In contrast to later traditional “metaphysics”, Aristotle recommended we start with the concrete, but then aim to dialectically rise to higher understanding, which is still of the concrete. In any inquiry, we should begin with the things closer to us, but as Wittgenstein said in a different context, we should ultimately aim to kick away the ladder upon which we climbed.
What Aristotle would have us eventually kick away is by no means the concrete itself, but only our preliminary understanding of it as a subject of immediate, simple reference. Beginnings are tentative, not certain. We reach more solid, richer understanding through development.
Aristotle’s discussion of “primary” substance in Categories has often been turned into a claim that individuals are ontologically more primary than form. This is to misunderstand what Categories is talking about. Aristotle explicitly says Categories will be about “things said without combination” [emphasis added], i.e., about what is expressed by kinds of apparently atomic sayings that are used in larger sayings.
The initial definition of substance in the strict or “primary” sense — which he will eventually kick away in the Metaphysics — is of a thing (said) “which is neither said of something underlying nor in something underlying”. (Aristotle often deliberately leaves it open whether he is talking about a referencing word or a referenced thing — or says one and implies the other — because in both cases, the primary concern is the inferential meaning of the reference.)
This initial definition is a negative one that suffices to distinguish substance from the other categories. By implication, it refers to something that is said simply of something, in the way that a proper name is. As examples, he gives (namings of) an individual human, or an individual horse.
“Socrates” would be said simply of Socrates, and would thus “be” — or refer to — a primary substance in this sense. The naming of Socrates is an apparently simple reference to what we might call an object. As Brandom has noted, this picks out a distinctive semantic and inferential role that applies only to references to singular things.
Aristotle then says that more universal namings or named things like “human” and “horse” are also “substances” — i.e., can also refer to singular objects — in a secondary sense, as in “that horse”. Then substance in general is further distinguished, by saying it is something A such that when something else B is said of it, both the naming and the “what-it-is” of B are said of the primary or secondary substance A. (See also Form; Things in Themselves; Definition.)
If a horse as such “is” a mammal of a certain description, then that horse must be a mammal of that description. If a mammal as such “is” warm-blooded, then that horse “is” warm-blooded.
These are neither factual nor ontological claims, but consequences of a rule of interpretation telling us what it means to say these kinds of things. Whether or not something is a substance in this sense is surely a key distinction, for it determines the validity or invalidity of a large class of inferences.
Based on the classification of A as an object reference and B as something said of A, we can make valid inferences about A from B.
When something else C is said of the non-substance B, by contrast, we still have a “naming” of B, but the “what-it-is” or substantive meaning of C does not apply to B itself, but only modifies it, because B is not an object reference. Applying the substantive meaning of C to B — i.e., making inferences about B from the meaning of C — would be invalid in this case.
Just because, say, warm-blooded as such “is” a quality, there is no valid inference that mammals “are” qualities, or that that horse “is” a quality. The concern here is with validity of a certain kind of inference and interpretation, not ontology (or epistemology, either).
In the Metaphysics, the initial referential notion of substance as something underlying is explicitly superseded through a far more elaborate development of “what it was to have been” a thing that emphasizes form, and ultimately actuality and potentiality. The appearance of what might be mistaken for a sort of referential foundationalism is removed. (See also Aristotelian Dialectic.)
I also think he wanted to suggest that practically, a kind of preliminary grasp of some actuality has to come first in understanding. Actuality is always concrete and particular, and said to be more primary. But potentiality too plays an irreducible role, in underwriting the relative persistence of something as the “same” something through change, which motivated the earlier talk about something underlying. The persistence of relatively stable identities of things depends on their counterfactual potentiality, which can only be apprehended in an inferential way. (See also Aristotelian Demonstration.)
It does make sense to say that things like actuality and substance inhere more in the individual than in the species, but that is due to the meanings of actuality and substance, not to an ontological status.