No doubt you know that Galileo had been convicted not long ago by the Inquisition, and that his opinion on the movement of the Earth had been condemn… - René Descartes

in order to seek truth, it is necessary once in the course of our life, to doubt, as far as possible, of all things.

But I cannot forget that, at other times I have been deceived in sleep by similar illusions; and, attentively considering those cases, I perceive so clearly that there exist no certain marks by which the state of waking can ever be distinguished from sleep, that I feel greatly astonished; and in amazement I almost persuade myself that I am now dreaming.

The whole force of the argument rests on the fact that I recognize that it would be impossible for me to exist, being of such a
nature as I am (namely, having in me the idea of God), unless God did
in fact exist. God, I say, that same being the idea of whom is in me: a
being having all those perfections that I cannot comprehend, but can
somehow touch with my thought, and a being subject to no defects
whatever. From these considerations it is quite obvious that he cannot be
a deceiver, for it is manifest by the light of nature that all fraud and
deception depend on some defect