Wittgenstein used to compare thinking with swimming: just as in swimming our bodies have a natural tendency to float on the surface so that it requires great physical exertion to plunge to the bottom, so in thinking it requires great mental exertion to force our minds away from the superficial, down into the depth of a philosophical problem.
—George Pitcher, The Philosophy of Wittgenstein
(Englewood Cliffs, NJ: Prentice-Hall, 1964)