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

Pour examiner la vérité, il est besoin, une fois dans sa vie, de mettre toutes choses en doute autant qu’il se peut.

It is thus quite certain that the constitution of the true religion, the ordinances of which are derived from God, must be incomparably superior to that of every other.

But immediately upon this I observed that, whilst I thus wished to think that all was false, it was absolutely necessary that I, who thus thought, should be somewhat; and as I observed that this truth, I think, therefore I am (cogito ergo sum), was so certain and of such evidence that no ground of doubt, however extravagant, could be alleged by the sceptics capable of shaking it, I concluded that I might, without scruple, accept it as the first principle of the philosophy of which I was in search … I thence concluded that I was a substance whose whole essence or nature consists only in thinking, and which, that it may exist, has need of no place, nor is dependent on any material thing; so that ‘I’, that is to say, the mind by which I am what I am, is wholly distinct from the body, and is even more easily known than the latter …