Thomas Aquinas
on Common Natures
Aquinas sorts out the metaphysics of forms in De ente et essentia (Being and Essence),
chapters III and IV (Hyman and Walsh, pp. 511-513). Aquinas rejects the Augustinian doctrines
of Universal Hylomorphism and the Plurality of Forms.
The "form of the part," or form which is a part, is the individual’s rational soul. Here, the
topic is the "essence" or the "form of the whole." The essence can be considered in several ways:
I. The essence, or form of the whole, can be considered as a part. This is humanity in the
example. "It only contains in its signification what belongs to man as man, with all determination
of matter set aside" (p. 511, ll. 6-7f.b.) It is "abstracted with precision." Humanity: (i) is
predicated of neither Socrates nor man; (ii) it signifies the essence; (iii) it has no esse, like
Avicenna’s "common nature in itself."
II. The essence, or form of the whole, can also be considered as a whole. In the example,
this is man. "It does not explicitly involve the determination of matter, but contains that
implicitly and indistinctly, just as the genus was said to contain the differentia" (p. 511, ll. 910f.b.). This is "abstraction without precision", i.e., not pre-scinded from signate matter. This can
be considered in several ways:
A. Considered "absolutely" or "according to its own nature" (p. 512, l. 15), it
includes only what is explicitly contained in its definition. It excludes only what is
incompatible with that definition. Everything else is neither included nor
excluded. Man, considered in this way, is rational and animal; it is neither one nor
many. It is: (i) predicated of many, but (ii) is underdetermined and with- out
real existence.
B. Considered according to its being, i.e., insofar as the "essence has being in this
one or that" (p. 512, l. 26).
1. In the individual, man, for example, has all the accidents of the
particular man. Considered this was, it includes whatever is in the
individual, so it does not include universality (predicability of many).
2. In the soul, i.e., "according to the being it has in the intellect" (p. 513, l.
1). This is "conducive to knowledge" but it is still "a certain particular
appearance" so it is not predicable of many.
What is predicated of many is IIA, but that has no Unity and no Being. What, then
grounds knowledge, i.e., what accounts for the formal identity of IIB-1 and IIB-2? And what
accounts for the "commonality" of common natures?
John Duns Scotus
on Common Natures
Scotus (1265-1308) discussed the nature, being, unity, and differences of common
natures in his Oxford Commentary on [Peter Lombard’s] Books of Sentences, particularly in
Book II, Distinction III (Hyman and Walsh, pp. 604ff.) The relevant background is provided by
Avicenna and Aquinas. Avicenna had concluded that universals (common natures) have no
unity but they do have some sort of lesser being -- violating the Augustinian identification of
Being and Oneness. Aquinas restored Augustine’s equation by arguing that since common
natures have no real unity, they can’t have any real being. Scotus, who did not follow Aquinas in
thinking of Being as act, adopted the other way of preserving the equation: if common natures
have some sort of lesser Being, they must have some sort of lesser Unity.
The three sorts of things Scotus considers are:
(i) individuals
(ii) common natures
(iii) concepts.
Each kind of thing has its own kind of being:
(i) individual things have esse existentia;
(ii) common natures have esse quidditativum.
(iii) the being of concepts (if any) is of no concern (they probably have none).
Each kind of being also has its own kind of unity:
(i) individual things have Numerical unity;
(ii) common natures have Real Minor (or Less-Than-Numerical) unity.
(iii) the unity of concepts is not important (and, if they have no real being,
then they have no real unity).
Each kind of being-one corresponds to a kind of not-being-one, a kind of difference:
(i) numerical unity is complemented by the Real Distinction;
(ii) real minor unity is complemented by the Formal Distinction;
(iii) the Distinction of Reason applies to concepts.
These distinctions naturally emerge from of Thomas’s disambiguation of Avicenna’s
criterion of differentiation: being-able-to-conceive-of-x-without-y. A proposed reconstruction:
x is really distinct from y iff it is possible that x exists without y,
or y exists without x, or both. [Aquinas accepted this distinction as
a valid criterion for metaphysical purposes.]
x is formally distinct from y iff it is possible to conceive of x
without conceiving of y. [Aquinas rejected this as a criterion.]
Scotus’s third distinction -- the distinction of reason -- seems to refer to two different ways of
understanding or conceptualizing one and the same thing. Examples:
Really Distinct: Socrates & Plato;
Formally Distinct: Socrates & humanity; humanity & animality;
Distinct by Reason: Socrates & Plato’s teacher.
Finally, Scotus distinguished three "Modes" of existence:
Singularity -- the mode of existence of individuals, requiring haecceitas;;
Universality -- being predicable of many (cf. Aristotle); only concepts have this;
Commonality -- being one-in-many (cf. Boethius); Common natures have this.