I think; therefore I am.
Illusory joy is often worth more than genuine sorrow.
Common sense is the most widely shared commodity in the world, for every man is convinced that he is well supplied with it.
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.
I desire to live in peace and to continue the life I have begun under the motto 'to live well you must live unseen
Dubium sapientiae initium. (Doubt is the origin of wisdom.)
Some years ago I was struck by the large number of falsehoods that I had accepted as true in my childhood, and by the highly doubtful nature of the whole edifice that I had subsequently based on them. I realized that it was necessary, once in the course of my life, to demolish everything completely and start again right from the foundations if I wanted to establish anything at all in the sciences that was stable and likely to last.
And thus, the actions of life often not allowing any delay, it is a truth very certain that, when it is not in our power to determine the most true opinions we ought to follow the most probable.
There is nothing more ancient than the truth.
Except our own thoughts, there is nothing absolutely in our power.
I am accustomed to sleep and in my dreams to imagine the same things that lunatics imagine when awake.
The two operations of our understanding, intuition and deduction, on which alone we have said we must rely in the acquisition of knowledge.
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.
Each problem that I solved became a rule, which served afterwards to solve other problems.
I am indeed amazed when I consider how weak my mind is and how prone to error.
An optimist may see a light where there is none, but why must the pessimist always run to blow it out?
The reading of all good books is like a conversation with the finest minds of past centuries.
Travelling is almost like talking with those of other centuries.
One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another.
In order to improve the mind, we ought less to learn, than to contemplate.