Scientific theories need reconstruction every now and then. If they didn't need reconstruction they would be facts, not theories. The more facts we know, the less radical become the changes in our theories. Hence they are becoming more and more constant. But take the theory of gravitation; it has not been changed in four hundred years.

The most important advance in the next fifty years will be in the realm of the spiritual - dealing with the spirit of thought.

Indeed, the most important part of engineering work-and also of other scientific work-is the determination of the method of attacking the problem, whatever it may be, whether an experimental investigation, or a theoretical calculation. ... It is by the choice of a suitable method of attack, that intricate problems are reduced to simple phenomena, and then easily solved.

I have succeeded in getting my actual work down to thirty minutes a day. That leaves me eighteen hours for engineering.

[I predict] the electricity generated by water power is the only thing that is going to keep future generations from freezing. Now we use coal whenever we produce electric power by steam engine, but there will be a time when there'll be no more coal to use. That time is not in the very distant future. ... Oil is too insignificant in its available supply to come into much consideration.

Mathematics is the most exact science, and its conclusions are capable of absolute proof. But this is so only because mathematics does not attempt to draw absolute conclusions. All mathematical truths are relative, conditional. In E. T. Bell Men of Mathematics, New York: Simona and Schuster, 1937.

The scientist is not much given to talking of the riddle of the universe. "Riddle" is not a scientific term. The conception of a riddle is "something which can he solved." And hence the scientist does not use that popular phrase. We don't know the why of anything. On that matter we are no further advanced than was the cavedweller. The scientist is contented if he can contribute something toward the knowledge of what is and how it is.