I am thinking, therefore I exist. (...) I was a substance whose whole essence or nature is solely to think, and which does not require any place, or depend on any material thing, in order to exist. Accordingly this 'I' - that is, the soul by which I am what I am - is entirely distinct from the body, and indeed is easier to know than the body, and would not fail to be whatever it is, even if the body did not exist.

For each of us there is a set limit to our intellectual powers which we cannot pass.

I am accustomed to sleep and in my dreams to imagine the same things that lunatics imagine when awake.

There is nothing more ancient than the truth.

When I consider this carefully, I find not a single property which with certainty separates the waking state from the dream. How can you be certain that your whole life is not a dream?

Even those who have the weakest souls could acquire absolute mastery over all their passions if we employed sufficient ingenuity in training and guiding them.

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