The postulates of empirical thought, which correspond to the categories of modality, are stated as follows: "1. That which agrees with the formal conditions of experience (according to perception and conceptions) is _possible_. 2. That which is connected with the material conditions of experience (sensation) is _actual_. 3. That of which the connexion with the actual is determined according to universal conditions of experience is _necessary_ (exists necessarily)."[1] [1] B. 265-6, M. 161. These principles, described as only 'explanations of the conceptions of possibility, actuality, and necessity as employed in experience', are really treated as principles by which we decide what is possible, what is actual, and what is necessary. The three conceptions involved do not, according to Kant, enlarge our knowledge of the nature of objects, but only 'express their relation to the faculty of knowledge'[2]; i. e. they only concern our ability to apprehend an object whose nature is already determined for us otherwise as at least possible, or as real, or as even necessary. Moreover, it is because these principles do not enlarge our knowledge of the nature of objects that they are called postulates; for a postulate in geometry, from which science the term is borrowed (e. g. that it is possible with a given line to describe a circle from a given point), does not augment the conception of the figure to which it relates, but only asserts the possibility of the conception itself.[3] The discussion of these principles is described, contrary to the terminology adopted in the case of the preceding principles, as 'explanation' and not as 'proof'. The discussion, however, certainly includes a proof of them, for it is Kant's main object to _prove_ that these principles constitute the general character of what can be asserted to be possible, actual, or necessary respectively. Again, as before, the basis of proof lies in a theory of knowledge, and in particular in Kant's theory of knowledge; for it consists in the principle that everything knowable must conform to the conditions involved in its being an object of possible experience. [2] B. 266, M. 161. Cf. B. 286-7, M. 173-4. [3] B. 286-7, M. 173-4.