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In Summa Theologica's article on "Whether God is altogether simple?" (and I believe in some other locations), Aquinas regularly uses the phrase "predicated of." For example:
The absolute simplicity of God may be shown in many ways... Fifth, because nothing composite can be predicated of any single one of its parts.
I don't understand exactly what this phrase means, though. Can someone explain what this argument means "in general", especially with regards to that specific phrase?
To predicate X of Y is to say that Y is X or that Y is an X. This terminology comes from syllogistic logic, where they tend to be loose about the distinction between a property, a class, and an individual, so in this definition, X could be any of those three.
To predicate a composite of one of its parts would be to take a composite object like a car and a part such as the hood and say, for example, "the hood is the car". So, what Aquinas is saying, roughly, is that no composite individual is identical with one of its parts.
There may be some subtlety in that passage that I don't understand, because he doesn't directly say that no composite individual is identical with its parts, what he says is more like "one can't form a true sentence that says a composite individual is identical with one of its parts." Whether this is significant, I don't know. I'm not familiar enough with Aquinas.
In modern logic, this conflation of properties, classes, and individuals is no longer done. There is a careful distinction between an object instantiating a property, an object being a member of a class, and an object being identical with another object.
A predicate (prædicare = "to assert") is, according to the Dictionary of Scholastic Philosophy (p. 95),
that which is affirmed or denied of a subject in a categorical proposition.
A proposition is (ibid., p. 99)
a statement making an affirmation or negation
and a categorical proposition is (idem):
one which makes an absolute statement about its subject.
A category or "predicament" is (ibid., p. 17):
the ultimate logical classification of all genera, species, and finite individuals.
Aristotle outlined his ten categories in his Categories.
Here's an example logical classification (ibid., p. 18):
These things can be predicated of any changeable being (ens mobile).
One can distinguish subject and predicate by looking at the extension of the terms. Subjects are of greater extension than that which is predicated of them.
Take "ball" and "red" as an example. The extension of "ball" is much less that that of "red [things]." Thus, "ball" could never be predicated of "[all] red [things]" as in
"[All] red [things]" is "ball."
This would imply the only red things are balls.
But "red" could certainly be predicated of "ball," as in
The ball is red.
This is because "[all] red [things]" has greater extension than "ball."
A predicate refers to a condition that some attribute or group of atrributes that some entity has.
Personally, I don't think the term, "absolutely simple" is well suited to describe what the scholastic theologians were attempting as the term, on the face of it, seems inapplicable to God. A much better term is given by the early Jain pholosophers, this is their doctrine of syadvada or conditioned predication. One example of this relating to God, would be the term describing:
From all perspectives, God is, and is indescribable.
This is much better than, "absolutely simple".
