Ockham [...] deplored the inaccuracy of the terms used in philosophy, and spent half his time trying to make them more precise. He resented the Gothic edifice of abstractions- one mounted upon the other like arches in superimposed tiers- that medieval thought had raised. We cannot find in his extant works precisely the famous formula that tradition called "Ockham's razor": entia non sunt multiplicanda praeter necessitatem - entities are not to be multiplied beyond need. But he expressed the principle in other terms again and again: pluralitas non est ponenda sine necessitate - a plurality (of entities or causes or factors) is not to be posited (or assumed) without necessity; and frustra fit per plura quod potest fieri per pauciora - it is vain to seek to accomplish or explain by assuming several entities or causes what can be explained by fewer.
The principle was not new; Aquinas had accepted it, Scotus had used it. But in Ockham's hands it became a deadly weapon, cutting away a hundred occult fancies and grandiose abstractions. Applying the principle to epistemology, Ockham judged it needless to assume, as the source and material of knowledge, anything more than sensations. From these arise memory (sensation revived), perception (sensation interpreted through memory), imagination (memories combined), anticipation (memory projected), thought (memories compared), and experience (memories interpreted through thought). "Nothing can be an object of the interior sense" (thought) "without having been an object of the exterior sense" (sensation); here is Locke's empiricism 300 years before Locke.
All that we ever perceive outside ourselves is individual entities- specific persons, places, things, actions, shapes, colors, tastes, odors, pressures, temperatures, sounds; and the words by which we denote these are "words of first intention" or primary intent, directly referring to what we interpret as external realities. By noting and abstracting the common features of similar entities so perceived, we may arrive at general or abstract ideas- man, virtue, height, sweetness, heat, music, eloquence; and the words by which we denote such abstractions are "words of second intention," referring to conceptions derived from perceptions. These "universals" are never experienced in sensation; they are termini, signa, nomina - terms, signs, names - for generalizations extremely useful (and dangerous) in thought or reason, in science, philosophy, and theology; they are not objects existing outside the mind. "Everything outside the mind is singular, numerically one."
Reason is magnificent, but its conclusions have meaning only in so far as they refer to experience- i.e., to the perception of individual entities, or the performance of individual acts; otherwise its conclusions are vain and perhaps deceptive abstractions. How much nonsense is talked or written by mistaking ideas for things, abstractions for realities! Abstract thought fulfills its function only when it leads to specific statements about specific things. From this "nominalism" Ockham moved with devastating recklessness into every field of philosophy and theology. Both metaphysics and science, he announced, are precarious generalizations, since our experience is only of individual entities in a narrowly restricted area and time; it is mere arrogance on our part to assume the universal and eternal validity of the general propositions and "natural laws" that we derive from this tiny sector of reality. Our knowledge is molded and limited by our means and ways of perceiving things (this is Kant before Kant); it is locked up in the prison of our minds, and it must not pretend to be the objective or ultimate truth about anything.
Will Durant - Story of Civilization, vol.06, pp.246-49