Chapter III of Etienne Gilson's Being and Some Philosophers is highly relevant to my ongoing discussion of common natures. Gilson appears to endorse the classic argument for the doctrine of common natures in the following passage (for the larger context see here):
Out of itself, animal is neither universal nor singular. Indeed, if, out of itself, it were universal, so that animality were universal qua animality, there could be no singular animal, but each and every animal would be a universal. If, on the contrary, animal were singular qua animal, there could be no more than a single animal, namely, the very singular to which animality belongs, and no other singular could be an animal. (77)
This passage contains two subarguments. We will have more than enough on our plates if we consider just the first. The first subargument, telescoped in the second sentence above, can be put as follows:
1. If animal has the property of being universal, then every animal would be a universal. But:
2. It is not the case that every animal is a universal. Therefore:
3. It is not the case that animal has the property of being universal.
This argument is valid in point of logical form, but are its premises true? Well, (2) is obviously true, but why should anyone think that (1) is true? It is surely not obvious that the properties of a nature must also be properties of the individuals of that nature.
There are two ways a nature N could have a property P. N could have P by including P within its quidditative content, or N could have P by instantiating P. There is having by inclusion and having by instantiation.
For example, 'Man is rational' on a charitable reading states that rationality is included within the content of the nature humanity. This implies that everything that falls under man falls under rational. Charitably interpreted, the sentence does not state that the nature humanity or the species man is rational. For no nature, as such, is capable of reasoning. It is the specimens of the species who are rational, not the species.
This shows that we must distinguish between inclusion and instantiation. Man includes rational; man does not instantiate rational.
Compare 'Man is rational' with 'Socrates is rational.' They are both true, but only if 'is' is taken to express different relatons in the two sentences. In the first it expresses inclusion; in the second, instantiation. The nature man does not instantiate rationality; it includes it. Socrates does not include rationality; he instantiates it.
The reason I balk at premise (1) is because it seems quite obviously to trade on a confusion of the two senses of 'is' lately distinguished. It confuses inclusion with instantiation. (1) encapuslates a non sequitur. It does not follow from a nature's being universal that everything having that nature is a universal. That every animal would be a universal would follow from humanity's being universal only if universality were included in humanity. But it is not: humanity instantiates universality. In Frege's jargon, universality is an Eigenschaft of humanity, not a Merkmal of it.
Since the first subargument fails, there is no need to examine the second. For if the first subargment fails, then the whole Avicennian-Thomist argument fails.
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