If I found any new truths in the sciences, I can say that they follow from, or depend on, five or six principal problems which I succeeded in solving and which I regard as so many battles where the fortunes of war were on my side.

When writing about transcendental issues, be transcendentally clear.

What then is the source of my errors? They are owing simply to the fact that, since the will extends further than the intellect, I do not contain the will within the same boundaries; rather, I also extend it to things I do not understand. Because the will is indifferent in regard to such matters, it easily turns away from the true and the good; and in this way I am deceived and I sin.

... moral certainty is certainty which is sufficient to regulate our behaviour, or which measures up to the certainty we have on matters relating to the conduct of life which we never normally doubt, though we know that it is possible, absolutely speaking, that they may be false.

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.

It is contrary to reasoning to say that there is a vacuum or space in which there is absolutely nothing.

There is a little gland in the brain in which the soul exercises its functions in a more particular way than in the other parts.