Rules necessary for definitions. Not to leave any terms at all obscure or ambiguous without definition; Not to employ in definitions any but terms pe… - Blaise Pascal

"Why are we not angry if we are told that we have a headache, and why are we angry if we are told that we reason badly, or choose wrongly?" The reason is that we are quite certain that we have not a headache, or are not lame, but we are not so sure that we make a true choice. So having assurance only because we see with our whole sight, it puts us into suspense and surprise when another with his whole sight sees the opposite, and still more so when a thousand others deride our choice. For we must prefer our own lights to those of so many others, and that is bold and difficult.

Rules necessary for demonstrations. To prove all propositions, and to employ nothing for their proof but axioms fully evident of themselves, or propositions already demonstrated or admitted; Never to take advantage of the ambiguity of terms by failing mentally to substitute definitions that restrict or explain them.

Todas las miserias de la humanidad se deben a una sola cosa: a que somos incapaces de estar sentados solos y en silencio en una habitación