§ 11. The problem of "a priori" intuition is solved. The pure "a priori" intuition of space and time is the basis of empirical "a posteriori" intuition. Synthetic "a priori" mathematical knowledge refers to empirically sensed objects. "A priori" intuition relates to the mere form of sensibility; it makes the appearance of objects possible. The "a priori" form of a phenomenal object is space and time. The "a posteriori" matter of a phenomenal object is sensation, which is not affected by pure, "a priori" intuition. The subjective "a priori" pure forms of sensation, namely space and time, are the basis of mathematics and of all of the objective " a posteriori" phenomena to which mathematics refers.