Plurality is never to be posited without necessity.

Quaestiones et decisiones in quattuor libros Sententiarum Petri Lombardi [Questions and the decisions of the Sentences of Peter Lombard] (1495), i, dist. 27, qu. 2, K; also in The Development of Logic (1962), by William Calvert Kneale, p. 243; similar statements were common among Scholastic philosophers, at least as early as John Duns (Duns Scotus). | As cited in "The Myth of Occam's Razor" by William Thorburn, in Mind, Vol. 27 (1918), 345–353.