Common sense

Aristotle, the first person known to have discussed "common sense", described it as the ability with which animals (including humans) process sense-perceptions, memories and imagination (φρονεῖν, phroneîn) in order to reach many types of basic judgments. In his scheme, only humans have real reasoned thinking (νοεῖν, noeîn), which takes them beyond their common sense.

The origin of the term is in the works of Aristotle. The best-known case is De Anima Book III, chapter 1, especially at line 425a27.[9] The passage is about how the animal mind converts raw sense perceptions from the five specialized sense perceptions, into 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 specific (ἴδια, idia) to it. 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" (κοινή, koiné) is a term opposed to specific or particular (idia). The Greek for these common sensibles is tá koiná (τά κοινᾰ́), which means shared or common things, and examples include the oneness of each thing, with its specific shape and size and so on, and the change or movement of each thing.[10] Distinct combinations of these properties are common to all perceived things.[11]

In this passage, Aristotle explained that concerning these koiná (such as movement) people have a sense — a "common sense" or sense of the common things (aísthēsis koinḕ) — and there is no specific (idéā) sense perception for movement and other koiná, because then we would not perceive the koiná at all, except by accident (κᾰτᾰ́ σῠμβεβηκός, katá sumbebēkós). As examples of perceiving by accident Aristotle mentions using the specific sense perception vision on its own to try to see that something is sweet, or to try to recognize a friend only by their distinctive color. Lee (2011, p. 31) 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 necessarily 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.[12] 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.[13] The common sense is where this comparison happens, and this must occur by comparing impressions (or symbols or markers; σημεῖον, sēmeîon, 'sign, mark') of what the specialist senses have perceived.[14] 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.[15]

The discussion was apparently intended to improve upon the account of Aristotle's friend and teacher Plato in his Socratic dialogue, the Theaetetus.[16] But Plato's dialogue presented an argument that recognising koiná is an active thinking process in the rational part of the human soul, making the senses instruments of the thinking part of man. Plato's Socrates says this kind of thinking is not a kind of sense at all. Aristotle, trying to give a more general account of the souls of all animals, not just humans, moved the act of perception out of the rational thinking soul into this sensus communis, which is something like a sense, and something like thinking, but not rational.[17]

Avicenna became one of the greatest medieval authorities concerning Aristotelian common sense, both in Islamic and Christian lands.

The passage is difficult to interpret and there is little consensus about the details.[18] Gregorić (2007, pp. 204–205) has argued that this may be because Aristotle did not use the term as a standardized technical term at all. For example, in some passages in his works, Aristotle seems to use the term to refer to the individual sense perceptions simply being common to all people, or common to various types of animals. There is also difficulty with trying to determine whether the common sense is truly separable from the individual sense perceptions and from imagination, in anything other than a conceptual way as a capability. Aristotle never fully spells out the relationship between the common sense and the imaginative faculty (φᾰντᾰσῐ́ᾱ, phantasíā), although the two clearly work together in animals, and not only humans, for example in order to enable a perception of time. They may even be the same.[15][17] Despite hints by Aristotle himself that they were united, early commentators such as Alexander of Aphrodisias and Al-Farabi felt they were distinct, but later, Avicenna emphasized the link, influencing future authors including Christian philosophers.[19][20] Gregorić (2007, p. 205) argues that Aristotle used the term "common sense" both to discuss the individual senses when these act as a unity, which Gregorić calls "the perceptual capacity of the soul", or the higher level "sensory capacity of the soul" that represents the senses and the imagination working as a unity. According to Gregorić, there appears to have been a standardization of the term koinḕ aísthēsis as a term for the perceptual capacity (not the higher level sensory capacity), which occurred by the time of Alexander of Aphrodisias at the latest.[21]

Compared to Plato, Aristotle's understanding of the soul (psūkhḗ) has an extra level of complexity in the form of the noûs 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 (λόγος, lógos) as well as the active intellect. The noûs 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 (lógos) 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 noûs.[22] Plato, on the other hand was apparently willing to allow that animals could have some level of thought, meaning that he did not have to explain their sometimes complex behavior with a strict division between high-level perception processing and the human-like thinking such as being able to form opinions.[23] Gregorić additionally argues that Aristotle can be interpreted as using the verbs phroneîn and noeîn to distinguish two types of thinking or awareness, the first being found in animals and the second unique to humans and involving reason.[24] 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 species sensibilis used for perception and imagination in animals, and the species intelligibilis or apprehendable forms used in the human intellect or noûs.

Aristotle also occasionally called the koinḕ aísthēsis (or one version of it) the prôton aisthētikón (πρῶτον αἰσθητῐκόν, lit. ''first of the senses''). (According to Gregorić, this is specifically in contexts where it refers to the higher order common sense that includes imagination.) Later philosophers developing this line of thought, such as Themistius, Galen, and Al-Farabi, called it the ruler of the senses or ruling sense, apparently a metaphor developed from a section of Plato's Timaeus (70b).[20] Augustine and some of the Arab writers, also called it the "inner sense".[19] 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, vis cogitativa, and memory. Avicenna, followed by Robert Grosseteste, Albert the Great, and Roger Bacon, argued for five internal senses: the common sense, imagination, fantasy, vis aestimativa, and memory."[25] 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.[19] 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.[26]

Heller-Roazen (2008) writes that "In 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."[27] It was "one of the most successful and resilient of Aristotelian notions".[28]

Marcus Aurelius, emperor and Stoic philosopher, and an important influence upon the concept of "humanist" common sense

"Sensus communis" is the Latin translation of the Greek koinḕ aísthēsis, which came to be recovered by Medieval scholastics when discussing Aristotelian theories of perception. In the earlier Latin of the Roman empire, the term had taken a distinct ethical detour, developing new shades of meaning. These especially Roman meanings were apparently influenced by several Stoic Greek terms with the word koinḗ (κοινή, 'common, shared'); not only koinḕ aísthēsis, but also such terms as koinós noûs (κοινός νοῦς, 'common mind/thought/reason'), koinḗ énnoia (κοινή ἔννοιᾰ), and koinonoēmosúnē, all of which involve noûs—something, at least in Aristotle, that would not be present in "lower" animals.[29]

• Koinḗ énnoia is a term from Stoic philosophy, a Greek philosophy, influenced by Aristotle, and influential in Rome. This refers to shared notions, or common conceptions, that are either in-born or imprinted by the senses on to the soul. Unfortunately few true Stoic texts survive, and our understanding of their technical terminology is limited.[30]
• Koinós noûs is a term found in Epictetus (III.vi.8), a Stoic philosopher. C.S. Lewis (1967, p. 146) believed this to be close to a modern English meaning of "common sense", "the elementary mental outfit of the normal man", something like intelligence. He noted that sensus could be a translation of noûs, (for example in the Vulgate Bible), but he only found one clear case of a Latin text showing this apparent meaning, a text by Phaedrus the fable writer.
• Koinonoēmosúnē is found only in the work of the emperor Marcus Aurelius (Meditations I.16), also known as a Stoic. He uses the word on its own in a list of things he learned from his adopted father. Anthony Ashley Cooper, 3rd Earl of Shaftesbury felt it represented the Stoic Greek original, which gave the special Roman meaning of sensus communis, especially when used to refer to someone's public spirit. He explained the change of meaning as being due to the specific way that Stoics understood perception and intellect, saying that one should "consider withal how small the distinction was in that Philosophy, between the ὑπόληψις [conjecture], and the vulgar αἴσθησις [perception]; how generally Passion was by those Philosophers brought under the Head of Opinion".[31]

Another link between Latin communis sensus and Aristotle's Greek was in rhetoric, a subject that Aristotle was the first to systematize. In rhetoric, a prudent speaker must take account of opinions (δόξαι, dóxai) that are widely held.[32] Aristotle referred to such commonly held beliefs not as koinaí dóxai (κοιναί δόξαι, lit. ''common opinions''), which is a term he used for self-evident logical axioms, but with other terms such as éndóxa (ἔνδόξα).

In his Rhetoric for example Aristotle mentions "koinōn [...] tàs písteis" or "common beliefs", saying that "our proofs and arguments must rest on generally accepted principles, [...] when speaking of converse with the multitude".[33] In a similar passage in his own work on rhetoric, De Oratore, Cicero wrote that "in oratory the very cardinal sin is to depart from the language of everyday life and the usage approved by the sense of the community." The sense of the community is in this case one translation of "communis sensus" in the Latin of Cicero.[34][35]

Whether the Latin writers such as Cicero deliberately used this Aristotelian term in a new more peculiarly Roman way, probably also influenced by Greek Stoicism, therefore remains a subject of discussion. Schaeffer (1990, p. 112) has proposed for example that the Roman republic maintained a very "oral" culture whereas in Aristotle's time rhetoric had come under heavy criticism from philosophers such as Socrates. Peters Agnew (2008) argues, in agreement with Shaftesbury, that the concept developed from the Stoic concept of ethical virtue, influenced by Aristotle, but emphasizing the role of both the individual perception, and shared communal understanding. But in any case a complex of ideas attached itself to the term, to be almost forgotten in the Middle Ages, and eventually returning into ethical discussion in 18th-century Europe, after Descartes.

As with other meanings of common sense, for the Romans of the classical era "it designates a sensibility shared by all, from which one may deduce a number of fundamental judgments, that need not, or cannot, be questioned by rational reflection".[36] But even though Cicero did at least once use the term in a manuscript on Plato's Timaeus (concerning a primordial "sense, one and common for all [...] connected with nature"), he and other Roman authors did not normally use it as a technical term limited to discussion about sense perception, as Aristotle apparently had in De Anima, and as the Scholastics later would in the Middle Ages.[37] Instead of referring to all animal judgment, it was used to describe pre-rational, widely shared human beliefs, and therefore it was a near equivalent to the concept of humanitas. This was a term that could be used by Romans to imply not only human nature, but also humane conduct, good breeding, refined manners, and so on.[38] Apart from Cicero, Quintilian, Lucretius, Seneca, Horace and some of the most influential Roman authors influenced by Aristotle's rhetoric and philosophy used the Latin term "sensus communis" in a range of such ways.[39] As C. S. Lewis wrote:

Quintilian says it is better to send a boy to school than to have a private tutor for him at home; for if he is kept away from the herd (congressus) how will he ever learn that sensus which we call communis? (I, ii, 20). On the lowest level it means tact. In Horace the man who talks to you when you obviously don't want to talk lacks communis sensus.[40]

Compared to Aristotle and his strictest medieval followers, these Roman authors were not so strict about the boundary between animal-like common sense and specially human reasoning. As discussed above, Aristotle had attempted to make a clear distinction between, on the one hand, imagination and the sense perception which both use the sensible koiná, and which animals also have; and, on the other hand, noûs (intellect) and reason, which perceives another type of koiná, the intelligible forms, which (according to Aristotle) only humans have. In other words, these Romans allowed that people could have animal-like shared understandings of reality, not just in terms of memories of sense perceptions, but in terms of the way they would tend to explain things, and in the language they use.[41]

René Descartes' illustration of perception. Sensations from the senses travel to sensus communis, seated in the pineal gland inside the brain, and from there to the immaterial spirit.

One of the last notable philosophers to accept something like the Aristotelian "common sense" was Descartes in the 17th century, but he also undermined it. He described this inner faculty when writing in Latin in his Meditations on first philosophy.[42] The common sense is the link between the body and its senses, and the true human mind, which according to Descartes must be purely immaterial. Unlike Aristotle, who had placed it in the heart, by the time of Descartes this faculty was thought to be in the brain, and he located it in the pineal gland.[43] Descartes' judgement of this common sense was that it was enough to persuade the human consciousness of the existence of physical things, but often in a very indistinct way. To get a more distinct understanding of things, it is more important to be methodical and mathematical.[44] This line of thought was taken further, if not by Descartes himself then by those he influenced, until the concept of a faculty or organ of common sense was itself rejected.

René Descartes is generally credited with making obsolete the notion that there was an actual faculty within the human brain that functioned as a sensus communis. The French philosopher did not fully reject the idea of the inner senses, which he appropriated from the Scholastics. But he distanced himself from the Aristotelian conception of a common sense faculty, abandoning it entirely by the time of his Passions of the Soul (1649).[45]

Contemporaries such as Gassendi and Hobbes went beyond Descartes in some ways in their rejection of Aristotelianism, rejecting explanations involving anything other than matter and motion, including the distinction between the animal-like judgement of sense perception, a special separate common sense, and the human mind or noûs, which Descartes had retained from Aristotelianism.[46] In contrast to Descartes who "found it unacceptable to assume that sensory representations may enter the mental realm from without"...

According to Hobbes [...] man is no different from the other animals. [...] Hobbes' philosophy constituted a more profound rupture with Peripatetic thought. He accepted mental representations but [...] "All sense is fancy", as Hobbes famously put it, with the only exception of extension and motion.[47]

René Descartes is the source of the most common way of understanding the "common sense" as a widely spread type of judgement.

But Descartes used two different terms in his work, not only the Latin term "sensus communis", but also the French term bon sens, with which he opens his Discourse on Method. And this second concept survived better. This work was written in French, and does not directly discuss the Aristotelian technical theory of perception. Bon sens is the equivalent of modern English "common sense" or "good sense". As the Aristotelian meaning of the Latin term began to be forgotten after Descartes, his discussion of bon sens gave a new way of defining sensus communis in various European languages (including Latin, even though Descartes himself did not translate bon sens as sensus communis, but treated them as two separate things).[48]

Schaeffer (1990, p. 2) writes that "Descartes is the source of the most common meaning of common sense today: practical judgment". Gilson noted that Descartes actually gave bon sens two related meanings, first the basic and widely shared ability to judge true and false, which he also calls raison (lit. ''reason''); and second, wisdom, the perfected version of the first. The Latin term Descartes uses, bona mens (lit. ''good mind''), derives from the Stoic author Seneca who only used it in the second sense. Descartes was being original.[49]

The idea that now became influential, developed in both the Latin and French works of Descartes, though coming from different directions, is that common good sense (and indeed sense perception) is not reliable enough for the new Cartesian method of skeptical reasoning.[50] The Cartesian project to replace common good sense with clearly defined mathematical reasoning was aimed at certainty, and not mere probability. It was promoted further by people such as Hobbes, Spinoza, and others and continues to have important impacts on everyday life. In France, the Netherlands, Belgium, Spain and Italy, it was in its initial florescence associated with the administration of Catholic empires of the competing Bourbon, and Habsburg dynasties, both seeking to centralize their power in a modern way, responding to Machiavellianism and Protestantism as part of the so-called counter reformation.[51]

Cartesian theory offered a justification for innovative social change achieved through the courts and administration, an ability to adapt the law to changing social conditions by making the basis for legislation "rational" rather than "traditional".[52]

So after Descartes, critical attention turned from Aristotle and his theory of perception, and more towards Descartes' own treatment of common good sense, concerning which several 18th-century authors found help in Roman literature.

Immanuel Kant proposed that sensus communis (German: Gemeinsinn) was a useful concept for understanding aesthetics, but he was critical of the Scottish school's appeals to ordinary widely shared common sense (gesunden Verstand) as a basis of real knowledge.

Immanuel Kant developed a new variant of the idea of sensus communis, noting how having a sensitivity for what opinions are widely shared and comprehensible gives a sort of standard for judgment, and objective discussion, at least in the field of aesthetics and taste:

The common Understanding of men [gemeine Menschenverstand], which, as the mere sound (not yet cultivated) Understanding, we regard as the least to be expected from any one claiming the name of man, has therefore the doubtful honour of being given the name of common sense [Namen des Gemeinsinnes] (sensus communis); and in such a way that by the name common (not merely in our language, where the word actually has a double signification, but in many others) we understand vulgar, that which is everywhere met with, the possession of which indicates absolutely no merit or superiority. But under the sensus communis we must include the Idea of a communal sense [eines gemeinschaftlichen Sinnes], i.e. of a faculty of judgement, which in its reflection takes account (a priori) of the mode of representation of all other men in thought; in order as it were to compare its judgement with the collective Reason of humanity, and thus to escape the illusion arising from the private conditions that could be so easily taken for objective, which would injuriously affect the judgement.[73]

Kant saw this concept as answering a particular need in his system: "the question of why aesthetic judgments are valid: since aesthetic judgments are a perfectly normal function of the same faculties of cognition involved in ordinary cognition, they will have the same universal validity as such ordinary acts of cognition".[74]

But Kant's overall approach was very different from those of Hume or Vico. Like Descartes, he rejected appeals to uncertain sense perception and common sense (except in the very specific way he describes concerning aesthetics), or the prejudices of one's "Weltanschauung", and tried to give a new way to certainty through methodical logic, and an assumption of a type of a priori knowledge. He was also not in agreement with Reid and the Scottish school, who he criticized in his Prolegomena to Any Future Metaphysics as using "the magic wand of common sense", and not properly confronting the "metaphysical" problem defined by Hume, which Kant wanted to be solved scientifically—the problem of how to use reason to consider how one ought to act.

Kant used different words to refer to his aesthetic sensus communis, for which he used Latin or else German Gemeinsinn, and the more general English meaning which he associated with Reid and his followers, for which he used various terms such as gemeinen Menscheverstand, gesunden Verstand, or gemeinen Verstand.[75]

According to Gadamer, in contrast to the "wealth of meaning" brought from the Roman tradition into humanism, Kant "developed his moral philosophy in explicit opposition to the doctrine of 'moral feeling' that had been worked out in English philosophy". The moral imperative "cannot be based on feeling, not even if one does not mean an individual's feeling but common moral sensibility".[76] For Kant, the sensus communis only applied to taste, and the meaning of taste was also narrowed as it was no longer understood as any kind of knowledge.[77] Taste, for Kant, is universal only in that it results from "the free play of all our cognitive powers", and is communal only in that it "abstracts from all subjective, private conditions such as attractiveness and emotion".[78]

Kant himself did not see himself as a relativist, and was aiming to give knowledge a more solid basis, but as Richard J. Bernstein remarks, reviewing this same critique of Gadamer:

Once we begin to question whether there is a common faculty of taste (a sensus communis), we are easily led down the path to relativism. And this is what did happen after Kant—so much so that today it is extraordinarily difficult to retrieve any idea of taste or aesthetic judgment that is more than the expression of personal preferences. Ironically (given Kant's intentions), the same tendency has worked itself out with a vengeance with regards to all judgments of value, including moral judgments.[79]

Among Catholic theologians, writers such as theologian François Fénelon and philosopher Claude Buffier (1661–1737) gave an anti-Cartesian defense of common sense as a foundation for knowledge. Other Catholic theologians took up this approach, and attempts were made to combine this with more traditional Thomism, for example Jean-Marie de Lamennais. This was similar to the approach of Thomas Reid, who for example was a direct influence on Théodore Jouffroy. This meant basing knowledge upon something uncertain, and irrational. Matteo Liberatore, seeking an approach more consistent with Aristotle and Aquinas, equated this foundational common sense with the koinaí dóxai of Aristotle, that correspond to the communes conceptiones of Aquinas.[53] In the twentieth century, this debate is especially associated with Étienne Gilson and Reginald Garrigou-Lagrange.[89] Gilson pointed out that Liberatore's approach means categorizing such common beliefs as the existence of God or the immortality of the soul, under the same heading as (in Aristotle and Aquinas) such logical beliefs as that it is impossible for something to exist and not exist at the same time. This, according to Gilson, is going beyond the original meaning. Concerning Liberatore he wrote:

Endeavours of this sort always end in defeat. In order to confer a technical philosophical value upon the common sense of orators and moralists it is necessary either to accept Reid's common sense as a sort of unjustified and unjustifiable instinct, which will destroy Thomism, or to reduce it to the Thomist intellect and reason, which will result in its being suppressed as a specifically distinct faculty of knowledge. In short, there can be no middle ground between Reid and St. Thomas.[53]

Gilson argued that Thomism avoided the problem of having to decide between Cartesian innate certainties and Reid's uncertain common sense, and that "as soon as the problem of the existence of the external world was presented in terms of common sense, Cartesianism was accepted".[89]

• McCarthy's advice-taker proposal of 1958 represents an early proposal to use logic for representing common-sense knowledge in mathematical logic and using an automated theorem prover to derive answers to questions expressed in logical form. Compare Leibniz's calculus ratiocinator and characteristica universalis.
• The Cyc project attempts to provide a basis of common-sense knowledge for artificial-intelligence systems.
• The Open Mind Common Sense project resembles the Cyc project, except that it, like other on-line collaborative projects depends on the contributions of thousands of individuals across the World Wide Web.

• Appeal to tradition – Logical fallacy in which a thesis is deemed correct on the basis of tradition
• Basic belief – Axioms under the epistemological view called foundationalism
• Common knowledge – Statement widely known to be true
• Commonsense reasoning – Branch of artificial intelligence aiming to create AI systems with "common sense"
• Conventional wisdom – Concepts and theories generally accepted by experts
• Counterintuitive – Quality of being surprising and contrary to intuition
• Dunning–Kruger effect – Cognitive bias about one's own skill
• Pre-theoretic belief – Topic in linguistics and philosophy
• Public opinion – Aggregate of individual attitudes or beliefs held by the adult population
• Social norm – Informal understanding of acceptable conduct