John Harsanyi Quote

If two objects or human beings show similar behaviour in all their relevant aspects open to observation, the assumption of some unobservable hidden difference between them must be regarded as a completely gratuitous hypothesis and one contrary to sound scientific method.

Harsanyi, J. C. (1955). "Cardinal Welfare, Individualistic Ethics, and Interpersonal Comparisons of Utility". J. Polit. Economy 63 (4): p. 317