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.

One does not ask whether a scientific theory is true, but only whether it is convenient.

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

A reality completely independent of the spirit that conceives it, sees it, or feels it, is an impossibility. A world so external as that, even if it existed, would be forever inaccessible to us.

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

A small error in the former will produce an enormous error in the latter.

Hypotheses are what we lack the least.