Sensation

SENSATION. - From sensus, sensatio, Gr. αἰσθησις, it is the original and fundamental manifestation of psychic life, through which the first form of the presence of things—of the world and of one’s own body—occurs in the consciousness of the living being. Beyond its biological significance as a means of preserving, developing, and defending life, in man it assumes a higher value as the initial process of knowledge that provides the matter and real contents of thought (cf. Sum. Theol., 2ᵃ-2ᵃᵉ, q. 67, a. 2, c.).

SUMMARY:

I. Sensation as a physio-psychic process

II. Classification of sensations: proper sensibles

III. Space, movement, time

IV. Sensation in the history of philosophy

V. sensation.

I. SENSATION AS A PHYSIO-PSYCHIC PROCESS

The condition of consciousness in the world is its being and manifesting itself in dependence on the body; the original moment in which this fundamental "situation" of consciousness is revealed is sensation, which presents itself as a fact of consciousness but with an intrinsic causal relationship to a corporeal fact (of the organism, and, through it, of the physical universe). Sensation thus presents two directions for investigation: a) the determination of its conditioning and its physio-psychic modalities (psychophysics and experimental psychology); b) the analysis of the state of consciousness as such and the meaning it assumes in relation to the tasks and ends of personal existence (gnoseology, pedagogy, ethics, etc.). As a "conscious reaction to an organic modification," sensation is a physio-psychic process, indeed the moment in which the transition between the physical and the psychic occurs with continuity of functions. The elements or conditions of this process are the organ, the stimulus, and the energy or psychic virtuality of the conscious subject.

The organ is, in a certain sense, the immediate corporeal subject of the faculty of sensing (αἰσθητήριον δὲ πρῶτον ἐν ᾧ ἡ τοιαύτη [αἰσθητική] δυνάμεις; Aristotle, De an., II, 12, 424 a 24). It is among the most differentiated structures of the living organism. It essentially consists of: a) a receptive apparatus (the peripheral nerve endings); b) the afferent nerve; c) the sensory center (a notion that today is believed to have not only anatomical but also physiological significance). Peripheral endings, nerve, and center constitute an anatomical-functional unity whose task is to receive the impression of the stimulus, react to it by elaborating it, and thus condition the conscious response. For the different senses, there is a constancy of nerve-central reaction expressed by J. Müller’s law (1801-58): a given organ always reacts to stimuli of different natures according to its specific sensation.

The stimulus is an action (and consequent modification) produced on the organ. This action may derive: a) from external reality, specifically from modifications of bodies that act either by contact of their mass (tactile sensations of resistance, pressure, etc.), or by chemical dissociation (sensations of taste and perhaps also of smell), or by transmission of vibratory movement (visual and acoustic sensations);

b) The stimulus-action may also derive from phenomena occurring within the organ itself (e.g., processes of dissolution or degeneration of tissues; currents of inter-cellular action-reaction in the centers, from which representative states reproduce elements or complexes of external sensations). Not every activity of the physical world is capable of acting as a sensory stimulus: electricity, radioactivity, magnetism do not act as specific stimuli on human sensibility. Moreover, even the specific stimulus (tactile, visual, auditory, etc.) is not capable of exciting the organ unless it reaches and surpasses minimum and maximum limits of intensity ("threshold" of the stimulus). The nature of the physiological process occurring within the nerve endings, the afferent nerve, and, even more, in the microstructure of the cerebral centers is obscure. It seems certain that the nervous current or wave of activity that travels through the sensory fibers is always accompanied by an electrical effect (cf. A. Edgar Douglas, The physical background of perception. Oxford-London 1952, Italian trans. Turin 1952, p. 24 ff.).

Psychic energy is the virtuality, proper to the living being, of reacting with a representation of consciousness to the peripheral-nerve-central modification. It is from this obscure virtuality—which is the first power of the being’s presence to itself—that sensation proceeds as knowledge. In its conclusive and formal moment, sensation is therefore the actualization and perfection of the sentient being as such, activity and spontaneity, formal and representative becoming of the subject, whose effective causality lies in the depth of life as a principle of immanence and consciousness (cf. Aristot., De an., II, 5, 416 b 33; St. Thomas, In De an., II, lect. 11-12, ed. Pirotta, n. 367 ff.).

In sensation, quality, quantity, and tone are to be distinguished. Quality is the proper representative content of sensation: color, sound, cold-hot, etc. (proper "sensibles"), movement, figure, form, size (common "sensibles"); cf. Aristotle, De an., II, 6 and St. Thomas, in hunc loc.). Qualities belong to sensation not as activity but as representation: "red," "cold" is not the sensing, the seeing (αἰσθησις), but the content or term of seeing and sensing (the "sensed," αἰσθητόν), whether or not such content has reality and consistency in itself independent of the representation. Quantity is the degree of intensity of a sensation; and it has its reason in the essential dependence of sensation itself on the stimulus, a physical and therefore quantitative fact. The relationship between the quantity of the stimulus and the intensity of sensation has been scientifically determined in the Fechner-Weber law: the quantitative variation of the stimulus necessary to determine a variation in the intensity of sensation is a constant ratio of the initial stimulus (in weight sensations, e.g., ca. 1/3). This law is also expressed in the formula: a geometric ratio of the stimulus corresponds to an arithmetic ratio of sensation. The only partial correspondence of the intensity of sensation to the quantitative variations of the stimulus is related to the practical purposes of the living being; but it also confirms the "approximate" value of sensation, which entails a certain "reduction," for biological ends, of the totality of the stimulus’s characteristics.

The "tone" is the affective element (of pleasure or displeasure) inherent, to varying degrees, in all sensations. Beyond the indicated fundamental properties, every sensation, as a process in time, has a duration, and spatial extension is essential to visual sensations.

II. CLASSIFICATION OF PROPER SENSIBLE SENSATIONS

The ancient classification, proceeding from an obvious consideration that takes into account the structural difference of peripheral organs and the qualitative difference of objects, distinguishes five kinds of sensations: tactile, gustatory, olfactory, auditory, and visual: «sensus esse quinque audire soleo, videndi, audiendi, olfaciendi, gustandi atque tangendi... Partitio ista antiquissima est et fere in contionibus celebrata» (St. Augustine, De quantitate animae, ch. 23, n. 41: PL 32, 1058; cf. Aristotle, De sensu et sensato, ch. 3 and De anima, II, 6, 418 a 7; Plato, Theaetetus, 156 b). However, Aristotle already observed that «the sense of touch contains within itself many different sensations» (Ἡ δ' ἄφη πλείους μὲν ἔχει διαφοράς: De anima, II, 6, 418 13; cf. II, 11, 422 b 17 and Sum. Theol., Iª, q. 78, a. 3 ad 3).

The modern classification, based on the triple criterion of the formal difference of qualities, the anatomical-functional diversity of the organ, and the specific difference of the physical stimulus, distinguishes the following principal forms of sensations: contact-pressure, temperature, weight, position, movement, pain; organic sensations; sensations of taste, smell; auditory and visual sensations.

1. Pressure-resistance sensations

The stimulus is a mechanical action of bodies on particular nerve endings in the skin and subcutaneous tissue, with an effect, it seems, of modifications in the osmotic state of the cells. The sensation is generally qualified according to the form and physical conditions of the object (sensations of hardness, roughness, smoothness, etc.). Because it provides the experience of an obstacle and resistance to one's own action, the pressure-resistance sensation contributes to founding the conviction of the existence of the external world. 2) Temperature sensations. These correspond to specific nerve endings (Pacinian and Meissner's corpuscles); the stimulus consists of molecular vibrations of the surrounding (air) or touching body. Temperature sensations have a subjective character: in a sensation of cold, heat, etc., the objectification of the stimulus is therefore null; where it intervenes (e.g., in the sensation of cold upon contact), it is due to concomitant tactile sensations. 3) Weight sensations (baresthesia). These are mixed sensations of contact and pressure, but they also possess a specific element; they are mostly associated with muscular and effort sensations. 4) Position sensations (bathesthesia). These report the position of the parts of one's own body. They probably originate in the joints but are contributed to by the general tone of innervation and principally by visuo-tactile representative associations. 5) Movement sensations. Also called kinetic sensations, these report the passive and active movement (ὁλοντικὴ) of one's own body and its limbs. They are principally of articular origin but also seem to be due to modifications occurring, following motor contraction, in muscles, tendons, and other dermal and endodermal tissues. They contribute to the clear perception of one's own body as a source of activity and thus to the fundamental sense of life. 6) Pain sensations (acute, dull, burning, punctate, continuous, etc.). These are caused by more or less profound alterations in tissues by mechanical, chemical, electrical agents, etc. The nerve endings are specific, distinct from those responding to pressure and temperature sensations. Pain sensations have a more or less unpleasant affective tone (when of minimal intensity, they may descend to a neutral affective level and even be slightly pleasant); they also have a teleological significance as signals of dangers and offenses to the integrity of the organism and to existence itself. Together with contact sensations, pain sensations are therefore among the most necessary for life («the sense of touch is the fundamental sense found in all animals... and of all it is the most necessary»: Aristotle, De anima, II, 2, 413 b 4 ff.; cf. St. Thomas, De veritate, q. 22, a. 5). 7) Organic sensations. These manifest, with a rich variety of forms, the functions and conditions, both normal and pathological, of the organism (sensations of hunger, thirst, well-being, fatigue, tension, relaxation, sexual sensations, etc.). The organ consists of nerve endings diffused throughout the body; the stimulus consists of physiological and microenergetic modifications in organs, tissues, and cells, determined by biological metabolism. Qualitatively simple, from a physiological standpoint they are complex sensations, difficult to analyze: they involve sensations of pressure, contact, temperature, muscular sensations, etc.; but their affective tone is specific and original. They condition the «behavior» of the animal and, in humans, also partly the higher affective states. Together with pain sensations, they serve as a phenomenological basis for the concepts of happiness, good, evil, etc. 8) Under the name of

Coenesthesia (κοινή αἰσθησις, common sensation) is the term modern psychology uses to denote the immediate awareness (which may also be understood as the basic and undifferentiated form of organic sensations) that an animal has of its own body and general state: an indistinct, somewhat obscure sensation, which can be described as a vague and diffuse internal tactile sensation, with a slightly but positively pleasant tone. Through it, the subject or the biological entity has its first and concrete self-presence, which other specific sensations have the task of intensifying and differentiating; coenesthesia thus stands as the experiential foundation of the very concept of existence and life.

9) Taste sensations. The organ consists of particular nerve endings on the tongue (fungiform papillae) and palate; the stimulus is chemically dissolved bodily substances. Already St. Thomas noted the affinity between taste and smell: «Odor enim et sapor quandam affinitatem habent» (In De sensu et de sensato, lect. 2, n. 23; cf. lect. 9, n. 118: «utrumque eorum est fere eadem passio»). Taste sensations are subjective affections: they therefore present no "external" objectification nor spatial determination.

10) Olfactory sensations. The organ consists of the nerve endings of the pituitary membrane; the stimulus is particles of bodies in a gaseous state, introduced through respiration. In some animal species (bees, etc.), olfactory sensations have a prevalence of meaning over all others. In humans, they are secondary («sensum olfactus peiorem habemus et per comparationem ad cetera animalia et per comparationem ad ceteros sensus qui in nobis sunt»: St. Thomas, ibid., lect. 9, n. 119). According to Pradines, taste and smell are "senses of need" insofar as «nous font connaître les choses comme répondant à nos besoins», that is, to the impulses that periodically drive us toward the search and appropriation of external conditions necessary for the conservation and perpetuation of our existence (Traité de psychol., I, 3rd ed., Paris 1948, p. 498). However, it must be admitted that such finality is common, though with different expressions, to all sensations (cf. St. Thomas, In De s. et de s., lect. 2, n. 24).

11) Auditory sensations. They derive from excitations produced by the vibrations of bodies, transmitted through the air or another elastic medium, on the nerve endings (Corti’s fibers) of the cochlear nerve. Homogeneous, regular vibrations produce clear, distinct "sounds"; irregular, mixed vibrations produce "noises," "clatters." Auditory sensations have a particularly synthetic character: elementary sounds (e.g., of the sea, wind) succeed in merging into a single vast auditory sensation. In relation to time, individual sounds, due to a certain "auditory memory," persist as sensations beyond their physical reality—a fact of extreme importance for language (v. LINGUAGGIO), which would be impossible if phonetic elements (syllables and words) did not continue within the representative unity of the sentence and period. Measure, rhythm, and proportion have furthermore conferred particular aesthetic value upon auditory sensations (music).

12) Visual sensations. They are determined by luminous vibrations that excite, with a probably electric-vibratory effect, the nerve endings (cones and rods) of the retina. Individual wavelengths produce elementary "colors"; the combination of wavelengths in the spectrum produces "white, light"; "black," which psychically has a positive value like every other chromatic sensation (the stimulus is either the limit of a chromatic sensation or an internal excitation of the nerve), physically corresponds only to the absence of vibrations. Particularly in visual sensations, the following should be noted:
a) Fusion phenomena: vibratory stimuli corresponding to different colors, if they simultaneously excite the retina, determine a psychically simple response.
b) Contrast phenomena: visual sensations close in space or time influence and modify each other.
c) Consecutive images: an intense visual sensation persists after the physical stimulus has ceased (the phenomenon is, to varying degrees, general for all sensations and is to be referred to the persistence of nervous excitation).
d) Binocular vision with the effect of a single image was already attributed by St. Thomas (In De s. et de s., lect. 19, n. 284) to the central unity of the organ.
e) All visual sensations are spatial. Due to their immense synthetic capacity (the visual angle can embrace an infinity of objects), visual sensations play a principal role, in sighted individuals, in the manifestation of the external world.

With the clear representation of distance (v. below), they stand in a certain way as an amplification in space of the tactile perceptual sphere. They thus constitute, from an objective standpoint, the highest perfection of sensibility, and visual terms and images have been adopted to indicate the activity itself and the values of thought (vision, idea, evidence, intuition, etc.).

The individual external sensations, due to their objective-functional discrimination, would not allow the subject that organic and unitary representation of objects necessary for effective biological behavior in the physical environment of existence. External sensations therefore continue into internal sensibility, which has the function of unification, representation, evocation with reference to time, and evaluation of the contents of external sensations (common sense, imagination, memory, reminiscence, estimative, cogitative; V. these entries, and cf. the lucid exposition in Sum. Theol., Iª, q. 78, a. 4).

III. SPACE, MOVEMENT, TIME

Space (figure, magnitude, form) and movement (rest), which Aristotelian-Thomistic psychology qualifies as "common sensibles" since they are the object of multiple senses, constitute a further level of objective determinations of sensibility.

2. Space

Empirical theories (Berkeley, Spencer, Taine, Lotze, Wundt) consider space a "secondary" datum, that is, derived from a constructive process of perception; nativist theories (Aristotle, St. Thomas, much of modern psychology) affirm, within certain limits, the "original" (native) character of spatial representation. Nativism should not be confused with "apriorism" (Kant), which understands space as a subjective form conditioning experience a priori. It seems difficult to deny the original character of spatial representation. The very empiricist explanation that derives space from the constructive movement of the organ does not, in reality, oppose nativism, since the movement of the eye along the points delimiting the object presupposes an extension that determines, as figure or form, but does not create as pure extension (furthermore, the extensive research of Gestalt- and Ganzheitpsychologie has shown that the same formal structures in many cases are given as global and unitary stimuli, so that even the visual apprehension of "form" often has an original character). The physiological antecedent of visual spatial sensation is the extended, spatially configured excitation of the stimulus on the retina. That the discontinuous extended, which material reality perhaps is, is represented by consciousness as continuous, is in accordance with the law of the "threshold" (already known and discussed by Aristotle and St. Thomas: cf. De sensu et sensato, ch. 6), whereby stimuli whose spatial distance is below the minimum distance discernible by the physiopsychic structure of the sense act as a single stimulus and thus cause a continuous visual representation (the continuous, therefore, as given in macroscopic experience, can be referred to the synthetic function of sensibility). The constructive activity of visual sensation is more fully expressed in the perception of the third dimension. Distance and depth do not seem to be the object of elementary sensations (a luminous point in darkness, without reference terms, cannot be evaluated in its distance), but are rather due to a perceptual elaboration (involving: binocular parallax; the different variation of the visual angle for near and far objects when the eye moves in their direction; the reference of the apparent size of an object to its known real size; the movement necessary to approach the object, etc.). Tactile spatial perception seems to be of an essentially genetic-motor nature. Pure contact sensation, not integrated by visuo-tactile representations (fantasy and memory generally intervene in the spatial construction of objects), does not provide, at least in distinct form, the third dimension and the form of objects: both are "constructed" through the representation produced by the tactile organ (hand) as it traces the surface delimiting the object: such is, as is well known, the spatial representation in the blind.

2. Movement. - Movement, as a change of position of external objects in space, is a modality of visuo-tactile stimulus and thus an original datum of sensation. But since, as pure variation of position, motion is relative and reversible, mere original visual sensation does not in itself refer to the real movement of an object. Even in sensations of movement, a physio-psychic constructive process may intervene: an immobile stimulus acting on the retina from two nearby positions in succession at an interval of time below the differential threshold is seen not as two images but as one, moving from the first to the second position (cinematography and stroboscopic apparent movement); in this case, movement is a pure perceptual datum. The tactile sensation of movement is primarily active, that is, dependent on the real movement of the organ (hand) over the object. More obscure—unless the habitual integration of internal representative sensibility intervenes—is the tactile sensation of passive movement (of the object over the organ).

3. Time. - Phenomenologically, time is the duration of a phenomenon (object, sensation, datum, representation). The "before" and the "after," the "beginning," the "lasting," the "ending" are essential modalities of experience, and therefore, in concrete terms, time is given with experience itself: "tempus non sentitur quasi aliqua res permanens proposita sensui, sicut videtur color, magnitudo, sed propter hoc sentitur quod sentitur aliquid quod est in tempore" (St. Thomas, In De sensu et sensato, lect. 18, n. 271). In the concrete content of perception, through an abstractive intuition of the spiritual consciousness, the real character of "duration" and "succession" is grasped, which thus come to be posited as universal representations. In its formality, time is therefore not an object of sensation, but as real duration, it is an objective character of all sensations. The Kantian doctrine of time and space as a priori forms of sensibility is therefore inexact. Time and space, as contents or concrete values, belong to the objects of real experience (time, moreover, to experience itself as successive psychic becoming in consciousness). The a priori of sensibility in relation to space and time is not a logical function, but only a constructive and organizational one; and transcendence for the sense is to be understood as the perceptive-elaborative relationship of physiopsychic structures to the real stimulus.

IV. SENSATION IN THE HISTORY OF PHILOSOPHY

The problem of sensation, its value and meaning, is fundamental in almost all philosophical currents. The Presocratics, who generally understood sensation in a passive and materialistic sense, sensism (v.), that is, the ultimate reduction of all knowledge to sensation. For Empedocles, sensible qualities are physical emanations of things: "colour is an influx of figures, proportional to the eye and perceptible" (Men., 76, d; cf. Aristot., De sensu et sensibilibus, 2, 438 a 4, and De anima, I, 2). Similarly, Democritus (v.), sensing means passively receiving corporeal images (the εἰδωλὰ that enter the organ: cf. Diog. Laert., IX, 44). But already in Protagoras, sensation is understood in an active sense: colour, heat, hardness, etc., "are nothing in themselves (καθ'αὐτὸ μεδὲν) but arise from a mutual agreement (of the object and the sense) and universally from movement" (Theaet., 157 a ff.; cf. 154 a, 156 b ff.). In Plato, sensation is an affection of the body that presents itself to the soul and reminiscence (v.) in the soul: "What is hidden from the soul (the affections of the body) you call insensitivity...; but when body and soul undergo the same affection (ἐν ἐνὶ πάθει) and are moved together, you rightly call this sensation" (Phil., 33 e 34 a; cf. 30 a). In Aristotle, the problem of sensation is developed with a wealth of analysis and observations. He overcomes the materialistic naturalism of the Presocratics with the concept of sensation as "the reception of forms or sensible images without matter" (De anima, II, 12, 424 a 18), a formula that seems to correspond to the expression in medieval terminology: "recipere intentionaliter, obiective." For Aristotle, sensation is, to some extent, knowledge: ἡ δ'αἴδησις γνῶσις τις (De generatione animalium, I, 23, 731 a 43); if it is to be called alteration, "since it is evident that sensation occurs through the body" (De sensu et sensibilibus, 1, 436 b 6), it is nevertheless a "special alteration" ἀλλοίωσις τις; ἡ ἕτερον γένος ἀλλοίωσως (De anima, II, 5, 416 b 34; 417 b 7); and while it involves a passive moment (πάσχειν, κινεῖσθαι: ibid., 416 b 34), it is formally an immanent and perfective act of the sensing being (εἰς αὐτὸ ἐπίδοσις καὶ εἰς ἐντελέχτων: ibid., 417 b 6). As for its biological significance, sensation is ordered to the preservation and defense of the animal (σωτηρίας ἕνεχνε: De sensu et sensibilibus, 1, 436 b 20), but in man, sensible knowledge is the objective condition of thought: "the intellect does not know external things except through sensation" (ibid., 6, 445 b 16-17; cf. De anima, III, 8, 432 a 8: a fundamental anti-Platonic motif in Aristotelian gnoseology). Stoicism, and even more so Epicureanism, resolve all spiritual activity into sensation, and thus posit sensation itself as the norm and criterion of truth (cf. Diog. Laert., VII, 4 ff., and X, 31: "Epicurus posits as the criterion of truth sensations, anticipations, affections"). Plotinus reaffirms sensation as an internal activity of the soul: "we understand sensations not as something we undergo (πάθη) but as activities concerning sensible impressions and as judgment" (Enn., III, 6, 1, 26). Augustine, following Plato, defines sensation as "passio corporis per seipsam non latens animam" (De quantitate animae, cap. 25, n. 48: PL 32, 1063; cf. cap. 23, n. 41; cap. 30, n. 58), that is, an affection of the body that immediately presents itself to consciousness: "Intendit se anima in tactum et eo calida, frigida, aspera, lenia, dura, mollia, levia, gravi sentit atque discernit" (ibid., cap. 33, n. 71: PL 32, 1074. Cf. De genesi ad litteram, XII, 24: PL 34, 475, where Augustine seems to attribute sensation only to the soul: "Neque enim corpus sentit, sed anima per corpus, quo velut nuntio utitur ad formandum in seipsa quod extrinsecus nuntiatur"; cf. Sum. Theol., 1ᵃ, q. 84, a. 6). The assertion "aliud est sentire, aliud cognoscere" (De quantitate animae, cap. 24, n. 45: PL 32, 1060) does not mean, in context, that sensation is not knowledge, but rather that there can be knowledge that is not sensation: sensation is true in itself, insofar as it is capable of grasping reality, but it does not possess that science "quam volunt intelligentia contineri remotamque a sensibus in mente vivere" (Contra academicos, III, cap. 11, n. 26; cf. De vera religione, cap. 33 and 36). The doctrine of St. Thomas on sensation is a deepening of Aristotelianism. Sensation is the first degree of knowledge, intrinsically conditioned by the bodily organ: "anima nihil sentit sine corpore, quia actio sentiendi non potest procedere ab anima nisi per organum corporale." As an act of a bodily organ, sense is a passive potency: "quae nata est immutari ab exteriori sensibili" (ibid., q. 78, a. 3 c.): an immutation that is not only material-physical but also spiritual-intentional: "ad operationem autem sensus requiritur immutatio spiritualis per quam intentio formae sensibilis fiat in organo sensus" (ibid.; cf. In De anima, II, lect. 2ᵃ, ed. Pirotta, n. 553). This "intentional" immutation (the "species impressa" of traditional terminology) is the stimulus itself insofar as it is an objective determination and formal principle of representation (cf. In De anima, loc. cit., where St. Thomas develops the Aristotelian concept of sensation as "receptio formae sine materia"). Sensation is not, however, a purely passive moment: "sentire, quantum ad ipsam receptionem speciei sensibilis nominat passionem; sed quantum ad actum consequentem ipsum sensum perfectum per speciem, nominat operationem" (In I Sent., dist. 40, q. 1, a. 1). The term of sensitive representation can only be a "form" individuated in time and space: "obiectum cuiuslibet sensitivae potentiae est forma prout in materia corporali existens" (Sum. Theol., 1ᵃ, q. 85, a. 1, c.; cf. ibid., q. 75, a. 6, c.); "sensus autem non cognoscit esse nisi sub hic et nunc" (ibid., 1ᵃ-2ᵃ, q. 15, a. 1). Therefore, neither the universal nor truth in the formal sense is to be sought in sensation (ibid., 1ᵃ, q. 17, a. 2, c.; cf. De veritate, q. 1, a. 11, c.). Nevertheless, the value of sensation for human knowledge is fundamental, for through it the presence of reality to consciousness is realized, and in sensible representations—which attain their highest richness and objective consistency in internal sensibility, particularly

Sensation (sensatio, from sensus; Gr. αἰσθησις) is the original manifestation through which reality is present to consciousness. In the representations of the senses—which attain their highest richness and objective consistency in internal sensibility, particularly in the cogitativa—the intellect finds the concrete content for the intuition and abstraction of universal values (cf. especially, on this fundamental point of Thomistic gnoseology, Sum. Theol., I, q. 84, a. 6-7, q. 85, a. 1; V. "CONOSCENZA: INDUZIONE"). During the Renaissance, sensation holds particular CAMPANELLO (v.). Feeling (which is not mere "passion" but an act of judgment: Metaph., I, Paris 1638, p. 42) is concrete and immediate knowledge, thus the foundation of true philosophy: « sentire est praesentis obiecti notitia » (Theol., X, cap. 3, a. 5); « duce sensu philosophandum esse existimamus. Eius enim cognitio omnium certissima est, quia fit obiecto praesente » (Prodromus, Frankfurt 1617, p. 27). In modern philosophy, Descartes, having reduced the essence of the subject to pure "thinking thing" (Med., VI), understands sensation as a "passive faculty of... receiving and recognizing sensible ideas" (ibid.). Organic sensations "ne sont autre chose que de certaines façons confuses de penser," dependent on the union and "mixture" of the spirit with the body, and to the sensations of the external world correspond in bodies certain properties, though dissimilar from the sensations themselves (ibid.; cf. Passiones an., I, 23, and Princ. philos., IV, 189). For Spinoza, "the ideas we have of external bodies indicate rather the constitution of our body than the nature of external bodies" (Eth., II, prop. 16, coroll. 2). According to Malebranche, sensation is « une modification de notre âme par rapport à ce qui se passe dans le corps auquel elle est unie ». Sensations, therefore, « ne sont que dans l'âme » (De la recherche de la vérité, I, cap. 13, §§ 1-2). In empiricism (v.), sensation is the primary source of knowledge, and its contents acquire the principal value of reality. For Locke, "the first capacity of human understanding consists in the soul's aptitude to receive the impressions made upon it either by external objects through the senses or by its own operations... Upon this foundation rest all our notions" (Essay, II, cap. 1, § 24). Berkeley, denying the existence of matter as a thing-in-itself, understands sensations as representations or "ideas" produced in our spirit by God Himself. Material things resolve into sensations: "light and colours, heat and cold, extension and figures—in a word, the things we see and feel—are nothing but so many sensations, notions, ideas, or impressions of the senses" (Principles, § 5). For Hume, sensations are the original perceptions of the mind, that is, impressions (Treatise, I, part 1, sect. 1, § 1). Sensations, together with "ideas," which are their residue in the mind, are ultimately the only thing of which we have absolute certainty ("nothing is really present to the mind but its own perceptions": ibid., part 4, sect. 2, § 5). The senses "do not present their impressions as images of something distinct or independent or external" (ibid., § 3), and "it will always be impossible to decide with certainty whether [impressions] arise immediately from the object, or are produced by the creative power of the mind, or are derived from the Author of our being" (ibid., part 3, sect. 5, § 1). In Kantian criticism, sensibility is "the capacity to receive representations through the way in which we are affected by objects," and sensation is "the action of an object upon the representative faculty, insofar as we are affected by it" (Kritik d. V. Vernunft, I, part 1, § 1). The representation of what is given in sensation is called by Kant empirical "intuition": it presupposes a formal element (space, which is a priori in the mind): for external sense, space; for internal sense (and for all sensation as subjective experience), time. Space and time are thus pure "intuitions," transcendental conditions for the possibility of representing objects in sensation (ibid., § 2 seq.). With Kantian doctrine, the creative activity of the subject is affirmed in relation to reality as the object of our experience (phenomenon).

In Hegelian idealism, sensation denotes the first determination of the spirit, still immediate and without development according to the antithesis of an objective against the subject. It is "the form of the dull agitation of the spirit in its unconscious and unintelligent individuality" (Encycl., § 400). Everything that occurs in spiritual consciousness and reason—Hegel asserts—has in some way "its source and origin in sensation," but only in the sense that sensation is the first and most immediate manner in which the contents of the spirit appear. Through sensation, the "I" of consciousness (which belongs to being-for-itself) is determined as spiritual corporeality; thus, feeling "is the healthy coexistence of the individual spirit in its corporeality" (ibid., § 401). Absolute interiority and immanence, therefore, of all determinations given through sensation: "Sensations are... mutations in the substantiality of the soul, posited in its being-for-itself, which is identical with substantiality" (ibid., § 402). ROSMINIANI (v.) distinguishes two modes of sensitive perception: a) there is the "fundamental and universal feeling" "through which we feel life to be in us" (Nuovo saggio, sect. 2, part 3, cap. 1, a. 1); it is a permanent and unified activity, "always vigilant and actualized to feel the state, whatever it may be, of our sensitive body" (ibid., a. 3) and results immediately "from the first individual union of the spirit with an animal body" (ibid.). The various sensations through which we subjectively feel our body and its parts are modifications of this fundamental feeling. The sentient soul, present throughout the body, also perceives the changes produced "on the sensitive parts by the force of an external body": "the perception of this passivity, which the sentient soul undergoes in a manner determined by the quality of the sensation, is the sensitive perception of bodies" (ibid., a. 4). Sensation takes on actualism (v.) GENTILI (v.). It is the initial and obscure moment from which the "I" begins the process toward its full spiritual reality as the unity of subject and object, that is, as thought, self-consciousness: "Pure feeling, sentiment, is thus the principle in which thought, at its first dawn, and reality, in its deepest root, coincide" (Introd. alla filos., cap. 4, n. 20, Milan-Rome 1923, p. 87). "That depth, dark in itself but which it [the "I"] illuminates with its eternal becoming, that first point from which it sets out on its eternal journey, the solid and unshakable foundation upon which it builds itself with its immanent activity, and in itself all that determined world which it comes to determine within itself from moment to moment—that is sense, which, seen from without, is the body, the very physical world" (ibid., cap. 5, n. 20, pp. 107-108).

BIBL.:
For the classical and medieval doctrine: S. Thomae Aquinatis Opera omnia, ed. Leonina, Rome 1882-; J. de Tonquédec, La critique de la connaissance, Paris 1929; E. Gilson, Le Thomisme, Paris 1942; M. Grabmann, Die Geschichte der scholastischen Methode, Freiburg 1909-11.
For the Renaissance: L. Blanchet, Campanella, Paris 1920; L. Firpo, Il processo di T. Campanella, Rome 1947.
For modern philosophy: E. Bréhier, Histoire de la philosophie, Paris 1928-32; N. Abbagnano, Storia della filosofia, Turin 1946-50; H. Höffding, Histoire de la philosophie moderne, Paris 1923-24.
For Kant: H. Cohen, Kants Theorie der Erfahrung, Berlin 1885; N. Kemp Smith, A Commentary to Kant's Critique of Pure Reason, London 1918.
For Hegel: J. Hyppolite, Genèse et structure de la Phénoménologie de l'Esprit, Paris 1946; G. Gentile, La filosofia di Marx, Florence 1899.
For Rosmini: G. Morando, La filosofia di A. Rosmini, Brescia 1934; C. Riva, Il problema della sensazione in Rosmini, Milan 1940.
For Gentile: U. Spirito, Il pensiero di G. Gentile, Florence 1937; A. Carlini, Il mito del realismo, Florence 1936.

A. Massolo

V. GNOSEOLOGICAL SIGNIFICANCE OF SENSATION

The idealistic conception, which understands sensation as the first form of the immanent spontaneity of the spirit, grasps one of the authentic meanings of sensing, namely its character of activity and subjective transcendence. Sensation, through which the first forms of consciousness in act are posited, loses its essentiality if reduced to pure passivity, with the same meaning as the actions-reactions of the physical world. Against the materialistic reduction of sensing to a mere cosmic process (not overcome by the so-called "psycho-physical parallelism" of Wundt and others, which makes the fact of consciousness a simple "epiphenomenon" [v.] of the physiological fact), the interpretation of sensation as a moment of spiritual subjectivity is correct. In reality, the gradual determinations that sensing assumes in the unfolding and affirmation of life in the world (from the most obscure "protopathic" sensitivity of unicellular animals, which is barely a glimmer of "presence" and "awareness," to the complex critical differentiations it assumes at the peak of its perfection in man) have their primary source in the transcendence of the sensing subject, which unfolds in multiple "forms" in living beings and posits itself, in man, as a spiritual principle, an ontological synthesis of sensing and thinking, of experience and self-consciousness. Sensation is, in this way, the immediate manifestation of the presence of the sensing principle in a body individuated in time and space, and it is determined in particular forms according to the structures of the sense organs and the action of the physical world upon them. Consequently, while it posits itself as an immanent form of being of consciousness, sensation is at the same time, in various ways according to the increasing degree of its qualitative differentiation, a revelation of the world, in the objective relations that the world itself assumes with the subject. Sensation - like every intentional form of consciousness - is therefore neither pure subjectivity nor pure objectivity: rather, it expresses (and in this direction the interpretation and perceptionism [v.] seem to reconcile) the determination of the physical world into "qualities" and "modes of representation" that have their formal principle in the expressive virtuality of the sensing principle, but are objectively conditioned - in their spatio-temporal meaning of "objects" and "things" - by the reality that becomes a physical stimulus of the organ. This accounts, on the one hand, for the characters of relativity of sensing (depending mainly on the "fluidity" of the conditions of the sense organs, already clearly noted by Greek thought: cf. the "ten tropes" of Aenesidemus in Sextus Empiricus, Pyrrh. hypoth., I, § 14 ff.; V. SCEPTICISM) and, on the other, for the insuperable persuasion of objective reality and "transcendence" with which sensation posits itself in human spiritual consciousness.

In order to precisely define the gnoseological significance of sensation, the following conclusions seem acceptable:

a) Pure sensation, while positing itself as a form of cognitive presence, is a form of initial and obscure knowledge that does not realize a full opposition between object and subject, self and non-self.

b) The world of sensation does not resolve within itself, phenomenism (v.) claims, entirely and without ontological residue, the world of reality; nor is it a simple reflection of a superior ideal reality (Plato). Not all aspects of the real are given in sensation, which is only a partial representation of the world, in some of its physical dynamic characters. The realistic significance of sensation lies in its value as a transcription in terms of consciousness of real phenomena and activities of matter, and in its conditioning and proportioning to them.

c) As the first contact of consciousness with reality, sensations are moments in which being is given, albeit in an unappreciated manner (not as being qua being, but in its concrete manifestations and physical activities). From this obscure givenness of being in sensation, intelligence can proceed to the determination of the ontological ground upon which the sensible object emerges and consists: moving in this direction, it seeks the principles and causes of corporeal being (metaphysics). From sensation, the intellect also proceeds to the determination of sensible objects and of the sensations themselves in their significance for the sensing subject: in this other direction, sensations reveal their biological, existential, aesthetic, social, and moral character (an ethical system, hedonism, resolves in the affective tone of sensations - pleasure, pain - the very moral values, good-evil), and thus also their essential function for man, spirit incarnate in a sensible body, in relation to the attainment of particular ends and of the ultimate end of existence.

d) Pure sensation as such, having no conceptual value, also has no meaning of "certainty" in the gnoseological sense of the term. Sensation is therefore not evidence of reality, of being, of existence, but awareness of "modes," "affections," "qualities." However, in man, due to the unity of the sensing and intelligent principle (sensation in man almost never presents itself in a "pure" state), the presence of the concrete existing entity is given with sensation in the judging consciousness (in some modes and forms of its spatio-temporal activity). The objects of sensation therefore possess a value of existence that nevertheless has different degrees: 1) There is a phenomenal-representative value of reality: it is proper to every object of sensation, but only insofar as it is "given," "represented": a weak reality, of the first degree, which also belongs to dreams, illusions, hallucinations, and all "false" representations of the senses. 2) A subjective-psychic value of reality: it is proper to every sensation insofar as it is a psychic fact of the subject: an ontological but "subjective" and "accidental" reality. 3) A transsubjective, transcendent value of reality, according to which the object of sensation reveals itself as having an existence of its own in the physical world. Such reality, globally, is the intentional reference of all sensibility: through it, the real "situation" of the sensing being (animal-man) in the world is explained and clarified; its existence with multiple relations of biological needs, necessities, dependence on physical reality. But in relation to the individual objects of sensation, transcendent reality (and its degree) is not universally determined and must be established with the criterion of the "critical convergence" of the various sensory planes, and together by the more or less positive and effective function that an object of sensation fulfills in relation to the existential necessities of the living being.

e) In pure sensation, as there is no formal evidence, so there is no formal truth or error: sensation is always determined by the encounter of an objective stimulus (external or internal) with the subjective conditions of the sense, and as such it is neither true nor false. However, due to the contingency of the conditions of the object and the subject, the sensitive representation may, to varying degrees, not correspond to the conditions of reality in itself (limit cases being hallucinations, and in general representations due to pathological affections of the organ): "sensum affici est ipsum sentire. Unde per hoc quod sensus ita nuntiant sicut afficiuntur, sequitur quod non decipiantur in iudicio quo iudicamus nos sentire aliquid. Sed ex hoc quod sensus aliter afficitur quam res sit, sequitur quod nobis nuntiant aliquando rem aliter quam sit" (Sum. Theol., 1°, q. 17, a. 2, c.). This intrinsic condition of sensibility does not entail a negative judgment on its value and positivity. Since, as St. Thomas further states, "naturus... sensibilium qualitatum intelligere non est sensus sed intellectus" (ibid., 1°, q. 78, a. 3, c.), determining the positive significance and at the same time the limits of sensation as a revelation of being belongs to thought, in which sensation is ultimately clarified and defined as knowledge.

BIBL.:
Per la concezione idealistica della s.: B. Croce, Filosofia dello spirito, Bari 1927; G. Gentile, Teoria generale dello spirito come atto puro, Firenze 1916; per la concezione fenomenistica: E. Mach, Analisi delle sensazioni, trad. it., Milano 1903; per la concezione realistica: J. Maritain, Distinguere per unire, trad. it., Brescia 1933; per la concezione tomistica: S. Tommaso, Summa Theol., I, q. 78-79; per la concezione scettica: Sesto Empirico, Pyrrh. hypoth., I, § 14 sgg.; per la concezione edonistica: Aristippo, frammenti in H. Diels, Fragmente der Vorsokratiker, Berlin 1903, II, pp. 280-285; per la concezione biologica: H. Bergson, L'évolution créatrice, Paris 1907; per la concezione estetica: B. Croce, Estetica come scienza dell'espressione e linguistica generale, Bari 1902; per la concezione sociale: G. Tarde, Les lois de l'imitation, Paris 1890.

A. Masnovo