“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).
Pluralitas non est ponenda sine necessitate.
As cited in "The Myth of Occam's Razor" by William Thorburn, in Mind, Vol. 27 (1918), 345–353.
Original
Numquam ponenda est pluralitas sine necessitate
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William of Ockham 13
medieval philosopher and theologian 1285–1349Related quotes
“Necessity gives the law without itself acknowledging one.”
Necessitas dat legem non ipsa accipit.
Maxim 444
Variant translation: Necessity knows no law except to conquer.
Necessitas non habet legem, "Necessity has no law", is apparently of medieval origin. See Necessity for further variants.
Sentences

The Art of Persuasion

“Love and compassion are necessities, not luxuries. Without them humanity cannot survive.”
As quoted in A Small Drop of Ink: A Collection of Inspirational and Moving Quotations of the Ages (2003) by Linda Pendleton.

“Civilization never recedes; the law of necessity ever forces it onwards.”
La civilisation ne recule jamais, et il semble qu’elle emprunte tous les droits à la nécessité.
Part III, ch. XVI
The Mysterious Island (1874)

Part V, Ch. 3 : 3rd Public Talk Madras 14th January 1968 "The Sacred" http://www.jiddu-krishnamurti.net/en/awakening-of-intelligence/1968-01-14-jiddu-krishnamurti-awakening-of-intelligence-the-sacred
1970s, The Awakening of Intelligence (1973)
Context: One can go on endlessly reading, discussing, piling up words upon words, without ever doing anything about it. It is like a man that is always ploughing, never sowing, and therefore never reaping. Most of us are in that position. And words, ideas, theories, have become much more important than actual living, which is acting, doing. I do not know if you have ever wondered why, throughout the world, ideas, formulas, concepts, have tremendous significance, not only scientifically but also theologically.

Source: The Anti-Christ/Ecce Homo/Twilight of the Idols/Other Writings

“People who pin their faith to a catchword never feel the necessity of understanding anything.”
in "Women and War" (May 1915)

“Good officers never engage in general actions unless induced by opportunity or obliged by necessity.”
Boni duces publico certamine numquam nisi ex occasione aut nimia necessitate confligunt.
De Re Militari (also Epitoma Rei Militaris), Book III, "Dispositions for Action"
Context: Punishment, and fear thereof, are necessary to keep soldiers in order in quarters; but in the field they are more influenced by hope and rewards. Good officers never engage in general actions unless induced by opportunity or obliged by necessity. (General Maxims)