Mathematics is of two kinds, Rigorous and Physical. The former is Narrow: the latter Bold and Broad. To have to stop to formulate rigorous demonstrations would put a stop to most physico-mathematical inquiries. Am I to refuse to eat because I do not fully understand the mechanism of digestion?

Why should I refuse a good dinner simply because I don't understand the digestive processes involved?

If it is love that makes the world go round, it is self-induction that makes electromagnetic waves go round the world.

Theory is the essence of facts. Without theory scientific knowledge would be only worthy of the madhouse.

Waves from moving sources: Adagio. Andante. Allegro moderato.

Criticized for using formal mathematical manipulations, without understanding how they worked: Should I refuse a good dinner simply because I do not understand the process of digestion?

The best result of mathematics is to be able to do without it.