The Sensus communis, or Common sense (Ancient Greek ), is the animal soul's proposed "inner sense" which perceives reality by monitoring and coordinating the five "external senses" (sight, hearing, taste, smell and touch).
The term Common Sense was first used this way by Aristotle, who was working through a problem which had previously been laid out by his teacher Plato, and explained instead by positing a level of conscious thought in animals. Aristotle's new explanation led to the creation of the new technical term "Common Sense", and a widely accepted hypothesis which lasted centuries in classical and medieval natural philosophy, and was translated and refined by Greek, Roman, Arabic and Christian authors.
While Aristotle originally believed that the Common Sense existed in the heart, it was later localized by Galen—together with imagination, cogitation, and memory—in the fluid-filled ventricles of the brain, a scheme known as the cell doctrine ().
Later philosophers developing Aristotle's line of thought, such as Themistius, Galen, and Al-Farabi, calling it the of the senses or sense, apparently a metaphor developed from a section of Plato's Timaeus (70b). Augustine and some of the Arab writers, also called it the "inner sense".
The concept of the inner senses, plural, was further developed in the Middle Ages. Under the influence of the great Persian philosophers Al-Farabi and Avicenna, several inner senses came to be listed. Thomas Aquinas and John of Jandun recognized four internal senses: the common sense, imagination, , and memory. Avicenna, followed by Robert Grosseteste, Albert the Great, and Roger Bacon, argued for five internal senses: the common sense, imagination, fantasy, , and memory." The "cogitative" or "estimative" capacity, , "enables the animal to extract vital information about its environment from the form processed by the common sense and imagination".
By the time of Descartes and Hobbes, in the 1600s, the inner senses had been standardized to five wits, which complemented the more well-known five "external" senses.. Under this medieval scheme the common sense was understood to be seated not in the heart, as Aristotle had thought, but in the anterior Galenic ventricle of the brain. The anatomist Andreas Vesalius found no connections between the anterior ventricle and the sensory nerves, leading to speculation about other parts of the brain into the 1600s, and Descartes..
Aristotelian philosophy
The origin of the term "common sense" (, ) is in the works of Aristotle, although the way he used the term had a very different meaning to the one most common in modern English. Although his model of perception is no longer commonly accepted, it has been called "one of the most successful and resilient of Aristotelian notions". In their own different ways, "the philosophers of medieval Latin and Arabic tradition, from Al-Farabi to Avicenna, Averroës, Albert, and Thomas, found in the De Anima and the Parva Naturalia the scattered elements of a coherent doctrine of the "central" faculty of the sensuous soul."
Plato as predecessor Aristotle's most well-known reference to the common sense addressed a problem posed by his older friend and teacher Plato in his Socratic dialogue, the Theaetetus, in which Plato already used the Greek term (, the "common" or "shared") to refer to what is common or shared in different sense perceptions—in the same way Aristotle later did. Plato described his own teacher Socrates arguing that knowledge can't be a result of sense perception on its own, because the soul () clearly doesn't use any sense organ to grasp what connects the different sensations it receives from the different sense organs. The senses on their own can't therefore account for the way in which the soul identifies real things, from diverse sense perceptions. The soul must therefore have its own ability to grasp the things which different sensations have in common——such as whether two particular sensory perceptions are connected to real things that are similar, identical, plural, or the same. Instead, Plato's Socrates convinces his interlocutor that "there is no special bodily organ for these things" and "the soul itself, through itself [] investigates the common features () of all things".Plato, Theaetetus, 185a–187a.
The in Plato and Aristotle are similar to what Robert Boyle and John Locke referred to as "primary qualities"., Scholars debate whether Plato saw the as being identical to his proposed "forms".
Aristotle however clearly distinguished the perceived by the common sense, from the forms or ideas seen by the intellect or ).See for example .
According to Gregorić, Plato's Socrates differs from Aristotle because he makes perception of these common things a kind of thinking, and not a kind of sense. Plato also indicated that it was not restricted to humans. In contrast, Aristotle, similarly trying to give a general account of the souls of all animals, not just humans, moved the act of perception out of the rational thinking soul, which he believed was unique to humans, into this , which is something like a sense, and something like thinking, but not rational.
Concerning the question of where the soul was in the body there had been a long debate among Greek physicians before the time of Plato and Aristotle, about whether this would be in the head or heart. Unlike Aristotle, Plato located it in the head.
Aristotle's explanation The only text of Aristotle which is certainly about the topic that later became an important medieval technical term is Aristotle's Book III, chapter 1, especially at line 425a27, which runs back over the argument of Plato's Socrates. Other places in the works of Aristotle where he used the same two words together include On Memory I.450a, Parts of Animals IV.10 686a, Metaphysics I.1 981b, and History of Animals I.3 489a, but he was not necessarily using them to refer to the same concept which became medieval Sensus communis.. This passage in De Anima is about how the animal mind converts raw sense perceptions from the five specialized "external" sense perceptions, into mental perceptions of real things moving and changing, which can be thought about. According to Aristotle's understanding of perception, each of the five senses perceives one type of "perceptible" or "sensible" which is not "common", but specific (, ) to itself. For example, sight can see colour. But Aristotle was explaining how the animal mind, not just the human mind, links and categorizes different tastes, colours, feelings, smells and sounds in order to perceive real things in terms of the "common sensibles" (or "common perceptibles"). In this discussion, "common" () is a term opposed to specific or particular ().
For Aristotle , the common or shared things, include not only the oneness of each thing, but also its specific shape and size and so on, and the change or movement of each thing. Aristotle lists change, shape, magnitude, number and unity, but he notes that we perceive shape, magnitude, and the rest by first being able to perceive change or movement (Greek uses one word for both: , ), and number is perceived by perceiving a lack of unity.De Anima 425a16, just before the famous mention of "common sense".
Like Plato, Aristotle explained that concerning these (such as movement) people have a sense within their soul, but he introduced a name for it— the "common sense" or sense of the common things (). He added detail to the argument of Plato's interlocutors, saying that there is no specific () sense perception for movement and other , because then we would not perceive the at all, except by accident (). Lee explains that "when I see Socrates, it is not insofar as he is Socrates that he is visible to my eye, but rather because he is coloured". So the normal five individual senses do sense the common perceptibles according to Aristotle (and Plato), but it is not something they can normally interpret correctly on their own. Aristotle proposes that the reason for having several senses is in fact that it increases the chances that we can distinguish and recognize things correctly, and not just occasionally or by accident.De Anima line 425a47, just after the famous mention of "common sense". Each sense is used to identify distinctions, such as sight identifying the difference between black and white, but, says Aristotle, all animals with perception must have "some one thing" that can distinguish black from sweet.De Anima column 427a. Plato, in his Theaetatus 185a–c uses the question of how to judge if sound or colour are salty. The common sense is where this comparison happens, and this must occur by comparing impressions (or symbols or markers; ) of what the specialist senses have perceived. The common sense is therefore also where a type of consciousness originates, "for it makes us aware of having sensations at all". And it receives physical picture imprints from the imaginative faculty, which are then memories that can be recollected.
Compared to Plato, Aristotle's understanding of the soul () has an extra level of complexity in the form of the or "intellect"—which is something only humans have and enables humans to perceive things differently from other animals. It works with images coming from the common sense and imagination, using reasoning () as well as the active intellect, both of which are unique to humans. The identifies the true forms of things, while the common sense identifies shared aspects of things. Though scholars have varying interpretations of the details, Aristotle's "common sense" was in any case not rational, in the sense that it implied no ability to explain the perception. Reason or rationality () exists only in man according to Aristotle, and yet some animals can perceive "common perceptibles" such as change and shape, and some even have imagination according to Aristotle. Animals with imagination come closest to having something like reasoning and .Posterior Analytics II.19. Plato, on the other hand was apparently willing to allow that animals could have some level of real thought, meaning that he did not have to explain their sometimes complex behavior within a strict division between high-level perception processing and the human-like thinking such as being able to form opinions.. Gregorić additionally argues that Aristotle can be interpreted as using the verbs and to distinguish two types of thinking or awareness, the first being found in animals and the second unique to humans and involving reason., Part II, chapter 3, which concerns a passage in De Partibus Animalium IV, but also refers to other passages in the corpus. See footnote 28. Therefore, in Aristotle (and the medieval Aristotelians) the universals used to identify and categorize things are divided into two. In medieval terminology these are the used for perception and imagination in animals, and the or apprehendable forms used in the human intellect or .
Variations in Aristotle
The passage in De Anima is difficult to interpret and there is little consensus about the details. Gregorić has argued that at least in this passage Aristotle did not use the term as a standardized technical term at all, and he believes that Aristotle used the term in several ways. Gregorić distinguishes four "notions": Aristotle sometimes uses the term to refer to individual sense perceptions, especially touch, simply because they are common in the sense of being widely shared, for example by most types of animals. citing Historia Animalium I.3 489a17, Nicomachean Ethics, III.10 118b, and Metaphysics I.1 981b14.