of ideas holds in brief that all the direct objects of mind are nothing but its own ideas. The sensation theory holds that the direct objects of sense are but one’s own sensations. The theory of ideas extends this notion from sense to all the other modes of awareness, to imagination, memory, and thought in all its forms. In this respect the theory of sensation may be regarded as a special case of the more general theory of ideas, and sensations, correspondingly, as a species of idea—the most important species, according to the British empiricists. The two theories thus arise together and flourish as virtually one.

For a reason I cannot discover, Descartes and his continental successors were strangely !oathe to formulate the theory of ideas, even less the theory of sensation. For brief statements of the two theories we must turn, therefore, not to the rationalists, but to the English empiricists. In the introduction to his Essay Concerning Human Understanding, Locke explains that the term “idea” stands “for whatsoever is the object of the understanding when a man thinks . . . or whatever it is which the mind can be employed about in thinking.” Berkeley is but echoing this surprising statement when he trenchantly asks: “and what do we perceive besides our own ideas or sensations?” (Principles, Par. 4. Berkeley’s italics). Hume, as we might expect, is even more explicit, although he avoids the term “idea,” this having for him a more restricted sense. It is the incontrovertible teaching of the “slightest philosophy,” he contends, “that nothing can ever be present to the mind but an image or perception, and that the senses are on!y the inlets, through which these images are conveyed, without being able to produce any immediate intercourse between the mind and the object . . . These are the obvious dietates of reason; and no man, who reflects, ever doubted that the existences, which we consider, when we say, this house and that tree, are nothing but perceptions in the mind . . . fleeting copies or representations” (Enquiry Concerning Human Understanding, Section XII, Part I).

It is interesting in passing to observe that Descartes’s argument for sensations has become for Hume a matter of the “slightest philosophy” involving only the “trite topics, employed by the sceptics in all ages, against the evidence of sense” (ibid.). For all their triteness, however, and the slightness of the philosophy which they involve, they are nonetheless incontrovertible arguments against the senses, which “no man, who reflects, ever doubted.”

Such is the conclusion of Descartes’s argument for sensations. In effect, it is a dual conclusion embracing both the theory of sensation and the theory of ideas. How remote this is from Galileo and the scientists of the age can best be seen by going back to the distinction between primary and secondary qualities. Using the distinction solely for its methodological virtue, the scientists were advancing by giant strides in acquiring objective knowledge of nature. Construing it as a philosophic theory of sensation, the philosophers, on the other hand, found themselves in a quandary: how on the subjective basis of ideas and impressions is such an objective knowledge of nature possible? What modern science was actually doing, modern philosophy was having trouble finding possible!

Turning now from the conclusion to the argument itself, there are a number of observations to be made. The first is that it is an argument. Now argument is not the normal means of establishing facts; facts are usually established by observation prior to argument. Sensations or sense impressions, however, are not observable facts and hence must be established by argument, as an inference from observable facts assumed as premises.

The second observation concerns these premises. Most of them are taken directly from experience. They assert such commonly observed facts as that we have physical bodies with sense organs, brains and nervous systems, that nerve fibers transmit motions or impulses, and that only by the normal functioning of this physical apparatus do we see things as colored, hear them as sounding, and so on. All this, I repeat, is quite empirical. But there is one premise, the most decisive of all, which is not empirical, not derived from observation. This is the premise that physical motions, neural impulses, actually cause such mental effects as impressions of sense. These effects are not observable, and it is also not observable that they are caused by (observable) disturbances in our nerve fibers. By this causal premise—or “causal theory of perception” as it is sometimes called—we have vaulted out of the observable into the unobservable and found ourselves at odds with naive experience.

But, you will demur, is not this causal premise just as empirical, in a way, as the rest? Is it not a matter of common observation that we have visual impressions only when our optic nerve is excited, auditory impressions only when our auditory nerve is excited, and so with al! our sense organs? The answer is “yes”—but only on the assumption that we actually have such things as visual, auditory, and the Other sense impressions. This, however, is just what we cannot assume; it is what we are trying to prove. If we stick by what is observable, all we can say is that we see when our optic nerve is excited, hear when our auditory nerve is excited, and so on. This is vastly different from saying that exciting our optic or auditory nerve causes visual or auditory impressions as mental effects. In the one instance we are talking about seeing and hearing, actually perceiving things with their primary and secondary qualities. In the other we are talking about sensations being caused in the mind or consciousness. It is empirically evident that we see and hear; it is not empirically evident that the colors we see and the sounds we hear are nothing but subjective sensation.

It is interesting to note what Descartes has done with this causal premise. To Galileo’s one causal link between object and sense organ Descartes has added a second causal link between sense organ (or nerve impulse) and sensation. This first link is observable; the second is not. Descartes, however, might argue that the second link is not wholly without empirica! warrant. He would invoke the similarity between sense perception and the sensations of tickling and pain, a similarity of which both he and Galileo made much. Admittedly tickling and pain are inner sensations and are usually caused by outer objects. Admittedly, too, in perceiving colors and sounds our eyes and ears are acted on by outer objects. In both instances outer objects act causally on parts of our bodies, on our sense organs or other nerve endings. But this one circumstance does not imply that tickling and pain are like color and sound, that as the former are inner effects of outer causes so too are the latter. We can assert this only if we assume that color and sound are subjective sense impressions. But, once again, this is just what is to be proved, not assumed.

The third observation is that there is a jarring incommensurability between physical cause and mental effect, and this in two respects. In the first respect we note a sameness on the side of the cause over against a great variety on the side of the effect. Neural impulses are so nearly alike for all nerve fibers that the fibers have been thought to be freely interchangeable throughout the whole nervous system—ideally, of course—without disturbing in the least its normal functioning. Thus the same nerve fiber and impulse in the optic nerve can cause a sensation of color and when transferred to the olfactory nerve can cause a sensation of odor. In spite of this sameness, neural impulses are said to cause sensations differing as widely as the myriad secondary qualities of things, a stupendous range of variation in both kind and degree. Descartes may be alluding to this remarkable circumstance when he asserts in the above passage that “we observe no such difference between the nerves as to lead us to judge that one set of them convey to the brain from the organs of the external senses anything different from another.” But if so, he does not seem to regard it as anomolous that causes so uniformly alike should produce effects so enormously varied.

The second respect we have already noted in the monadic opposition between physical and mental, res extensa and res cogitans. Descartes sought to obviate this disparity by allowing mind and body to interact, to act causally on each other through the pineal gland. In bringing them thus into causal connection Descartes erroneously assumed that the direction of motion is not essential, i.e., that motion is not a vector quantity. This error in mechanics alone would have vitiated his causal premise. But still more influential was the metaphysical disparity between thought and extension, their monadic opposition. This rendered a causal connection between them inherently unthinkable.

In view of these difficulties it is not surprising that Descartes’s causal premise should fall under attack. Malebranche, Spinoza, Leibnitz, and Berkeley all assailed it vigorously, each for slightly different reasons, and rejected it outright. “Nothing,” says Hume, “can be more inexplicable than the manner in which body should so operate upon mind as ever to convey an image of itself to a substance, supposed of so different, and even contrary a nature. It is a question of fact, whether the perceptions of the senses be produced by external objects, resembling them: How shall this question be determined? By experience surely; as in all other questions of a like nature. But here experience is, and must be entirely silent. The mind has never any thing present to it but the perceptions, and cannot possibly reach any experience of their connection with objects. The supposition of such a connection is, therefore, without any foundation in reasoning” (Enquiry, Sect. XII, Part I).

After Descartes the causal premise was dropped as a philoSophie tenet. It was abandoned almost at once, and with it went the Cartesian argument for sensations, the one direct “proof” that the secondary qualities of things must be in sooth but subjective sensations in the perceiving mind. The argument, I repeat, because of its causal premise, lost its philosophic standing. But it did not lose its popular acceptance. It became known to every thoughtful and literate person; and its appeal was obvious, and lasting. Although banned from high philosophy, it was generally admitted into what Hume called the “slightest philosophy.” Holding here its sway, as it still does, Descartes’s argument was covertly accepted even by those who overtly denied its validity.

It is not surprising, then, that for all its ostensive rejection the argument for sensations should persist and its conclusion become the more secure. One would think that in impugning the argument for sensations, the theory of sensation would lose its hold. It did not. It flourished under the aegis of the new theory of ideas. Under this aegis it lost entirely its sense of being a theory, an inference from premises, a conclusion from argument. It became instead a “fundamental assumption,” an axiom manifest to the “slightest philosophy.” Even for Hume, and in spite of his searching scepticism, it is simply beyond doubt that the mind has never anything present to it but its own impressions and ideas.

But Hume had one luminous moment on this score, which occurred in the Enquiry at the close of his survey of the irreconcilable conflict between experience and reasoning. When they are experiencing, “men are carried, by a natural instinct or prepossession, to repose faith in their senses,” that is, “they always suppose the very images, presented by the senses, to be the external objects, and never entertain any suspicion, that the one are nothing but representations of the other. . . . But this universal and primary opinion of all men is soon destroyed by the slightest philosophy, which teaches us, that nothing can ever be presented to the mind but an image or perception. . . . Thus the first philosophical objection to the evidence of sense or to the opinion of external existence consists in this, that such an opinion, if rested on natural instinct, is contrary to reason, and if referred to reason, is contrary to natural instinct, and at the same time carries no rational evidence with it, to convince an impartial enquirer. The second objection goes farther, and represents this opinion as contrary to reason: at least, if it be a principle of reason, that all sensible qualities are in the mind, not in the object” (ibid.).

The italics are mine, but the insight is Hume’s. It would be Hume who thus probed to the core of the matter: “If it be a principle of reason that all sensible qualities are in the mind, not in the object!” I cannot but wonder what the subsequent course of philosophy might have been had Hume explored this “if” as he explored so many other alleged “principles of reason.” Most of these principles turned out to be groundless pretensions, especially when they pertained to matters of fact and existence; only in mathematics, which deals with the “relations of ideas,” is reason to be followed; elsewhere it should be but the slave of passion. But it is a question of fact and existence whether the sensible qualities of things are in the mind or the object; hence it is presumably beyond the province of reason. Why then does Hume in this one instance grant to reason a competence which he denies it in all similar instances? Whatever the answer, Hume unfortunately stopped at the threshold of an “enquiry” which might well have revolutionized modern philosophy.

As it was, he remained fettered to the theory of ideas and sensations. Kant, conceivably, might have loosed these fetters when, as he himself remarked, he was aroused by Hume from his dogmatic slumbers. But to what did Hume awaken him? To the theory of ideas and sensations! Kant’s awakening was a fettering more complete than Hume’s. What had been for Hume a matter of some question—slight, to be sure—became for Kant the dogmatic foundation of the “critical” philosophy. In laying this new foundation Kant likened it to the Copernican révolution in astronomy. “Hitherto men had assumed that all our knowledge must conform to objects; we shall now assume that objects must conform to our knowledge” (K.d.r.v., в XVI). This assumption, moreover, “will be proved, not hypothetically, but apodictically” in the body of the Critique of Pure Reason, presumably as Copernicus in his great book “proved” his hypothesis (K.d.r.v., B XXII п.).

The whole sense of this “Copernican revolution” is, of course, that the direct objects of mind are nothing but its own ideas or Vorstellungen. Of course, too, this “assumption” or theory is not “proved” in the body of the Critique; as with Hume it was simply taken for granted, a “fundamental assumption.” On one important occasion, where Kant refers to this fundamental assumption, he mentions it most casually. “In such wise,” he says by way of summary (A 158, в 197), “are synthetic judgments a priori possible, if we refer the formal conditions of intuition a priori, the synthesis of imagination, and the necessary unity of this latter in a transcendental apperception, to a possible experiential knowledge in general and say I or assert, sagen ן: the conditions of the possibility of experience in general are at the same time the conditions of the possibility of the objects of experience and have thereby objective validity in a synthetic judgment a priori.” The italics are Kant’s and are plainly intended to point up the two members of an equation which to him is all-important. The equation is that of “experience” and “object of experience.” They are the same, of course, only to the extent that they are both “ideas” or Vorstellungen. Only to the extent that they are thus the same can we “say” that the “conditions of the possibility” underlying the one are the same as those underlying the other.

In a philosophy which vaunts itself on being “critical” it is remarkable that a principle so foundational should require only to be “said.” Nowhere is it proved or even elucidated, let alone questioned. It is beyond “criticism.” It needs only the barest mention; obviously because it is self-evident, one of Whitehead’s “fundamental assumptions.”

It is appropriate at this juncture to take note of the inherent ambiguity in the term “idea.” It stands for both experience and object, or perception and object, and may be conveniently used to signify either or both. I shall have more to say of this later. Here I wish merely to note the ambiguity of “idea” and that it rejects, by implication at least, the common-sense distinction between awareness and object. This distinction, presumably a naive product of “natural instinct or prepossession,” conceals from the vulgar—as Berkely might have put it—the more learned doctrine that as “ideas” awareness and object are the same, a doctrine taught by the “slightest philosophy,” that is, the theory of sensation and ideas!

Such was Descartes’s argument for sensations. The argument itself, because of its causal premise, lost its philosophic standing, although not its secret appeal. The conclusion of the argument, on the other hand, lost nothing; rather it gained in standing and swept the field. It became a “fundamental assumption” of modern philosophy, where, in the notions of “sensation” and “idea,” it enjoyed an acceptance as complete as that accorded any other doctrine in the history of western thought.

But if the opinion prevailed that the sensation-idea doctrine required no support beyond itself, still the need was occasionally felt to say something in defense of the strange thesis that the secondary qualities of things are but subjective impressions in the mind. Naturally Descartes was the first to recognize this need. He responded to it with the causal argument which we have just been considering, the only direct argument I know of. He gave us also an indirect argument which we have not yet considered. After Descartes, however, the need was no longer felt on the continent; only the British empiricists seem to have recognized it, perhaps because they have never felt quite as comfortable in defying common sense as the continental rationalists. Of these, Berkeley was the most responsive. He offered an indirect argument for sensations, indirect because the direct argument was unacceptable to him, as we noted above.

To these indirect arguments for sensations I shall now turn. Because they are indirect, they will be found to lack, as one might expect, the force and vigor of the direct argument—the one argument that will prevail, covertly if not overtly, as long as the belief in sensations persists.

*Cambridge University Press, 1931, Vol. 1, pp. 294-6.