A hundred thousand million Stars make one Galaxy; A hundred thousand million Galaxies make one Universe. The figures may not be very trustworthy, but I think they give a correct impression.

Don't believe the results of experiments until they're confirmed by theory.

It is reasonable to hope that in the not too distant future we shall be competent to understand so simple a thing as a star.

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.

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.

[When thinking about the new relativity and quantum theories] I have felt a homesickness for the paths of physical science where there are ore or less discernible handrails to keep us from the worst morasses of foolishness.

Man is slightly nearer to the atom than to the star. ... From his central position man can survey the grandest works of Nature with the astronomer, or the minutest works with the physicist. ... [K]nowledge of the stars leads through the atom; and important knowledge of the atom has been reached through the stars.