But I cannot forget that, at other times I have been deceived in sleep by similar illusions; and, attentively considering those cases, I perceive so clearly that there exist no certain marks by which the state of waking can ever be distinguished from sleep, that I feel greatly astonished; and in amazement I almost persuade myself that I am now dreaming.

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.

There is nothing more ancient than the truth.

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

I should consider that I know nothing about physics if I were able to explain only how things might be, and were unable to demonstrate that they could not be otherwise.

And thereby make ourselves, as it were, the lords and masters of nature.

Intuitive knowledge is an illumination of the soul, whereby it beholds in the light of God those things which it pleases Him to reveal to us by a direct impression of divine clearness.