s 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.

To understand these principles and the proof of them, we must notice
certain preliminary considerations. In the _first_ place, the very
problem of distinguishing the possible, the actual, and the necessary
presupposes the existence of distinctions which may prove open to
question. It presupposes that something may be possible without being
actual, and again that something may be actual without being
necessary. In the _second_ place, Kant's mode of approaching the
problem assumes that we can begin with a conception of an object,
e. g. of a man with six toes, and then ask whether the object of it is
possible, whether, if possible, it is also actual, and whether, if
actual, it is also necessary. In other words, it assumes the
possibility of separating what is conceived from what is possible, and
therefore _a fortiori_ from what is actual,[4] and from what is
necessary. _Thirdly_, in this context, as in most others, Kant in
speaking of a conception is thinking, to use Locke's phraseology, not
of a 'simple' conception, such as that of equality or of redness, but
of a 'complex' conception, such as that of a centaur, or of a triangle
in the sense of a three-sided three-angled figure. It is the
apprehension of a 'complex' of elements.[5] _Fourthly_, what is said
to be possible, real, or necessary is not the conception but the
corresponding object. The question is not, for instance, whether the
conception of a triangle or of a centaur is possible, actual, or
necessary, but whether a triangle or a centaur is possible, actual, or
necessary. Kant sometimes speaks loosely of conceptions as
possible,[6] but the terms which he normally and, from the point of
view of his theory, rightly applies to conceptions are 'objectively
real' and 'fictit