It is far easier for a woman to lead a blameless life than it is for a man; all she has to do is to avoid sexual intercourse like the plague.
I remember that Jack Lemmon, who is one of my favorite actors of all time, says that the day he stops being nervous is the day he should leave the business.
Finally there are simple ideas of which no definition can be given; there are also axioms or postulates, or in a word primary principles, which cannot be proved and have no need of proof.