Naive realism is 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, they are out of me. But then how do explain the epistemological problems that lead to idealism? In reality it is a false problem. When for example transcendental idealists argue that the true reality of things' exists independently of our intellect "formulate expressions unclear: how do you know that there is something that can not be known with the intellect?
Critical realism argues, instead, that bodies exist as a reality independent of the subject. It is called critical realism because unlike the previous one does not refuse to ask any epistemological problem. Admitted, in fact, that bodies exist, as claimed by the naive realism, it should be explained why are present in them some subjective elements (e.g. color). The response of critical realism is that the bodies have primary quality and secondary qualities: the first - the most important - are those that exist by itself; others - less important, called indeed secondary - are subjective and negligible. The image of the real world - for critical realists - is not given by our senses, but by what is out of the laborious analysis of physics.
Compared to what has been mentioned in the beginning on the form of knowledge next to common sense, there are forms of thought more appropriate to grasp the deeper layers, complex and elusive of reality and better suited to compete with the growing sense of the unspeakable and unthinkable. With their help we can go deeper: a) the person that tries, he will know and thinks; b) experience as analogic source rather than univocal of knowledge; c) knowledge as "organization" instead of "accumulation" of data; d) logic as an imperfect and relative tool; e) doubt and self-criticism as systematic components of scientific thought; f) the unavoidable incompleteness, partiality and provisional nature of any theory; g) the unsurpassed ‘partisanship’ and uncertainty of any knowledge.
For this deepening, the theory of knowledge is instrumental because it deals with the analysis of the fundamentals, limits and validity of human knowledge, essentially understood as a relationship between knower and known object. The survey through reason produces a pure knowledge, which is opposed to the unstable reality perceived by the senses. Aristotle agreed with Plato in the affirmation of the possibility of a scientific and true knowledge but it denied that it dealt with essences separated from the world of experience. According to Aristotle all knowledge comes directly from practical experience, through abstraction of the peculiar traits of a species, or indirectly, through the deduction of conclusions starting from premises already known according to the rules of logic.
Let us examine the main consequences of the epistemological theories on the metaphysical view of the world:
1) The first problem arising from the metaphysical theory of knowledge has to do with the theory of knowledge developed by Plato and concerns ideal objects.
Plato argues that our knowledge comes only by reason and not by the senses. But who knows what the reason is? According to Plato, it captures entities that are only in thought and are inaccessible both to the perception and the representation: it is ideas. An example of ideas or entities that can be grasped only conceptually is the following:
Beauty, goodness understood as concepts of beauty and goodness, and not as specific objects (objects beautiful or good deeds) that can be perceived with the senses.
From these reflections on knowledge, Plato drew a very particular vision of the world whereby only the world of ideas is the true reality, while the world of individual things is made from copies of ideas; the world of copies, which are gathered by the senses, is not the true reality.
The epistemological and metaphysical doctrine of Plato caused a big problem:
it is a challenge to 'common sense' and this has attracted a lot of criticism; many have found it difficult to accept. Given, in fact, that we frequently use abstract concepts to acquire knowledge and that relative concepts are really something very important for the functioning of our mind, one might wonder what is their ontological reality: how they exist? On what plane of reality? What consistency have these general ideas so frequently used?
In the Middle Ages was much debated the question of the existence of general objects, known to history as the problem of universals, because in Latin general objects were called "universal".
In this regard, various positions have been developed:
Realism: the universal actually exists, but as ideal object in a world separate from the real one;
Conceptualism: the universal exists only in our minds;
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 philosophy realism is the belief that there is a reality independent of our conceptual schemes, from our linguistic practices and beliefs. The philosophers who declare themselves realists typically think that truth consists in some form of correspondence of thoughts to reality. In a broader sense, typical of Scholastica, realists are those that give reality to entities of thought.
While acknowledging a practical utility to 'common sense', especially as a guide of judgment in the ethical-moral field (ability to apply the rules to particular cases), Kant devalues the role in terms of epistemological and cognitive point of view, in favor of the ‘pure’ use of intellect (faculty to determine the rules and concepts). The notion of 'common sense' is back however in the center of the reflection of Kant's Critique of Judgment, with specific reference to the founding of the judgments of taste; in these, in fact, the claim of a universality unbound by rules and concepts would pre-suppose the idea of a shared feeling, which in general makes it possible to communicate the experience.
The critical line initiated by Kant undergoes a sharp radicalization in the post-Kantian idealism, especially by Hegel, who interprets 'common sense' as a form of "immediate knowledge," in which expresses itself the attitude of ordinary consciousness , which "maintains its view that the truth rests on the sensitive reality, that thoughts are only thoughts [...] and that reason as it is in and of itself does not give out that dream."