Abstraction is the process of limiting or reducing information, and while all abstraction is essentially the same process, it may be useful to delineate a couple of different types of abstraction.
We see (or somehow observe) some empirical event or phenomenon. We see a cat, for instance. The question is not, and never was, “How do we know it’s a cat?” The question is, “Why - for what purpose - do we call it a cat?”
Purpose is the operative concept here. “How do we know it’s a cat?” has caused much confusion, evidently. The question is fundamentally not about knowledge but about purpose. Asking the wrong question is ostensibly what led Plato astray and what leads people astray to this day.
We have a purpose when we abstract. The reason we abstract is to place an object or idea in a group - in a category. In a class. For a particular purpose. Taxonomy has a particular purpose.
It should already be obvious why an error in or about abstraction will lead to errors in logic and in philosophy in general, but probably it’s not obvious at all to many.
We abstract, perform abstractions for particular purposes, and in doing so retain only the information that suits that purpose. That purpose, in general, is to conceptually place the objects or ideas from which we create the abstraction in a particular group or class.
So “cat”, as a label for a class of objects, is useful to us. If we see an ad on Craigslist titled “cats for free”, we have enough of an idea about the contents of that ad to know if it’s worth our time to click on the ad and read it. This by no means limits the number or types of classes that these cats may belong to. Later, one of the cats may belong to the class “my pets” or “Dave’s pets”, for instance, and still belong to the class “cats”. And these class labels would be employed as they are useful, if at all. But the part to understand here is that there is nothing intrinsic to these cats that dictates what classes they belong to.
So the label “cat” is the device by which we group together individuals, by ignoring some of the information we have about them and including only that information that suits our present purpose, to form a conceptual group that we have some purpose in forming. In the end, that is all that abstraction is. But this means that an abstraction (here meaning the product of the process of abstraction) is conceptual only - it is not taken to be real - even if the object itself is. It’s the product of a purely mental process, and a product of language. Without a label, there is no class, no group and no abstraction. Abstraction is a feature of language.
Anyone not with me so far? There’s a lot more. Abstraction can be performed on not just things we see, but on other ideas as well. Some abstractions are of other concepts alone, and not of objects that we experience in the material world. More on that later.