Whitehead once advised, “When you are criticizing the philosophy of an epoch, do not chiefly direct your attention to those intellectual positions which its exponents feel it necessary explicitly to defend. There will be some fundamental assumptions which adherents of all the variant systems within the epoch unconsciously presuppose.”* Because they are presupposed, more or less unconsciously, these assumptions are seldom if ever mentioned; they are simply taken for granted, like the air we breathe. Yet, because they are fundamental, they are always present, constituting—again like the air we breathe—what might be called the atmosphere of an epoch.

One of the fundamental assumptions which characterize the epoch known as “modern philosophy” is the notion of “sensation” or “sense impression” This notion takes it for granted that in all sense perception there occur inside the perceiving subject, as a consequence of the stimulation of his sense organs, private sensations or impressions corresponding to the sensible qualities perceived: visual sensations corresponding to the colors we perceive with our eyes, auditory sensations corresponding to the sounds we hear with our ears, tactual sensations corresponding to the qualities we perceive by touch, olfactory sensations corresponding to the odors we smell, and gustatory sensations corresponding to the flavors we taste—to mention only the five traditional senses.

The notion goes further than this. It holds that the sensation not only corresponds to the perceived quality; it actually is this quality as perceived. For the quality CIS perceived, as the immediate sensory content of the perception, is private to the perceiver and hence must be one with the sensation itself. Naturally, something must be added or done to the private sensation so that what originally occurs as an inner impression can be perceived as a public quality.

One thing more: sensations are “inner” in exactly the same sense as aches and pains and ticklings and kinesthetic sensations. These are all “sensations” in a wider sense, consorting together as subjective contents of our stream of consciousness. Only those sensations, however, which are also sense impressions, i.e., derive somehow from our sense organs, are acted upon by the mind so as to be perceived as outer qualities of things; the others are not.

Sensation in this sense is manifestly not a simple notion; it is rather a theory, whose central tenet is that the sensuous qualities of external things are really but internal sensations in the perceiving mind. This is the “theory of sensation” which I propose to study in the following pages. The theory itself was first set forth by Descartes and by him made the starting point of an exciting new philosophic venture. At the hands of Descartes’s successors, however, the theory quickly lost its status as a theory and became almost at once one of those fundamental assumptions which, as Whitehead remarked, come to be more or less unconsciously presupposed by the exponents of an epoch. What happened here is that the theory as theory “went underground,” so to speak, or became so “sedimented” in the term “sensation”—also in the term “idea,” as we shall see shortly—that it rarely thereafter came explicitly to utterance. My aim is to bring it to utterance and to examine it carefully, in a systematic and historical vein.

Having uncovered the central tenet or thesis of the theory, that the sensuous qualities of sense objects are really but subjective sensations in the perceiving mind, let us see what is further involved.

First to be noted is that the thesis rests on a distinction between two kinds of qualities or properties which all things appear to possess on becoming objects of sense perception. There are 1) those qualities which may be called “sensible” because they are associated directly and uniquely with our various organs of sense: colors with the eyes; sounds with the ears; smoothness with touch; odors with the nose; and flavors with the tongue. In contrast with these sensible qualities there are 2) what may be called the mathematical properties of external things: shape, size, weight, position, motion, number, and the like, no one of which is confined, like the sensible qualities, to one organ or department of sense. On the contrary, these mathematical properties would seem to be quite independent of any of our senses; for when one or more of our senses fail, as they occasionally do with heavy colds or old age, then things “lose” their odors and flavors, or their colors and sounds, but never their mathematical properties. These latter would thus seem to be independent of our senses, whereas the sensible qualities would seem to be altogether dependent.

This distinction between the sensible qualities of things and their mathematical properties is better known to us, in terms made famous by John Locke, as that between the “secondary qualities” and the “primary qualities” of external objects, the secondary being the sensible qualities, the primary being the mathematical qualities. 1 shall prefer the more familiar Lockean version, but with the clear understanding that it signifies nothing more than what has so far been explicitly stated, that the secondary qualities are dependent on our sense organs, the primary qualities not.

Plainly the theory of sensation assumes or presupposes this distinction between the primary and secondary qualities of empirical things. But just as plainly it also assumes something more. For in declaring that the secondary qualities are but “subjective” sensations inside our minds, the sensation theory is going well beyond the distinction; it is “subjectivizing” the secondary qualities, or “reducing” them to sensations. Now since it is by this subjectivizing reduction that we pass from the distinction to the theory, plainly the distinction must be prior to the theory and independent of it; so that one may hold to the distinction between primary and secondary qualities without subscribing to the theory of sensation.

The reason for this subjectivizing reduction I shall examine later on. Here I wish only to point out that it leads from the distinction to the very important thesis that the secondary qualities are not really qualities of outer things at all, but actually only subjective impressions, sensations, “ideas,” sense data inside the perceiving mind or consciousness. Taken at its face value—and it is meant to be taken literally—this is an astonishing utterance at which common sense can only stand agape. For what it says is that the color of eyes, the sound of voices, the softness and warmth of flesh, the fragrance of flowers, the flavor of foods, in short, all the sensible qualities which things possess for us in our daily lives, by which they are identified and sought or shunned, are not really “out there” attaching to things, but private sensations inside our minds. If common experience, the only experience we have, is to be trusted, this is simply incomprehensible. For what experience reveals is precisely the opposite: that these sensible qualities of things are not “in the mind,” but “out there” in things themselves; and, indeed, so fused with primary qualities—colors with shapes and motions, sounds with distances and directions, hardness with surfaces and edges—that the two kinds of qualities would seem to be utterly inseparable in fact even though distinguishable in thought. By no effort can we perceive things otherwise than as possessing both primary and secondary qualities. And by no effort can we perceive things otherwise than by our organs of sense. Moreover, since it is by the same organs of sense that we perceive both primary and secondary qualities, must not the primary qualities be in a way just as “sensible” as the secondary, and in a similar way just as sense-dependent?

Plainly the evidence of experience is that the primary and secondary qualities as perceived are inseparable and are on precisely the same footing, as Berkeley rightly insisted. This, however, is not to deny a certain difference between them as qualities of objects. Experience confirms this difference or distinction—up to a point. But to what point? This question, too, I shall defer. My concern here is with the evidence of experience which confirms, up to an unspecified point, the distinction between primary and secondary qualities, but does not confirm the thesis of the sensation theory. Experience flatly opposes the separation of the quali- ties which this theory entails and the subjectivizing of the secondary.

Because experience opposes the theory, the theory in its turn must oppose experience; it must discredit experience. This it does by calling experience “naive,” “uncritical,” the source of “naive realism.” Every student of philosophy soon learns that to be naive or uncritical about direct experience and its evidence is to be unphilosophical and that naive realism is intellectually dis- reputable. For, plainly, if direct experience is a matter of private sensations, its evidence must be equally private and—to this extent, at least—unacceptable. But this “if” is just the point at issue, the point where experience in its common-sense capacity opposes the theory of sensation. For the theory, however, this is no issue at all; it is simply beyond question, axiomatically evident, that experience begins with subjective sensations.

And so the theory of sensation, serenely confident of its critical ascendancy, was moved to dispense with naive experience and to resort to “reason” alone. Under the guidance of reason the theory burgeoned in a brilliant display of metaphysics. At the hands of Descartes and his immediate successors this metaphysical outburst flared up and then partly subsided, leaving as a residue another one of those “fundamental assumptions” of which Whitehead spoke. This second predilection I shall call “monadism,” for reasons now to be related.

It all began with Descartes, who saw at once that his new theory of sensation had in tow two metaphysical corollaries. The first of these corollaries is that external things literally possess only primary qualities, that the external world is literally an insensible, mathematical reality—a res extensa (extended thing or substance), as Descartes called it—utterly devoid of all secondary qualities. This notion was of great importance to Descartes, for he was confident that only if the world were a res extensa could it be a proper object for the new mathematical approach to nature. We have dropped Descartes’s term but retained its sense in our notion of the “physical world,” which means the world just as it is depicted by natural science. Notice that this “physical world”—or res extensa—is not merely an aspect or facet of a richer empirical world of experience; it is not this “richer” world shorn, for methodological reasons only, of its sensible vestments. Rather it is the empirical world as it really, metaphysically is, just as it is disclosed to the initiates of natural science. In this “scientific” world the secondary qualities have no possible place, not even in our sense organs or brains, for these too are “physical” objects and possess only primary qualities. The secondary qualities have been read out of the world altogether and must find lodging elsewhere. The most obvious place is the human mind.

The second metaphysical corollary is that the human mind is the lodging place of the secondary qualities, and hence must be a substance completely different from res extensa, as different as the nonextended or nonspatial from the extended or spatial, as that which thinks from that which cannot think. The mind on this assumption must exist by itself quite outside the physical world, and must be as closed to this world, and as self-contained, as the world is to it. Indeed, it must be a kind of inner world of intangible “ideas” over against the outer world of tangible things. Descartes called it a res cogitans (thinking thing or substance), to contrast it as sharply as possible with res extensa.

The important point here is the contrast between res cogitans and res extensa. This contrast is so extreme that it can hardly be exaggerated. With it Descartes introduced into modern philosophy an unparalleled opposition or “dualism,” a metaphysical split so final, so absolute, so unbridgeable, that it destroys the very possibility of the severed members forming a single metaphysical whole. No real togetherness is possible between res cogitans and res extensa; they have no possible meeting ground. Hence they can form not one ultimate whole, but two separate wholes—two at the very least. The successors of Descartes found this intolerable and set about trying to overcome the dualism, trying to find beneath the dualism a conciliating unity (monism). One thinks at once of Spinoza’s parallelism, of the idealisms of Leibnitz, Berkeley, Kant, Hegel, of the materialism of Hobbes and Marx—to mention but a conspicuous few.

This “revolt against dualism” is perhaps the dominant metaphysical theme in modern philosophy. From Spinoza to the present it recurs without sign of abating. It recurs because that which it opposes persists, because the dualism which Descartes introduced has become one of the pivotal centers on which modern philosophy turns, one of those fundamental assumptions which, like the theory of sensation, are unconsciously assumed by all who would philosophize in the modern vein. This assumption has no accepted designation, like “sensation”; and Descartes’s terms, res cogitans and res extensa, have long been abandoned. Because it is so unprecedented and extreme 1 shall call the dualism not Cartesian, but “monadic”; and I shall call its tacit, yet common, acceptance “monadism.”

Leibnitz, who introduced the term “monad” from the Greek (meaning “one” or “the one”), did not speak of nature as a monad; he confined the term to souls and minds. In applying it to nature as well, I am suggesting that if minds are monads, nature too must be a monad. My reason for saying this is that the basic sense of “monad,” or an integral part thereof, is that of a closed, self-contained order “without windows,” i.e., into which nothing can enter and from which nothing can escape, in a word, a “conservative system.” Now we are quite accustomed to viewing nature as a conservative system; nearly every high school graduate has been told that the quantity of mass (or “matter”) and energy in the universe is constant or conserved. Leibnitz is holding in effect that souls and minds are similarly conservative with respect to the sum total of their contents, sensations, ideas, and the like. All I am adding is that the two notions go insepàãàÛó together, that in separating mind and nature in the fashion of Descartes we are making both equally conservative, both equally “closed” to each other. Taking “monad” in this sense of complete closedness—with of course, some apologies to Leibnitz—then clearly if mind is a monad, nature is a monad—or, if you prefer, a counter-monad.

Obviously by “monadism” I do not mean an express adherence to the view that mind and nature are monads. Still less do I signify an acceptance of Leibnitz’s celebrated theory of monads. Monadism does not even involve the use of the word “monad”; the word may be quite unknown. Monadism is rather the deeply sedimented predilection which predisposes us all, or nearly all, to take it simply for granted that mind and nature, or more commonly “psychical” and “physical,” signify two distinct and separate orders of existence, two monads. Thus the familiar term “physical nature”—a curious etymological redundance—almost invariably stands for an order utterly closed to psychical mind or consciousness. By almost universal consent things psychical or mental, being inaccessible to the methods of natural inquiry, have no place at all in the “objective” world of science. Conversely, things psychical or psychological comprise a (dubious) “subjective” realm from which things physical are with equal finality excluded. And so we have the familiar opposition of psychical and physical with its many variations, thought and extension, subjective and objective, subject and object, thought and things, mind and matter, soul and body, inner consciousness and outer object, self and other, value and fact, and so on—all expressing covertly one and the same “monadic” opposition between mind and nature which was first propounded by Descartes.

The important point here is the monadic nature of the opposition. The contrasting pairs just enumerated are as old as western thought. The members of each pair were early distinguished one from the other, and several were found to be in tension, even conflict. In a loose sense they are all “opposites”; and the members of each pair may be said to stand in a kind of “opposition.” But in no instance, I believe, is the traditional opposition conceived in the Cartesian sense of utter separation and exclusion—“closedness,” as I have called it, in the fashion of monads. This irreconcilability is what is new and Cartesian and modern. This is what lies at the core of monadism.

The hold of this monadic view on the modern mind is astonishing, and the more so when we look at it against the classical background from which it emerged. Consider for a moment the meanings of the terms “physical” and “psychical” prior to Descartes. The one derives from physis, which means “nature” (natura in Latin); the other derives from psyche, which means “soul” (anima or animus in Latin) and which we at times transliterate into English. Aristotle at the far end of the classical tradition could hold that man’s psyche is in a way his physis (nature). St. Thomas at the nearer end, agreeing with Aristotle, could speak of an influxus physicus in the psyche or anima—a position unalterably opposed by Descartes’s immediate successors. There is no such opposition here between physical and psychical as we find after the advent of the sensation theory and its metaphysical corollaries.

What happened here, terminologically, is that “physical” came to be virtually synonymous with “material.” Matter, you will recall, had been liberated by Descartes from all involvement with “form” and proclaimed a substance in its own right, a res extensa. At the same time form—at least as mind or soul (mens sive animus)—was similarly liberated from matter and proclaimed a substance in its own right, a res cogitans. Form as accident disappeared in the distinction between primary and secondary qualities, becoming as primary the essential or defining predicates of matter, and reducing as secondary to subjective contents of mind, sensations. What little was left of form was tarred with the caption “occult quality” and read out of the new movement together with potency and act. Thus the classical union of “form” and “matter” as the polar constituents of substance was dissolved at a stroke and replaced with the brand new opposition and separation of incorporeal and corporeal substance.

It is easy to see in this opposition how “psychical” took its stand with incorporeal mind. It is not equally easy to see how “physical” took its stand with corporeal matter and became opposed to “psychical.” The explanation, possibly, lies in the increasing use of “physical” to modify “world” as in the expression “physical world.” On becoming a material substance the world, continuing to be designated as “physical,” probably gave to “physical” the exclusive connotation of “material” or “corporeal.” At all events, a vast terminological convulsion occurred in which a host of distinctions carefully refined and conciliated over the centuries was suddenly suppressed, transformed, and gathered up into one overriding metaphysical opposition and separation, the unbridgeable monadic dualism of psychical mind and physical nature.

All this is quite explicit with Descartes. He was fully aware that in subjectivizing the secondary qualities and separating them from the primary, the sensation theory was committed via its metaphysical corollaries to the unprecedented and irreconcilable opposition of res cogitans and res extensa. Descartes was not fully aware of the monadic nature of this opposition; he still held that mind and body could interact. Spinoza and Leibnitz corrected this at once. With them the dualism attained its sharpest formulation and then went underground, becoming one of those “fundamental assumptions’’ which characterize the modern outlook. On becoming thus implicit the dualism lost much of its sharpness, as do most tenets that come to be simply taken for granted. But for all its want of sharpness its edge is keenly felt in the predilection that psychical mind and physical nature are “closed” to each other, that never the twain shall meet and merge and form a real unity, comparable to the classical union of form and matter. To this “closed” theory of mind and nature I shall later oppose an “open” theory.

By way of summary, we have uncovered two “fundamental assumptions,” two of the most powerful in the framework of modern thought. The more basic of the two is the theory of sensation, inasmuch as it gave rise to monadism. This priority is not manifest in their role as fundamental assumptions; they would seem to be rather coeval companions holding a joint sway. But they are not coeval in origin; the sensation theory is prior. Because it is prior, it is that on which our attention naturally comes principally to focus. The question now arises as to the origin of the sensation theory. What are its grounds? By what arguments and from what evidence does it derive? What, in a word, is its rationale?

In raising this question it is important to note once again that although the theory of sensation rests on the distinction between primary and secondary qualities, it is not necessarily entailed by this distinction. The distinction is logically independent of the theory of sensation. To get from the distincton to the theory a further assumption must be made; the secondary qualities must be subjectivized. If we refrain from making this assumption, we may then hold to the distinction, as every thoughtful scientist must, without subscribing to the theory. In this event, the distinction is taken not as a philosophic gambit but as a methodological aid to natural inquiry. It was so taken by Kepler and Galileo. They did not use it to support a philosophic theory of sensation. Nor did anybody else before Descartes. It was Deseartes, and Descartes alone, who first gave to the world the theory of sensation. It is for this reason that I speak of the theory as “Cartesian” and its attendant monadism as “modern.”

I stress this point, for as legatees of Descartes we are not in the habit of regarding the distinction as independent—both logically and chronologically—of the theory; we tend to forget that there was a time when the distinction was drawn without any thought of the theory. In our Cartesian heritage the two are so closely associated that the bare mention ®f the distinction calls at once to mind the theory of subjectivity. Even such excellent historians as Cassirer and Burtt—to mention but two of many—simply take it for granted that when Kepler and Galileo distinguish between primary and secondary qualities they are also proclaiming the “subjectivity” of the latter. If by “subjective” Cassirer and Burtt mean what the word has come to mean only since Descartes, namely, that the secondary qualities are but sensations in the mind, then Kepler and Galileo proclaimed no such thing. This I shall attempt to show in the next chapter, after which I shall turn to Descartes’s arguments for the theory of sensation.

*Quoted in w. K. c. Guthrie, The Greek Philosophers (New York: Harper, 1950), p. 11.