pnrjulius comments on Rationality Quotes June 2012 - Less Wrong
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I recall a math teacher in high school explaining that often, in the course of doing a proof, one simply gets stuck and doesn't know where to go next, and a good thing to do at that point is to switch to working backwards from the conclusion in the general direction of the premise; sometimes the two paths can be made to meet in the middle. Usually this results in a step the two paths join involving doing something completely mystifying, like dividing both sides of an equation by the square root of .78pi.
"Of course, someone is bound to ask why you did that," he continued. "So you look at them completely deadpan and reply 'Isn't it obvious?'"
I have forgotten everything I learned in that class. I remember that anecdote, though.
The standard proof of the Product Rule in calculus has this form. You add and subtract the same quantity, and then this allows you to regroup some things. But who would have thought to do that?
--Richard Hamming