On one occasion when [William] Smart found him engrossed with his fundamental theory, he asked Eddington how many people he thought would understand what he was writing-after a pause came the reply, 'Perhaps seven.'

Science is one thing, wisdom is another. Science is an edged tool, with which men play like children, and cut their own fingers.

Never accept a fact until it has been verified by theory.

But it is necessary to insist more strongly than usual that what I am putting before you is a model-the Bohr model atom-because later I shall take you to a profounder level of representation in which the electron instead of being confined to a particular locality is distributed in a sort of probability haze all over the atom.

Electrical force is defined as something which causes motion of electrical charge; an electrical charge is something which exerts electric force.

Proof is an idol before which the mathematician tortures himself.

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.