Philosophers deal with abstractions. Being reflective, they also have come up with theories about what these abstractions are. Aristotle is no exception, and indeed gave what came to be a canonical account of abstraction. Here I shall investigate what Aristotle thinks abstraction is. I shall conclude that Aristotle views abstraction as selective attention.
As its very name suggests, abstracting (ἀφαιρέω) consists in taking away something from an object. The root verb, αἱρέω, suggests additionally a sense of grasping or of choosing, of taking for oneself something of what lies ready to hand.
These lexical meanings leave open a wide range of conceptions of ‘abstraction’. Does the abstraction consist in taking out something and discarding the rest? Or does it consist in taking away something and keeping what is left? We can call the first one the selection view, and the latter the subtraction view. The Greek gives an ambiguity between the two because ‘ἀφαίρεσις’, being a verbal noun, could be derived from the active form ‘ἀφαιρεῖν’, which generally does have the sense of ‘removal’, or from the middle form ‘ἀφαιρείσθαι’, which generally has the sense of ‘take away for oneself’ or ‘steal’. On linguistic grounds of common usage, the selective reading of ‘ἀφαίρεσις’ has the advantage, as the middle voice forms are far more common than the active voice forms. Yet, as Aristotle is a philosopher, and philosophy stretches or distorts the ordinary usage of language, the philological evidence does not settle the issue. For that, we must turn to Aristotle’s texts.
Bäck, Allan, "The Concept of Abstraction" (2006). The Society for Ancient Greek Philosophy Newsletter. 376.