History of Western Philosophy - Bertrand Russell [269]
Occam is best known for a maxim which is not to be found in his works, but has acquired the name of 'Occam's razor'. This maxim says: 'Entities are not to be multiplied without necessity.' Although he did not say this, he said something which has much the same effect, namely: 'It is vain to do with more what can be done with fewer.' That is to say, if everything in some science can be interpreted without assuming this or that hypothetical entity, there is no ground for assuming it. I have myself found this a most fruitful principle in logical analysis.
In logic, though apparently not in metaphysics, Occam was a nominalist; the nominalists of the fifteenth century4 looked upon him as the founder of their school. He thought that Aristotle had been misinterpreted by the Scotists, and that this misinterpretation was due partly to the influence of Augustine, partly to Avicenna, but partly to an earlier cause, Porphyry's treatise on Aristotle's Categories. Porphyry in this treatise raised three questions: (1) Are genera and species substances? (2) Are they corporeal or incorporeal? (3) If the latter, are they in sensible things or separated from them? He raised these questions as relevant to Aristotle's Categories, and thus led the Middle Ages to interpret the Organon too metaphysically. Aquinas had attempted to
undo this error, but it had been reintroduced by Duns Scotus. The result had been that logic and theory of knowledge had become dependent on metaphysics and theology. Occam set to work to separate them again.
For Occam, logic is an instrument for the philosophy of nature, which can be independent of metaphysics. Logic is the analysis of discursive science; science is about things, but logic is not. Things are individual, but among terms there are universals; logic treats of universals, while science uses them without discussing them. Logic is concerned with terms or concepts, not as psychical states, but as having meaning. 'Man is a species' is not a proposition of logic, because it requires a knowledge of man. Logic deals with things fabricated by the mind within itself, which cannot exist except through the existence of reason. A concept is a natural sign, a word is a conventional sign. We must distinguish when we are speaking of the word as a thing, and when we are using it as having meaning, otherwise we may fall into fallacies such as: 'Man is a species, Socrates is a man, therefore Socrates is a species.'
Terms which point at things are called 'terms of first intention'; terms which point at terms are called 'terms of second intention'. The terms in science are of first intention; in logic, of second. Metaphysical terms are peculiar in that they signify both things signified by words of first intention and things signified by words of second intention. There are exactly six metaphysical terms: being, thing, something, one, true, good.5 These terms have the peculiarity that they can all be predicated of each other. But logic can be pursued independently of them.
Understanding is of things, not of forms produced by the mind; these are not what is understood, but that by which things are understood. Universals, in logic, are only terms or concepts predicable of many other terms or concepts. Universal, genus, species are terms of second intention, and therefore cannot mean things. But since one and being are convertible, if a universal existed, it would be one, and an individual thing. A universal is merely a sign of many things. As to this, Occam agrees with Aquinas, as against Averroes, Avicenna,