There was a time when we wanted to be told what an electron is. The question was never answered. No familiar conceptions can be woven around the electron; it belongs to the waiting list.

Whatever else there may be in our nature, responsibility toward truth is one of its attributes.

Observation and theory get on best when they are mixed together, both helping one another in the pursuit of truth. It is a good rule not to put overmuch confidence in a theory until it has been confirmed by observation. I hope I shall not shock the experimental physicists too much if I add that it is also a good rule not to put overmuch confidence in the observational results that are put forward until they have been confirmed by theory.

Our model of Nature should not be like a building-a handsome structure for the populace to admire, until in the course of time some one takes away a corner stone and the edifice comes toppling down. It 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.

Who will observe the observers?

It is a primitive form of thought that things exist or do not exist.

In the most modern theories of physics probability seems to have replaced aether as "the nominative of the verb 'to undulate'."