A logical theory may be tested by its capacity for dealing with puzzles, and it is a wholesome plan, in thinking about logic, to stock the mind with as many puzzles as possible, since these serve much the same purpose as is served by experiments in physical science.

"On Denoting", Mind, Vol. 14, No. 56 (October 1905), pp. 479–493; as reprinted in Logic and Knowledge: Essays, 1901–1950, (1956)