1. Do not leave undefined any terms at all obscure or ambiguous.
2. Use in definitions only terms perfectly well known or already explained.
3. Ask only that evident things be granted as axioms.
4. Prove all propositions, using for their proof only axioms that are perfectly self-evident or propositions already demonstrated or granted.
5. Never get caught in the ambiguity of terms by failing to substitute in thought the definitions which restrict or explain them.
Blaise Pascal (1623-1662)
Pascal's thinking remains instructive to this day for all forms of analysis.
Source: Pascal, B (1952), On Geometrical Demonstration (On the Geometrical Mind) (R Scofield, Trans). In R M Hutchins & M Adler (Eds), Great Books of the Western World (Vol 33, pp. 430-446), Chicago, IL: Encyclopedia Britannica.