There was an absence in our teaching system, as there is, I believe, in most high school teaching systems, to consider mathematics and science as exact. “It is so, it is provable, it is indubitable!” All of it is true. But it misses the point. The interesting thing in the exact sciences is what is not yet known, what is in doubt, and that process of doubt, of contradiction, which actually occurs as science changes from century to century, should be reproduced in every student’s mind. And I think, as a matter of fact, it is being reproduced in every good student’s mind.