“If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found the object of his search. I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him ninety per cent of his labor.” -- Nikola Tesla

Edit: Please note that I have quoted Edison many times when encouraging people to not give up in the face of failure. So I am not against the idea that one needs to keep working at some things. But the idea of "keeping at it" can be a trap that prevents you from reassessing your approach and finding something better.