Laudator Temporis Acti

Charles Lutwidge Dodgson (1832-1898, also known as Lewis Carroll), letter to Edith Rix:

When you have made a thorough and reasonably long effort, to understand a thing, and still feel puzzled by it, stop, you will only hurt yourself by going on. Put it aside till the next morning; and if then you can’t make it out, and have no one to explain it to you, put it aside entirely, and go back to that part of the subject which you do understand. When I was reading Mathematics for University honours, I would sometimes, after working a week or two at some new book, and mastering ten or twenty pages, get into a hopeless muddle, and find it just as bad the next morning. My rule was to begin the book again. And perhaps in another fortnight I had come to the old difficulty with impetus enough to get over it. Or perhaps not. I have several books that I have begun over and over again.

My second hint shall be—Never leave an unsolved difficulty behind. I mean, don’t go any further in that book till the difficulty is conquered. In this point, Mathematics differs entirely from most other subjects. Suppose you are reading an Italian book, and come to a hopelessly obscure sentence—don’t waste too much time on it, skip it, and go on; you will do very well without it. But if you skip a mathematical difficulty, it is sure to crop up again: you will find some other proof depending on it, and you will only get deeper and deeper into the mud.

My third hint is, only go on working so long as the brain is quite clear. The moment you feel the ideas getting confused leave off and rest, or your penalty will be that you will never learn Mathematics at all!

George Clausen (1852-1944),
Twilight Interior
(Reading by Lamplight)