There is no point in saying that one should not doubt or one should believe. Just to say ‘I believe’ does not mean that you understand and see. When a student works on a mathematical problem, he comes to a stage beyond which he does not know how to proceed, and where he is in doubt and perplexity. As long as he has this doubt, he cannot proceed. If he wants to proceed, he must resolve this doubt. And there are ways of resolving that doubt. Just to say ‘I believe’, or ‘I do not doubt’ will certainly not solve the problem. To force oneself to believe and to accept a thing without understanding is political, and not spiritual or intellectual.
Walpola Rahula
--Jean le Rond d’Alembert on infinitesimals (as quoted in Mathematics: the loss of certainty, by Morris Kline)