e perception on
which the _a priori_ synthetic judgement is based] contains that which
is bound to be found in pure perception, since, as _a priori_
perception, it is inseparably connected with the conception _before
all experience_ or individual sense-perception."

This passage is evidently based upon the account which Kant gives in
the _Doctrine of Method_ of the method of geometry.[35] According to
this account, in order to apprehend, for instance, that a three-sided
figure must have three angles, we must draw in imagination or on paper
an individual figure corresponding to the conception of a three-sided
figure. We then see that the very nature of the act of construction
involves that the figure constructed must possess three angles as well
as three sides. Hence, perception being that by which we apprehend the
individual, a perception is involved in the act by which we form a
geometrical judgement, and the perception can be called _a priori_, in
that it is guided by our _a priori_ apprehension of the necessary
nature of the act of construction, and therefore of the figure
constructed.

[35] B. 740 ff., M. 434 ff. Compare especially the following:
"_Philosophical_ knowledge is _knowledge of reason_ by means
of _conceptions_; mathematical knowledge is knowledge by
means of the _construction_ of conceptions. But the
_construction_ of a conception means the _a priori_
presentation of a perception corresponding to it. The
construction of a conception therefore demands a
_non-empirical_ perception, which, therefore, as a
perception, is an _individual_ object, but which none the
less, as the construction of a conception (a universal
representation), must express in the representation universal
validity for all possible perceptions which come under that
conception. Thus I construct a triangle by presenting the
object corresponding to the conception, either by mere
imagination in pure perception, or also, in accordance with
pure perception, on paper in empirical perception, but in
both cases completely _a priori_, without having borrowed
the pattern of it from any experience. The individual drawn
figure is empirical, but nevertheless serves to indicate the
conception without prejudice to its universality, because in
this empirical perception we always attend only to the act of
construction of the conception, to which many determinatio