Assumed that we use habitually abstract concepts and that these concepts are really something very important for the functioning of our mind, it is legitimate to ask what is their ontological reality. Plato argues that our knowledge comes only by reason and not by the senses. But what reason knows?
A brief synthesis of the content of the theory of knowledge would help, at least from the modern age. Then, it is a choice to start from the assertion that the theory of knowledge has been consolidated in that period by the philosophical speculations of Kant, who dealt with the analysis of the fundamentals, limits and the validity of knowledge, essentially understood as a relationship between knower and known object.
Thus I start from the consideration that naive realism be the theory that supports the position closer to common sense, that which every man adheres spontaneously before any philosophical reflection on knowledge: the bodies exist, are there out of us.
But then how we explain the epistemological problems that lead to idealism? In reality it is a false problem. When, for example, the transcendental idealists argue that the true reality of things exists independently of our intellect, they formulate expressions unclear: how do you know that there is something that can not be known with the intellect?
As it is well acknowledged, concepts can be derived from experience, or are ‘pure’, being 'a priori' in the intellect. Unlike Aristotle, for whom categories are principles of logical thinking and reality, the Kantian’s are modes of the intellect that play a transcendental ordering of phenomena; they precede any experience and at the same time have value and meaning only when applied to experience itself. The intellect uses them to unify the many sensitive data, ensuring the universality of knowledge.
Now, referring to the discussion of abstract concepts, in The Middle Ages it has been much debated the issue of the existence of general objects, known to history as problem of universals. The philosopher Frédéric Cossutta asserts that: “ it is precisely the concept which is the intermediary between the image and form, between the living and the abstract. Philosophy makes a variety of uses, but there is no philosophy that does not refer, if necessary, beyond its limits, to universalizing abstraction (see Leibniz's project of "universal characteristic"). But, if the abstraction is meaningless, or the picture and subjectivity extend outside of any form, philosophy decreed its own death.
We use frequently abstract concepts in our knowledge and these concepts are really something very important for the functioning of our mind. Then, it is legitimate to ask what is their ontological reality: how are there? On what plane of reality? In this regard, various positions have been developed:
Realism : The universal actually exists, but as an ideal object in a separate world from the real one. That there are horses we see and there are - on a different plane of reality - the ones we do not see (Platonic position). These horses have autonomous reality and ideals exist even if there were no horse perceived with the senses.
Conceptualism: the universal exists only in our minds, it is its simple abstraction created by grouping the most general features of many real horses. Unlike realists, conceptualists argue that the abstract idea of the horse would not arise in our minds if we had never seen real horses by whose observation we could derive the similarities that allow us to formulate the concept of abstract horse.
Nominalism: refusing the realistic position, according to which universals exist as ideal objects, nominalists approach the conceptualists, but go even further because they argue that it is not conceivable any form of abstraction (in fact I can not think of a horse in general, i.e. that has neither a certain color or a certain height, etc.) and, therefore, I conclude that the universal can not exist even as a concept.
After Kant, with the birth of German epistemology, ontology seemed to gain the upper hand, although in Fichte and Schelling these two disciplines remain still on a equal level, because the Idea from which they do arise can only be grasped with an intuitive act (similar to the Neoplatonist One).
It will be with Hegel that the ontology will be permanently absorbed by Gnoseology. Hegel in fact built a system that had the logical claim to be also ontological. The categories of knowledge, which in Kant were purely "formal", become together "form and content": they are logical-ontological categories. Hegel is therefore at the opposite of Parmenides and Plotinus: knowledge to him does not happen at the immediate and intuitive level, but is the result of a rational mediation, it is the consequence of a process by which reason comes to deduce all the reality from itself.
Only in the twentieth century Heidegger tried to restore the supremacy of ontology, claiming that ‘Being’ can never be reduced to an object, because it always transcends us. To presume to be able to deduce it rationally, giving it a predicate, was the fundamental error of Western metaphysics.