What then are the criteria of a science? When is our thought knowledge, and of objective reality? In the first place, there must be a given something--a sens_-datum_--an 'impression' as Hume might have said. If there be no impression, therefore, there can be no scientific idea, no real knowledge. There must be the primary touch--the feeling--the affection--the _je ne sais quoi_ of contact with reality. Secondly, what is given can only be received if taken up by the recipient, and in such measure as he is able to appropriate it. The given is received in a certain mode. In the present case, the sensation is apprehended and perceived under the forms of space and time. Perception, in other words, whatever may be its special quality or its sensuous material, is always an act of dating and localisation. The distinction between the mere lump of feeling or sensibility and the perception is that the latter implies a field of extended and mutually excluding parts of space, and a series of points of time, both field and series being continuous, and, so far as inexhaustibility goes, infinite. Thirdly, even in the reception of the given there is a piece of action and spontaneity. If the more passive recipiency be called Sense, this active element in the adaptation may be termed Intellect. Intellect is a power or process of choice, selection, comparison, distinguishing and dividing, analysis and synthesis, affirmation and negation, numeration, of judgment and doubt, of connexion and disjunction, differentiation and integration. Its general aspect is by Kant sometimes described as Judgment--the act of thought which correlates by distinguishing; sometimes as Apperception, and the unity of apperception. It is, i. e., an active unity and a synthetic energy; it unifies, and always unifies. It links perception to perception, correlating one with another--interpreting one by another; estimating the knowledge-value of one by the rest. It thus 'apperceives.' It is a faculty of association and consociation of ideas. But the association is inward and 'ideal' union: the one idea interpenetrates and fuses with the other, even while it remains distinct.
Kant's work may be described--in its first stage--as an analysis and a criticism of experience. The term Experience is an ambiguous one. It sometimes means what has been called the 'raw material' of experience: the crude, indigested mass of poured-in _matter_-of-knowledge. If there be such a shapeless lump anywhere,--which has to be considered presently--it, at any rate, is not on Kant's view properly entitled to the name of Experience. The Given must be felt and apprehended: and--to put the point paradoxically--to be felt it must be more than felt,--it must be perceived. It must, in other words, be projected--set in space and time: let out of the mere dull inner subjectivity of feeling into the clear and distinct outer subjectivity of perception. But, again, to be perceived, it must be apperceived: to be set in time and space, it must first of all be in the hands of the unifying consciousness, which is the lord of time and space. For in so far as space and time mean a place and an order--in so far as they mean more than an empty inconceivable receptacle for bulks of sensation, in the same degree do they presuppose an intellectual, synthetic genius, which is in all its perceptions one and the same,--the fundamental, original unity of consciousness. And this analysis of experience is transcendental.' Beginning with the assumed datum--the object of or in experience--it shows that this object which is supposed to be _there_--to exist by itself and wait for perception--is created by and in the very act which apprehends it. Climbing up and rising above its habitual absorption _in_ the _thing_, consciousness (that of the philosophic observer and analyst) sees the thing in the act of making, and watches its growth.