The cause of suffering is intensity of will; the less we exercise will, the less we shall suffer.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“The best of myths is that of Nirvana (which Schopenhauer interprets as extinction) . This, he agrees, is contrary to Christian doctrine, but “the ancient wisdom of the human race will not be displaced by what happened in Galilee.” The cause of suffering is intensity of will; the less we exercise will, the less we shall suffer. And here knowledge turns out to be useful after all, provided it is knowledge of a certain sort. The distinction between one man and another is part of the phenomenal world, and disappears when the world is seen truly.” source