William Thomson, Lord Kelvin

Do not imagine that mathematics is hard and crabbed, and repulsive to common sense. It is merely the etherealization of common sense.

I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.

Heavier-than-air flying machines are impossible.

In science there is only physics; all the rest is stamp collecting.

Large increases in cost with questionable increases in performance can be tolerated only in race horses and women.

Science is bound, by the everlasting vow of honour, to face fearlessly every problem which can be fairly presented to it.

The atheistic idea is so nonsensical that I cannot put it into words.

The true measure of a man is what he would do if he knew he would never be caught.