Je pense, donc je suis. - René Descartes

And let him do his best at deception, he will never bring it about that I am nothing so long as I shall think that I am something.

Whence then come my errors? They come from the sole fact that since the will is much wider in its range and compass than the understanding, I do not restrain it within the same bounds, but extend it also to things which I do not understand: and as the will is of itself indifferent to these, it easily falls into error and sin, and chooses the evil for the good, or the false for the true.

The first precept was never to accept a thing as true until I knew it as such without a single doubt.