Arthur Latham Perry, Elements of Political Economy (1869).A theory that does not work well in practice is a bad theory. The way to tell whether a theory is good or bad is to test it by practice. Everything that is done at all, unless by mere chance, is done on some theory; and it is certainly better that things should be done on a good theory than on a bad one. What makes a theory good? Simply because it corresponds with and explains the facts.