...we ought also to consider as false all that is doubtful.

If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.

Let whoever can do so deceive me, he will never bring it about that I am nothing, so long as I continue to think I am something.

For, occupied incessantly with the consideration of the limits prescribed to their power by nature, they [philosophers of former times] became so entirely convinced that nothing was at their disposal except their own thoughts, that this conviction was of itself sufficient to prevent their entertaining any desire of other objects; and over their thoughts they acquired a sway so absolute, that they had some ground on this account for esteeming themselves more rich and more powerful, more free and more happy, than other men who, whatever be the favors heaped on them by nature and fortune, if destitute of this philosophy, can never command the realization of all their desires.

...we ought not meanwhile to make use of doubt in the conduct of life.

I suppose therefore that all things I see are illusions; I believe that nothing has ever existed of everything my lying memory tells me. I think I have no senses. I believe that body, shape, extension, motion, location are functions. What is there then that can be taken as true? Perhaps only this one thing, that nothing at all is certain.