These categories are "pure" conceptions of the understanding, in as much as they are independent of all that is contingent in sense. They are not derived from what is called the "matter" of sense, or from particular, variable sensations. However, they are not independent of the universal and necessary form of sense. Again, Kant, in the "Transcendental Logic," is professedly engaged with the search for an answer to the second main question of the Critique, How is pure physical science, or sensible knowledge, possible? Kant, now, has said, and, with reference to the kind of knowledge mentioned in the foregoing question, has said truly, that "thoughts, without the content which perception supplies, are empty." This is not less true of "pure" thoughts, than of any others. The content which the pure conceptions, as categories of pure physical science or sensible knowledge, cannot derive from the matter of sense, they must and do derive from its pure form. And in this relation between the pure conceptions of the understanding and their pure content there is involved, as we shall see, the most intimate community of nature and origin between sense, on its formal side (space and time), and the understanding itself. For Kant, space and time are a priori intuitions. Out of a total of six arguments in favor of space as "a priori" intuition, Kant presents four of them in the Metaphysical Exposition of space: two argue for space "a priori" and two for space as intuition.