Kant said: there are two *a priori* intuitions — space and time. There are also categories, and “the number of the categories in each class is always the same, namely, three”, like unity-plurality-modality, or possibility-existence-necessity. It would be fun to have three *a priori* intuitions, but only two exist, sigh. Really though?

Kant probably did not realize: there is a third one — probability, to wit, certainty of our experience. Just like space, probability precedes any experience. Every object is uncertain as much as it is extended.

The three *a priori* intuitions are related — infinite and undirected space, infinite and directed time, finite and undirected probability. Physics knows of *uncertainty principle*, we are uncertain about relation of time and space: both time and space cannot be intuited with certainty. Probability is as basic and fundamental as time and space for our cognition.

Just like geometry deals with *a priori* intuition of space, and mathematical analysis — with intuition of time, theory of probability deals with intuition of probability. There is philosophical justification for studying uncertainty, probability, and bayesian inference.