It is one thing for the human mind to extract from the phenomena of nature the laws which it has itself put into them; it may be a far harder thing to extract laws over which it has no control. It is even possible that laws which have not their origin in the mind may be irrational, and we can never succeed in formulating them.

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

To the pure geometer the radius of curvature is an incidental characteristic - like the grin of the Cheshire cat. To the physicist it is an indispensable characteristic. It would be going too far to say that to the physicist the cat is merely incidental to the grin. Physics is concerned with interrelatedness such as the interrelatedness of cats and grins. In this case the "cat without a grin" and the "grin without a cat" are equally set aside as purely mathematical phantasies.

We often think that when we have completed our study of one we know all about two, because 'two' is 'one and one.' We forget that we still have to make a study of 'and.'

In Einstein's theory of relativity the observer is a man who sets out in quest of truth armed with a measuring-rod. In quantum theory he sets out with a sieve.

I am aware that many critics consider the conditions in the stars not sufficiently extreme . . . the stars are not hot enough. The critics lay themselves open to an obvious retort: we tell them to go and find a hotter place.

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.'