A couple of elementary ‘facts of life’ stimulate our reflection. The first is that “all humans want by their nature to know”, as Aristotle says at the beginning of his Metaphysics. The second is that, in order to understand what we ‘see’, we normally introduce something that we do not ‘see’. These two facts are testified by a basic evidence which we do not understand here in its reductive empiricist sense as a structure of sensible perceptions, but rather in the more pregnant sense of ‘phenomenological evidence’, by which we refer to the capability of immediate conscious apprehension of facts with which human beings are endowed. In other words, these facts are not ‘supported’ by the said phenomenological evidence, but are the content of that evidence. This is why they are certainly accepted by commonsense but cannot be qualified as ‘naive’ beliefs of common sense (with the implicit understanding that they are ‘uncritical’). This cannot avoid, however, that the meaning of the statements expressing the phenomenological evidence depends on the meaning of the terms occurring in these statements (i.e. on the sense of the concepts associated with the corresponding linguistic terms). In the case of the first ‘Aristotelian’ statement the concept whose sense has to be clarified is “to know”. This sense must be counted among the most primitive in any linguistic context since it denotes the specific action thanks to which a certain entity establishes a peculiar relation with the ‘world’, an action in which the entity which knows ‘identifies’ itself in a certain sense with the entity which is known, though remaining ontologically distinct from it. Or, to put it differently, an action through which an entity is capable of ‘interiorizing’ or ‘assimilating’ other entities without destroying them. (Therefore, this action is different, e.g., from feeding, in which a plant interiorizes and assimilates the substances of the soil by destroying their original compounds, or animals feed on plants or other animals by eating them and destroying them). The medieval philosophy had elaborated the concept of intentional identity to characterize that peculiar way of identification and interiorization that preserves ontological distinction, which is typical of knowing and is present only in certain animals and, at its highest level, in humans.
We shall use the term “cognition” to denote this sense of knowledge understood as an action, because in English “knowledge” is rather used to denote the result of such an action. In this most basic concept of cognition, which is present in common sense and ordinary language, we can easily recognize the idea of the ontological independence (i.e. the independence of existence) of the known entity from the action of the knowing entity. If we agree to call “reality” (for our present purpose) the target of the knowing action (cognition), we can recognize in this ontological independence of reality from cognition the situation defended by ontological realism. The negation of this thesis (i.e. ontological anti-realism) can succeed only by showing that the concept of knowing as used in ordinary language is inconsistent (a little like the concept of a square circle) because the alleged independence of reality from cognition does not obtain, is simply an arbitrary unconscious imagination and illusion: what is actually the case is that reality is posited by the knowing action. This is the thesis advocated by idealism, which is the direct negation of ontological realism (also called sometimes “metaphysical realism”). Leaving aside the conspicuous difficulties one finds in the cumbersome speculations of the idealist philosophers when they try to prove that reality is posited by thinking and is ultimately ontologically identical with thinking itself, a more modest but radical criticism can be leveled against the idealistic proposal: in order to show that the real relation between reality and thinking is that of an ontological dependence, one must consider this relation as something that thinking ‘considers’ as distinct from its own action, that is, that such consideration is not an action of self-consciousness. This amounts to saying that the idealist position, if taken seriously, is self-defeating since it requires for its defense that the relation thinking-reality be considered as a content in itself ‘about which’ we develop our ‘considerations’ (that are nothing but acts of thinking).