bes is required in order that we may
learn that it exists along with some other body. In the second place,
that for which the principle of interaction is really required is not,
as Kant supposes, the determination of the coexistence of an
unperceived body with a perceived body, but the determination of that
unperceived state of a body already known to exist which is coexistent
with a perceived state of a perceived body. As has been pointed out,
if we perceive A and B alternately in the states [alpha]_{1}
[beta]_{2} [alpha]_{3} [beta]_{4} ... we need the thought of
interaction to determine the nature of [beta]_{1} [alpha]_{2}
[beta]_{3} [alpha]_{4} ... Thus it appears that Kant in his
vindication of the third analogy omits altogether to notice the one
process which really presupposes it.

[55] B. 233-4, M. 142.

[56] B. 259, M. 157.

CHAPTER XIII

THE POSTULATES OF EMPIRICAL THOUGHT

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
principle