Dubium sapientiae initium. (Doubt is the origin of wisdom.)
The greatest minds are capable of the greatest vices as well as of the greatest virtues.
Everything is self-evident.
When it is not in our power to follow what is true, we ought to follow what is most probable.
The first precept was never to accept a thing as true until I knew it as such without a single doubt.
There is nothing so strange and so unbelievable that it has not been said by one philosopher or another.