determining why the
one mode of combination was exercised in any given case rather than
the other. If, several kinds of synthesis being allowed, this
difficulty be avoided by the supposition that, not being incompatible,
they are all exercised together, we have the alternative task of
explaining how the same manifold can be combined in each of these
ways. As a matter of fact, Kant thinks of manifolds of different kinds
as combined or related in different ways; thus events are related
causally and quantities quantitatively. But since, on Kant's view, the
manifold as given is unrelated and all combination comes from the
mind, the mind should not be held capable of combining manifolds of
different kinds differently. Otherwise the manifold would in its own
nature imply the need of a particular kind of synthesis, and would
therefore not be unrelated.

Suppose, however, we waive the difficulty involved in the plurality of
the categories. There remains the equally fundamental difficulty that
any single principle of synthesis contains in itself no ground for the
different ways of its application.[2] Suppose it to be conceded that
in the apprehension of definite shapes we combine the manifold in
accordance with the conception of figure, and, for the purpose of the
argument, that the conception of figure can be treated as equivalent
to the category of quantity. It is plain that we apprehend different
shapes, e. g. lines[3] and triangles[4], of which, if we take into
account differences of relative length of sides, there is an infinite
variety, and houses,[5] which may also have an infinite variety of
shape. But there is nothing in the mind's capacity of relating the
manifold by way of figure to determine it to combine a given manifold
into a figure of one kind rather than into a figure of any other kind;
for to combine the manifold into a particular shape, there is needed
not merely the thought of a figure in general, but the thought of a
definite figure. No 'cue' can be furnished by the manifold itself, for
any such cue would involve the conception of a particular figure, and
would therefore imply that the particular synthesis was implicit in
the manifold itself, in which case it would not be true that all
synthesis comes from the mind.

[2] Cf. p. 207.

[3] B. 137, M. 85.

[4] A. 105, Mah. 199.

[5] B. 162, M. 99.

This difficulty takes a somewhat different form in the case of the
categories of relation