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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 than 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 recency, 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 from 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?

2. TRANSCENDENTAL ANALYTIC

So we pass from the wide field of sensation and perception to the dark and narrow chamber of thought; from “trancendental esthetic” to “transcendental logic.” And first to the naming and analysis of those elements in our thought which are not so much given to the mind by perception as given to perception by the mind; those levers which raise the “perceptual” knowledge of objects into the “conceptual” knowledge of relationships, sequences, and laws; those tools of the mind which refine experience into science. Just as perceptions arranged sensations around objects in space and time, so conception arranges perceptions (objects and events) about the ideas of cause, unity, reciprocal relation, necessity, contingency, etc.; these and other “categories” are the structure into which perceptions are received, and by which they are classified and moulded into the ordered concepts of thought. These are the very essence and character of the mind; mind is the coördination of experience.

And here again observe the activity of this mind that was, to Locke and Hume, mere “passive wax” under the blows of sense-experience. Consider a system of thought like Aristotle’s; is it conceivable that this almost cosmic ordering of data should have come by the automatic, anarchistic spontaneity of the data themselves? See this magnificent card-catalogue in the library, intelligently ordered into sequence by human purpose. Then picture all these card-cases thrown upon the floor, all these cards scattered pell-mell into riotous disorder. Can you now conceive these scattered cards pulling themselves up, Münchausen-like, from their disarray, passing quietly into their alphabetical and topical places in their proper boxes, and each box into its fit place in the rack,—until all should be order and sense and purpose again? What a miracle-story these sceptics have given us after all!

Sensation is unorganized stimulus, perception is organized sensation, conception is organized perception, science is organized knowledge, wisdom is organized life: each is a greater degree of order, and sequence, and unity. Whence this order, this sequence, this unity? Not from the things themselves; for they are known to us only by sensations that come through a thousand channels at once in disorderly multitude; it is our purpose that put order and sequence and unity upon this importunate lawlessness; it is ourselves, our personalities, our minds, that bring light upon these seas. Locke was wrong when he said, “There is nothing in the intellect except what was first in the senses”; Leibnitz was right when he added,—“nothing, except the intellect itself.” “Perceptions without conceptions,” says Kant, “are blind.” If perceptions wove themselves automatically into ordered thought, if mind were not an active effort hammering out order from chaos, how could the same experience leave one man mediocre, and in a more active and tireless soul be raised to the light of wisdom and the beautiful logic of truth?

The world, then, has order, not of itself, but because the thought that knows the world is itself an ordering, the first stage in that classification of experience which at last is science and philosophy. The laws of thought are also the laws of things, for things are known to us only through this thought that must obey these laws, since it and they are one; in effect, as Hegel was to say, the laws of logic and the laws of nature are one, and logic and metaphysics merge. The generalized principles of science are necessary because they are ultimately laws of thought that are involved and presupposed in every experience, past, present, and to come. Science is absolute, and truth is everlasting.

3. TRANSCENDENTAL DIALECTIC

Nevertheless, this certainty, this absoluteness, of the highest generalizations of logic and science, is, paradoxically, limited and relative: limited strictly to the field of actual experience, and relative strictly to our human mode of experience. For if our analysis has been correct, the world as we know it is a construction, a finished product, almost—one might say—a manufactured article, to which the mind contributes as much by its moulding forms as the thing contributes by its stimuli. (So we perceive the top of the table as round, whereas our sensation is of an ellipse.) The object as it appears to us is a phenomenon, an appearance, perhaps very different from the external object before it came within the ken of our senses; what that original object was we can never know; the “thing-in-itself” may be an object of thought or inference (a “noumenon”), but it cannot be experienced,—for in being experienced it would be changed by its passage through sense and thought. “It remains completely unknown to us what objects may be by themselves and apart from the receptivity of our senses. We know nothing but our manner of perceiving them; that manner being peculiar to us, and not necessarily shared by every being, though, no doubt, by every human being.”20 The moon as known to us is merely a bundle of sensations (as Hume saw), unified (as Hume did not see) by our native mental structure through the elaboration of sensations into perceptions, and of these into conceptions or ideas; in result, the moon is for us merely our ideas.21