In the nineteenth century the problem of the universal was considered as the central theme of medieval philosophy. One meets a typical medieval form of comparison between Platonism and Aristotelism, which paradoxically developed in the absence of Platonic texts. The problem arises with regard to the relationship between language and reality when, approaching the study of logic, there were questions about the universal terms (those indicating genus and species) starting from the text that was read throughout the Middle Ages at baseline logic as an introduction to the categories that are specifically supreme classes of every possible predicate, which can order the whole reality : Isagoge Porfirio translated by Boethius. L '"Isagoge" is a brief introduction to the "Category" of Aristotle, in which Porfirio, coded the doctrine of the five predicable (genus, species, difference, right, and accident), building a logical hierarchical structure (which is known as the "Tree of Porfirio").

The question of the relationship between language and reality in general had been placed by Aristotle in 'De interpretatione': where the philosopher says that "the sounds of voice are symbols of affections that take place in the soul, and these are the same for everyone," wants to understand that these 'illnesses' is a psychological 'imprint' of realities which exists outside of the soul, or consider their mental universality as an expression of the common psychic structure of men?

The grid issue on which develops the medieval debate on universals is all contained in these three questions linked: the possible answers are traditionally referred to as realism, nominalism and conceptualism.

For conceptualists, as opposed to the realist, the concept does not simply reflect the essence, or the notion of form, (universal in re, according to scholastic terminology) of the thing, but it is a construction of the human mind. As opposed to the nominalist, who claim that of the universal exists only the name, the conceptualists support the existence of universal concepts or general ideas in our minds: the universal is a mental content that exists post rem.

If for example, I think the dolphin, I express an idea to which I refer to a mammal, my mind a universal form that is different from what they had in mind the old thinking that the dolphin conceived it as a simple fish. So the universal, according to conceptualists is a training of the mind that exists and changes through experience.

Nominalism is the doctrine of philosophers called nominales, which represented one of the most important currents of Scholastica. The nominalist doctrine is positioned in the so-called problem of universals arguing that the concepts, terms of general and those in philosophy are called universal, not possess their own existence prior to or disconnected from things, and there are no outside or in things but they are understood to be only names. Nominalism has conventionally been divided into extreme, like that of Roscellinus, and in moderate as that of Abelard.

Nominalism is opposed to conceptualism and to philosophical realism, position which holds that the general used terms, such as "tree" and "green" are forms of general scope which exist in a world of abstractions independent from the world of objects physically defined. This position is particularly drawn to Plato.

Universals are abstract signs that can be predicated of concrete subjects, individuals that only them are real, while the universal concepts only exist post rem, such as verbal conventions associated with specific objects, or in the imagination or memory of those who speak.

Moderate Nominalism is the philosophical position of mold nominalist put forth by William of Ockham in the fourteenth century, which defined as the universal concepts of our mind, expressed through a name. Therefore, he argued, we must get rid of the universals (Ockham's Razor) as part of the knowledge as being unnecessary, leading to multiply indefinitely the search for truth

I would add what I believe to be a significant element. From the unitary concept of the individual and from the value of 'sign' recognized to the universal derive some consequences. First of all, in opposition to G. Duns Scotus, it is refused any distinction that is not the real one: the intellect, dividing, i.e. separating signs, follows the actual distinction between things. Moreover, it is rejected any ‘entification’ of relations, because, he says, in reality the only absolute things are substance and quality. The mental order and the vocal order are coordinated voice, but both refer to the real order meaning it. If the subject of human knowledge are, as taught by Ockam, propositions, it should be noted that they are composed of terms (mental, vocal and written) and that the terms are signs of things. The terms reveal the reality; diversity and convenience of things manifests itself, in fact, in the diversity and convenience between the terms.

There have been adverse reactions to conceptualism. McDowell, the most influential advocate of conceptualism, argued that the perception should be attributed conceptual content, because otherwise we could not guarantee the intentionality of thought to the world. Against this, the philosopher Luca Barlassina showed that conceptualism does not enjoy all of the epistemological advantages that McDowell attributes.

To this it must be added that conceptualism has proved very weak in psychological terms, because the thesis of the conceptual content of perception undergoes a series of objections that it seriously undermines the empirical plausibility. Therefore, by combining the psychological inadequacy of conceptualism and its epistemological uselessness, Barlassina and the current of thought to which he belongs conclude that, at least in the case of perceptual states, the perspective advanced by McDowell should be replaced by a non-conceptualist theory . On the other hand, having established the falsity of conceptualism does not mean to have identified which of the form of non-conceptualism that have been screened by Barlassina are up to the task. For this reason, there is still much work to do to be able to develop a satisfactory of non-conceptualist theory. Such work will require not only the contribution of philosophers, but also of psychologists, linguists and neuroscientists.