When introduced at the wrong time or place, good logic may be the worst enemy of good teaching.

In order to solve this differential equation you look at it until a solution occurs to you.

It is better to solve one problem five different ways, than to solve five problems one way.

The principle is so perfectly general that no particular application of it is possible.

John von Neumann was the only student I was ever afraid of.

An idea which can be used only once is a trick. If one can use it more than once it becomes a method.

I am intentionally avoiding the standard term which, by the way, did not exist in Euler's time. One of the ugliest outgrowths of the "new math" was the premature introduction of technical terms.