Philosophy of mathematics deals with a number of issues as, for instance, the question as to the essence of the mathematics and its objects (the so-called ontological issue). Then there is also the epistemological question of whether mathematical knowledge is possible in the classical sense, and the question as to what the validity of mathematical theorems and theories is based on. Yet another issue is the question as to the interaction between mathematics and physics on the one hand and between mathematics and philosophy on the other.