variable sequences of which you may be forever certain; and naturally you must not expect to "see" your soul, even with the eyes of the internal sense. Let us grant that absolute certainty of knowledge is impossible if all knowledge comes from sensation, from an independent external world which owes us no promise of regularity of behavior. But what if we have knowledge that is independent of sense-experience, knowledge whose truth is certain to us even before experience -à priori? Then absolute truth, and absolute science, would become possible, would it not? Is there such absolute knowledge? This is the problem of the first Critique. "My question is, what we can hope to achieve with reason, when all the material and assistance of experience are taken away." The Critique becomes a detailed biology of thought, an examination of the origin and evolution of concepts, an analysis of the inherited structure of the mind. This, as Kant believes, is the entire problem of metaphysics. "In this book I have chiefly aimed at completeness; and I venture to maintain that there ought not to be one single metaphysical problem that has not been solved here, or to the solution of which the key at least has not here been supplied." 2 Exegi monumentum aere perennius! With such egotism nature spurs us on to creation.

The Critique comes to the point at once. "Experience is by no means the only field to which our understanding can be confined. Experience tells us what is, but not that it must be necessarily what it is and not otherwise. It therefore never gives us any really general truths; and our reason, which is particularly anxious for that class of knowledge, is roused by it rather than satisfied. General truths, which at the same time bear the character of an inward necessity, must be independent of experience, clear and certain in themselves." 3 That is to say, they must be true no matter what our later ex

1 Critique of Pure Reason, pref. p. xxiv.

2 Ibid., p. xxiii.

3 Ibid., p. 1.

perience may be; true even before experience; true à priori. "How far we can advance independently of all experience, in à priori knowledge, is shown by the brilliant example of mathematics." 1 Mathematical knowledge is necessary and certain; we cannot conceive of future experience violating it. We may believe that the sun will "rise" in the west to-morrow, or that some day, in some conceivable asbestos world, fire will not burn stick; but we cannot for the life of us believe that two times two will ever make anything else than four. Such truths are true before experience; they do not depend on experience past, present, or to come. Therefore they are absolute and necessary truths; it is inconceivable that they should ever become untrue. But whence do we get this character of absoluteness and necessity? Not from experience; for experience gives us nothing but separate sensations and events, which may alter their sequence in the future. These truths derive their necessary character from the inherent structure of our minds, from the natural and inevitable manner in which our minds must operate. For the mind of man (and here at last is the great thesis of Kant) is not passive wax upon which experience and sensation write their absolute and yet whimsical will; nor is it a mere abstract name for the series or group of mental states; it is an active organ which moulds and coördinates sensations into ideas, an organ which transforms the chaotic multiplicity of experience into the ordered unity of thought.

But how?

1. Transcendental Esthetic

The effort to answer this question, to study the inherent structure of the mind, or the innate laws of thought, is what Kant calls "transcendental philosophy," because it is a prob

1 P. 4.

2 "Radical empiricism" (James, Dewey, etc.) enters the controversy at this point, and argues, against both Hume and Kant, that experience gives us relations and sequences as well as sensations and events.

lem transcending sense-experience. "I call knowledge transcendental which is occupied not so much with objects, as with our à priori concepts of objects." 1-with our modes of correlating our experience into knowledge. There are two grades or stages in this process of working up the raw material of sensation into the finished product of thought. The first stage is the coördination of sensations by applying to them the forms of perception-space and time; the second stage is the coördination of the perceptions so developed, by applying to them the forms of conception-the "categories" of thought. Kant, using the word esthetic in its original and etymological sense, as connoting sensation or feeling, calls the study of the first of these stages "Transcendental Esthetic"; and using the word logic as meaning the science of the forms of thought, he calls the study of the second stage "Transcendental Logic." These are terrible words, which will take meaning as the argument proceeds; once over this hill, the road to Kant will be comparatively clear.

Now just what is meant by sensations and perceptions?— and how does the mind change the former into the latter? By itself a sensation is merely the awareness of a stimulus; we have a taste on the tongue, an odor in the nostrils, a sound in the ears, a temperature on the skin, a flash of light on the retina, a pressure on the fingers: it is the raw crude beginning of experience; it is what the infant has in the early days of its groping mental life; it is not yet knowledge. But let these various sensations group themselves about an object in space and time-say this apple; let the odor in the nostrils, and the taste on the tongue, the light on the retina, the shape-revealing pressure on the fingers and the hand, unite and group themselves about this "thing": and there is now an awareness not so much of a stimulus as of a specific object; there is a perception. Sensation has passed into knowledge.

But again, was this passage, this grouping, automatic? i Critique of Pure Reason, p. 10.

Did the sensations of themselves, spontaneously and naturally, fall into a cluster and an order, and so become perception? Yes, said Locke and Hume; not at all, says Kant.

For these varied sensations come to us through varied channels of sense, through a thousand "afferent nerves" that pass from skin and eye and ear and tongue into the brain; what a medley of messengers they must be as they crowd into the chambers of the mind, calling for attention! No wonder Plato spoke of "the rabble of the senses." And left to themselves, they remain rabble, a chaotic "manifold," pitifully impotent, waiting to be ordered into meaning and purpose and power. As readily might the messages brought to a general from a thousand sectors of the battle-line weave themselves unaided into comprehension and command. No; there is a law-giver for this mob, a directing and coördinating power that does not merely receive, but takes these atoms of sensation and moulds them into sense.

Observe, first, that not all of the messages are accepted. Myriad forces play upon your body at this moment; a storm of stimuli beats down upon the nerve-endings which, amoebalike, you put forth to experience the external world: but not all that call are chosen; only those sensations are selected that can be moulded into perceptions suited to your present purpose, or that bring those imperious messages of danger which are always relevant. The clock is ticking, and you do not hear it; but that same ticking, not louder than before, will be heard at once if your purpose wills it so. The mother asleep at her infant's cradle is deaf to the turmoil of life about her; but let the little one move, and the mother gropes her way back to waking attention like a diver rising hurriedly to the surface of the sea. Let the purpose be addition, and the stimulus "two and three" brings the response, "five"; let the purpose be multiplication, and the same stimulus, the same auditory sensations, "two and three," bring the response, "six." Association of sensations or ideas is not merely by contiguity in

space or time, nor by similarity, nor by decency, frequency or intensity of experience; it is above all determined by the purpose of the mind. Sensations and thoughts are servants, they await our call, they do not come unless we need them. There is an agent of selection and direction that uses them and is their master. In addition to the sensations and the ideas there is the mind.

This agent of selection and coördination, Kant thinks, uses first of all two simple methods for the classification of the material presented to it: the sense of space, and the sense of time. As the general arranges the messages brought him according to the place for which they come, and the time at which they were written, and so finds an order and a system for them all; so the mind allocates its sensations in space and time, attributes them to this object here or that object there, to this present time or to that past. Space and time are not things perceived, but modes of perception, ways of putting sense into sensation; space and time are organs of perception. They are à priori, because all ordered experience involves and presupposes them. Without them, sensations could never grow into perceptions. They are à priori because it is inconceivable that we should ever have any future experience that will not also involve them. And because they are à priori, their laws, which are the laws of mathematics, are à priori, absolute and necessary, world without end. It is not merely probable, it is certain that we shall never find a straight line that is not the shortest distance between two points. Mathematics, at least, is saved from the dissolvent scepticism of David Hume.

Can all the sciences be similarly saved? Yes, if their basic principle, the law of causality—that a given cause must always be followed by a given effect-can be shown, like space and time, to be so inherent in all the processes of understanding that no future experience can be conceived that would violate or escape it. Is causality, too, à priori, an indispensable prerequisite and condition of all thought?