All that is not thought is pure nothingness; since we can think only thoughts, and all the words we use to speak of things can express only thoughts, to say there is something other than thought is therefore an affirmation which can have no meaning.

Science is built up with facts, as a house is with stones. But a collection of facts is no more a science than a heap of stones is a house.

Doubting everything and believing everything are two equally convenient solutions that guard us from having to think

Ideas rose in clouds; I felt them collide until pairs interlocked, so to speak, making a stable combination.

Mathematicians do not deal in objects, but in relations between objects; thus, they are free to replace some objects by others so long as the relations remain unchanged. Content to them is irrelevant: they are interested in form only.

Just as houses are made of stones, so is science made of facts.

A cat is witty, he has nerve, he knows how to do precisely the right thing at the right moment.