Looking Back date their knowledge and develop their ability to solve problems. A good teacher should understand and impress on his students the view that no problem whatever is com pletely exhausted. There remains always something to do; with sufficient study and penetration, we could improve any solution, and, in any case, we can always improve our understanding of the solution. The student has now carried through his plan He has written down the solution, checking each step. Thus, he should have good reasons to believe that his is Nevertheless, errors are always possible, especially if the argument is long and involved. Hence, verifications are desirable. Especially, if there is some rapid and in tuitive procedure to test either the result or the argument it should not be overlooked. Can you check the result? Can you check the argument? In order to convince ourselves of the presence or of the quality of an object, we like to see and to touch it. And as we prefer perception through two different senses, so we prefer conviction by two different proofs: Can you de rive the result diferently? We prefer, of course, a short and intuitive argument to a long and heavy one: Can you see it at a glance? One of the first and foremost duties of the teacher is not to give his students the impression that mathematical problems have little connection with each other, and no connection at all with anything else. We have a natural opportunity to investigate the connections of a problem when looking back at its solution. The students will find looking back at the solution really interesting if they have made an honest effort, and have the consciousness of having done well. Then they are eager to see what else they could accomplish with that effort, and how the could do equally well another time. The teacher should encourage the students to imagine cases in which they