"Now that quantity which is apprehended only as unity, and in which plurality can be represented only by approximation to negation = 0, I call an _intensive quantity_. Every reality, therefore, in a phenomenon has intensive quantity, that is, a degree."[7]
In other words, 'We can lay down _a priori_ that all sensations have a certain degree of intensity, and that between a sensation of a given intensity and the total absence of sensation there is possible an infinite number of sensations varying in intensity from nothing to that degree of intensity. Therefore the real, which corresponds to sensation, can also be said _a priori_ to admit of an infinite variety of degree.'
Though the principle established is of little intrinsic importance, the account of it is noticeable for two reasons. In the first place, although Kant clearly means by the 'real corresponding to sensation' a body in space, and regards it as a phenomenon, it is impossible to see how he can avoid the charge that he in fact treats it as a thing in itself.[8] For the correspondence must consist in the fact that the real causes or excites sensation in us, and therefore the real, i. e. a body in space, is implied to be a thing in itself. In fact, Kant himself speaks of considering the real in the phenomenon as the cause of sensation,[9] and, in a passage added in the second edition, after proving that sensation must have an intensive quantity, he says that, corresponding to the intensive quantity of sensation, an intensive quantity, i. e. _a degree of influence on sense_, must be attributed to all objects of sense-perception.[10] The difficulty of consistently maintaining that the real, which corresponds to sensation, is a phenomenon is, of course, due to the impossibility of distinguishing between reality and appearance within phenomena.[11]
[10] B. 208, M. 126. The italics are mine. Cf. from the same passage, "Phenomena contain, over and above perception, the materials for some object (through which is represented something existing in space and time), i. e. they contain the real of sensation as a merely subjective representation of which we can only become conscious that _the subject is affected_, and which we relate _to an object in general_." (The italics are mine.)