Some initial suggestions :
- Simo Knuuttila & Jaakko Hintikka (editors), THE LOGIC OF BEING : Historical Studies (1986), at least : BENSON MATES, Identity and Predication in Plato, page 29-on, and JAAKKO HINTIKKA, The Varieties of Being in Aristotle, page 81-on.
For Arsitotle's theory of substance and predication, see Aristotle's Categories :
given the above interpretation of the said-of and present-in relation, a primary substance is a particular that is non-accidental.
Thus, according to this interpretation, Socrates (the individual) is a substance.
Aristotle's categorialism is firmly anti-Platonic. Whereas Plato treated the abstract as more real than material particulars, in the Categories Aristotle takes material particulars as ontological bedrock — to the extent that being a primary substance makes something more real than anything else, entities such as Socrates and a horse are the most real entities in Aristotle's worldview [emphasis added].
Due to the Platonic background of many of A's thesis, see Mates' study, page 29 :
In recent times this situation has been analyzed on the basis of the assumption that the verb "to be" has at least two senses, viz., the predicative sense, as in "Socrates is human", and the identity sense, as in "Socrates is the husband of Xanthippe". Plato's critics castigate him for being unaware of the distinction, while his defenders believe that he was perfectly well aware of it and that the allegedly self-predicative statements are to be understood as assertions of identity.
And see Hintikka :
Almost all twentieth-century philosophers in English-speaking countries have
followed Frege and Russell and claimed that the words for being in natural languages - "is", "ist", etc. - are ambiguous between the is of predication, the is of existence, the is of identity, and the generic is.
For Aristotle discussion of "sameness", see Topics, Book I: 103a32, and see Beck [page 182] :
So he admits self-predication, in the sense of having the expressions contained in the subject and the predicate signify the same item in a category. [...] Aristotle says that in cases of sameness in number we uses two appellations of the same thing.
and Beck, [page 184] :
The aspect theory can handle identity claims: 'Socrates is Socrates' becomes
'Socrates exists as (identical to) Socrates'; 'this man is Socrates'
becomes 'this man exists as (the same as) Socrates'. Such statements are
harmless but not too informative. We can see why Aristotle would not
think that the identity relation has central importance in predication. [...] Aristotle allows predication at best only of expressions signifying singulars in accidental categories (taken abstractly). He does not seem to allow the predication of singulars in the category of substance. Still his remarks on numerical sameness to offer a way of analyzing such singular statements as 'the one drinking hemlock is Socrates' without predicating a singular term: not 'is Socrates'; rather take the predicate as either 'the same as Socrates' or, better, 'being Socrates' (sc., 'the essence of Socrates' — suggested by the predicative position).
For A, strictly speaking, a singular term (the name of an individual) cannot be predicated:
Of all the things which exist some are such that they cannot be predicated of
anything else truly and universally, e.g. Cleon and Callias, i.e. the individual
and sensible, but other things may be predicated of them (for each of these is
both man and animal).[An.Pr., Book I, 43a32-3]