Doubt is the origin of wisdom

So far, I have been a spectator in this theater which is the world, but I am now about to mount the stage, and I come forward masked.

All is to be doubted.

Everything is self-evident.

For to be possessed of a vigorous mind is not enough; the prime requisite is rightly to apply it.

Each problem that I solved became a rule, which served afterwards to solve other problems.

Bad books engender bad habits, but bad habits engender good books.