[24] In this manner it is proved, that the synthesis of apprehension, which is empirical, must necessarily be conformable to the synthesis of apperception, which is intellectual, and contained à priori in the category. It is one and the same spontaneity which at one time, under the name of imagination, at another under that of understanding, produces conjunction in the manifold of intuition. To take another example, when I perceive the freezing of water, I apprehend two states (fluidity and solidity), which, as such, stand toward each other mutually in a relation of time. But in the time, which I place as an internal intuition, at the foundation of this phenomenon, I represent to myself synthetical unity of the manifold, without which the aforesaid relation could not be given in an intuition as determined (in regard to the succession of time). Now this synthetical unity, as the à priori condition under which I conjoin the manifold of an intuition, is, if I make abstraction of the permanent form of my internal intuition (that is to say, of time), the category of cause, by means of which, when applied to my sensibility, I determine everything that occurs according to relations of time. Consequently apprehension in such an event, and the event itself, as far as regards the possibility of its perception, stands under the conception of the relation of cause and effect: and so in all other cases. Categories are conceptions which prescribe laws à priori to phenomena, consequently to nature as the complex of all phenomena (natura materialiter spectata). And now the question arises—inasmuch as these categories are not derived from nature, and do not regulate themselves according to her as their model (for in that case they would be empirical)—how it is conceivable that nature must regulate herself according to them, in other words, how the categories can determine à priori the synthesis of the manifold of nature, and yet not derive their origin from her. The following is the solution of this enigma.