For he found that what he had to do was not to trace what he saw in this figure, or even trace its mere con cept, and read off, as it were, from the properties of the figure; but rather that he had to produce the latter from what he himself thought into the object and presented (through construction) according to a priori concepts, and that in order to know something securely a priori he had to ascribe to the thing nothing except what followed nec essarily from what he himself had put into it in accordance with its concept