[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 determinations, e. g. the magnitude of the sides and of the angles, are wholly indifferent, and accordingly abstract from these differences, which do not change the conception of the triangle." The account in the _Prolegomena_, however, differs from that of the _Doctrine of Method_ in one important respect. It asserts that the perception involved in a mathematical judgement not only may, but must, be pure, i. e. must be a perception in which no spatial object is present, and it implies that the perception must take place _before_ all experience of actual objects.[36] Hence _a priori_, applied to perception, has here primarily, if not exclusively, the temporal meaning that the perception takes place _antecedently to all experience_.[37] [36] This becomes more explicit in § 8 and ff. [37] This is also, and more obviously, implied in §§ 8-11.