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.