Ask a philosopher "What is philosophy?" or a historian "What is history?" and they will have no difficulty in giving an answer. Neither of them, in fact, can pursue his own discipline without knowing what he is searching for. But ask a mathematician "What is mathematics?" and he may justifiably reply that he does not know the answer but this does not stop him from doing mathematics.

François Lasserre, 'Pi in the sky' - John D Barrow