God is a challenge because there is no proof of his existence and therefore the search must continue.

These machines have no common sense; they have not yet learned to "think," and they do exactly as they are told, no more and no less. This fact is the hardest concept to grasp when one first tries to use a computer.

The best practice is inspired by theory.

Programming is the art of telling another human being what one wants the computer to do.

...One of the most important lessons, perhaps, is the fact that SOFTWARE IS HARD. From now on I shall have significantly greater respect for every successful software tool that I encounter. During the past decade I was surprised to learn that the writing of programs for TeX and Metafont proved to be much more difficult than all the other things I had done (like proving theorems or writing books). The creation of good software demand a significiantly higher standard of accuracy than those other things do, and it requires a longer attention span than other intellectual tasks.

I define UNIX as 30 definitions of regular expressions living under one roof.

The whole thing that makes a mathematician’s life worthwhile is that he gets the grudging admiration of three or four colleagues.