The whole strenuous intellectual work of an industrious research worker would appear, after all, in vain and hopeless, if he were not occasionally through some striking facts to find that he had, at the end of his all criss-cross journeys, at last accomplished at least one step which was conclusively nearer the truth.

Nobel Lectures, Physics 1901-1921, Nobel lecture for award received in 1918 (p. 407)