Our model of Nature... should be like an engine with movable parts. We need not fix the position of any one lever; that is to be adjusted from time to time as the latest observations indicate. The aim of the theorist is to know the train of wheels which the lever sets in motion — that binding of the parts which is the soul of the engine.

The Internal Constitution of Stars, Nature, Volume 106, Number 2603, 2 September 1920 (p. 20)