Rene Descartes

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.

Doubt is the origin of wisdom

Common sense is the most widely shared commodity in the world, for every man is convinced that he is well supplied with it.

Except our own thoughts, there is nothing absolutely in our power

It is not enough to have a good mind; the main thing is to use it well.

To know what people really think, pay attention to what they do, rather than what they say

The greatest minds are capable of the greatest vices as well as of the greatest virtues.