In striking contrast to Plato's use of idea is that of John Locke. In his Introduction to An Essay Concerning Human Understanding, Locke defines "idea" as "that term which, I think, serves best to stand for whatsoever is the object of the understanding when a man thinks, I have used it to express whatever is meant by phantasm, notion, species, or whatever it is which the mind can be employed about in thinking; and I could not avoid frequently using it." He said he regarded the book necessary to examine our own abilities and see what objects our understandings were, or were not, fitted to deal with. In his philosophy other outstanding figures followed in his footsteps — Hume and Kant in the 18th century, Arthur Schopenhauer in the 19th century, and Bertrand Russell, Ludwig Wittgenstein, and Karl Popper in the 20th century. Locke always believed in "good sense" — not pushing things to extremes and on taking fully into account the plain facts of the matter. He considered his common-sense ideas "good-tempered, moderate, and down-to-earth."