In scholastic psychology, the central sense (sensus communis ) is an internal organic power for acquiring sense knowledge, distinct from the external and other internal senses. Its function is to grasp all the stimuli known through the external senses, to compare them, unifying or distinguishing among them, and to know the very activity of the external senses, that is, to be conscious of sensation. Its organ is to be found in the sensory and psychosensory zones of the cerebral cortex, in the associative cortical zones, and in the long and short associative fibers that link these zones.
Necessity. At the close of his study on the external senses, Aristotle concluded that there was need for a superior function of sense knowledge to explain certain activities closely associated with external sensation, but not themselves reducible to the operations of the external senses. These activities are (1) consciousness of the operation of the external senses and (2) knowledge of the similarities and differences between the objects of the various external senses. Each sense knows only the sensibles that are proper to it. Sight is aware of color but not of sound, just as hearing is aware of sound but not of color. Again, sight is not audible and therefore is unknowable through hearing. Hearing is not colored and cannot be grasped by sight. No direct exchange is then possible between the external senses, either at the level of their proper object or of their specific activity. Moreover, no external sense can know its own sensation. In fact, not being colored, sight is not visible. The same is true of the other senses. For to know its own sensation, the sense would have to double itself in such a way that the sensory organ would become detached from itself as knowing subject to consider itself as object of its knowledge—an
impossible dissociation at the level of organic operative powers. Therefore these two kinds of operations—discerning among the sensible objects of the various senses and being conscious of sensation—require an organic cognitive power superior to the external senses, common to all, and central as regards their specific operation. This is the central sense.
Operation and Functions. The roles of the central sense in elaborating sense knowledge can best be discussed in terms of its various functions in sensation, in perception, and in consciousness.
Sensation. sensation here is understood as knowledge of the action exerted upon sensory receptors by a specific stimulus, whatever be the nature of the object causing the stimulation. According to St. thomas aquinas (In lib. de sensu 19.282–296) the central sense, as a superior power, uses the external senses as instruments to know sensible things. For it is from the central sense, as from a common source, that the power to sense (vis sentiendi ) diffuses itself into the external senses. This explains why the stimulations of all the senses converge and terminate at the central sense (In 3 de anim. 3.599–613). Therefore no expressed species is needed in the sensus communis (see species, intentional). The central sense collects the data of the various external senses in a global way. From this comes its aptitude for comparing the stimulations of the various senses, for grouping and synthesizing these, as well as for distinguishing and dissociating them.
To explain how the central sense operates in the distinction and synthesis of sensation, St. Thomas takes a rather obscure Aristotelian comparison and likens the central sense to a point toward which various lines converge (In 3 de anim. 3.599–613, 12.773). Just as the point can be considered indivisible in itself, or as multiple as the terminus of various lines, so it is with the central sense. To the extent that it integrates explicitly and actually the activity of each of the external senses, giving it special attention, the central sense can discern the likenesses and differences among sensations and among objects of the various external senses. To the extent that it operates at its own level, which surpasses that of the external senses (De ver. 15.1 ad 3; Summa Theologiae 1a, 57.2), it can synthesize the objects of the various senses and reconstitute the unity of the stimuli affecting the knowing subject.
Perception. Perception designates the identification by the knowing subject of the object that is the source of the stimulus affecting the sensory receptors to bring on sensation. The identification takes place with dependence upon the global cognitive and affective context of the subject, within which it acquires a specific meaning. It presupposes a holistic organization of all qualitative and quantitative data supplied by sensation. Perception thus implies the discovery of values in the object that the external senses cannot recognize, and permits the establishing of a functional contact between the object and the knowing subject. Given this unifying and discriminating function, the central sense plays an important role in elaborating the knowledge that makes perception possible. Some scholastics attribute this elaboration entirely to the central sense. It seems that St. Thomas makes it the task rather of all the internal senses (without excluding intellect), while recognizing the preponderant role of the cogitative power (particularly in the elaboration of the experimentum, with the concurrence of memory). For the cogitative power alone perceives the individual existing in its ineffable singularity and detects in it values that escape the external senses and the central sense (In 2 de anim. 13.396–398). Thanks to its organizing activity, the central sense prepares the sensory matter through which the superior powers (cogitative, then intellect) better perceive the object's profound reality. Since the sensory data centralized by the sensus communis in some way already reveal, as impressions produced by the accidents of the object, the nature of that object, there can be no doubt of the importance of this first organization of knowledge effected by the central sense.
Consciousness. The central sense initiates the conscious awareness of the whole object and begins perceiving the distinction existing between subject and object. This awareness, however, remains as frail and as limited as the sensation upon which it is based. In fact, the central sense grasps in sensible objects only the forms of energy (qualities affected with a certain quantity) that stimulate the sensory receptors, without perceiving the singular existent being as such, for this is the proper object of the cogitative power. It follows that the distinction recognized by the central sense between sensations and their specific content is again found at the accidental level, qualitatively and quantitatively, without reaching the level of the concrete substance, which is perceived only by the cogitative power. This occurs whether the substantial reality of the object or that of the subject, the ego, is concerned. The contribution of the central sense to the total consciousness of the subject is thus constituted by the perception of the distinction between, on the one hand, the continuing flux of sensations (as activities following each other, without implying the grasping of an underlying, subsisting subject) and, on the other hand, sensory impressions relating to the accidental properties of the objects affecting the senses. But without this first distinction, it would be impossible for the subject to arrive at a total consciousness of himself as a subsistent being distinct from every other existing being. This consciousness is deepened at the cogitative level (with which memory is associated) and at the level of intelligence. (see consciousness.)
Unconscious Knowledge. Although a partial function of consciousness, the central sense is an organic power subject to the limitations and imperfections inherent in every organic faculty. Thus a stimulation may remain below the threshold required to transmit sufficient disturbance to the organ of the central sense to make it aware of the stimulation. The stimulation will, however, have excited the external sense in which it leaves its trace to the point where a subsequent stimulation, even subliminal, may reactivate it, and, by summation, finally arrive at the threshold needed to awaken the central sense. On the other hand, the energy available for sensory perception, as for every vital operation, is necessarily limited; thus an increased expenditure of this energy on a given perception proportionately reduces the energy available for other purposes that play only a secondary role in consciousness (cf. De ver. 13.3). For example, while concentrating its attention on the work of a given sensation, the central sense is not able to give equal attention to another sensation. The latter escapes its vigilance, even though it is perceived confusedly. It may, in its turn, emerge at the level of consciousness if it becomes the object of special attention. Consequently, even though in principle the central sense can perceive all sensations, the beam of its clear and distinct attention cannot be simultaneously applied with equal effectiveness to all sensations. A goodly number thus remain at the edge of conscious perception. Further research on the organic structure of the central sense is needed before the phenomenon of unconscious sensations can be more fully explained.
Role in Sleep. To the degree that it implies a loss of consciousness, sleep requires a corresponding inhibition of the central sense and of all the senses whose thresholds increase as a result. If a sensory stimulation is strong enough, it can go beyond the threshold of sensation without reaching consciousness as a sensation, because the central sense is bound in sleep and does not perform its proper functions. Such a stimulation is eventually integrated in disguised fashion within some oneiric content of the imagination. Reducing the inhibition of sensitiveness can bring a corresponding freedom to the central sense. This allows the subject a certain discrimination between dream images and sensations brought on by stimuli coming from extrasubjective reality. However, as long as some inhibition persists, perception from the central sense remains proportionately handicapped. The subject still confuses dreams and reality, not clearly distinguishing between reality and its representation in the imagination (Summa Theologiae 1a, 84.8 ad 2).
See Also: sense knowledge; knowledge, process of.
Bibliography: m. stock, "Sense Consciousness according to St. Thomas," Thomist 21 (1958) 415–486. b. j. muller-thym, "Common Sense, Perfection of the Order of Pure Sensibility," Thomist 2 (1940) 315–343. e. j. ryan, The Role of the "Sensus Communis" in the Psychology of St. Thomas (Carthagena, Ohio 1951). m. de corte, "Notes exégétiques sur la théorie aristotélicienne du 'Sensus communis'," New Scholasticism 6 (1932) 187–214. e. barbado, "La conciencia sensitiva según S. Tomàs," Ciencia tomista 30 (1924) 169–203.
[a. m. perreault]