Thomas Aquinas, Summa Theologiae

Objection 1: It would seem inaccurate to distinguish five exterior senses. But there are many kinds of accidents. Therefore, as powers are distinguished by their objects, it seems that the senses are multiplied according to the number of the kinds of accidents.

Objection 2: Further, magnitude and shape, and other things which are called "common sensibles," are "not sensibles by accident," but are contradistinguished from them by the Philosopher (De Anima ii, 6). Now the diversity of objects, as such, diversifies the powers. Since, therefore, magnitude and shape are further from color than sound is, it seems that there is much more need for another sensitive power than can grasp magnitude or shape than for that which grasps color or sound.

Objection 3: Further, one sense regards one contrariety; as sight regards white and black. But the sense of touch grasps several contraries; such as hot or cold, damp or dry, and suchlike. Therefore it is not a single sense but several. Therefore there are more than five senses.

Objection 4: Further, a species is not divided against its genus. But taste is a kind of touch. Therefore it should not be classed as a distinct sense of touch.

On the contrary, The Philosopher says (De Anima iii, 1): "There is no other besides the five senses."

I answer that, The reason of the distinction and number of the senses has been assigned by some to the organs in which one or other of the elements preponderate, as water, air, or the like. By others it has been assigned to the medium, which is either in conjunction or extrinsic and is either water or air, or such like. Others have ascribed it to the various natures of the sensible qualities, according as such quality belongs to a simple body or results from complexity. But none of these explanations is apt. For the powers are not for the organs, but the organs for the powers; wherefore there are not various powers for the reason that there are various organs; on the contrary, for this has nature provided a variety of organs, that they might be adapted to various powers. In the same way nature provided various mediums for the various senses, according to the convenience of the acts of the powers. And to be cognizant of the natures of sensible qualities does not pertain to the senses, but to the intellect.

The reason of the number and distinction of the exterior senses must therefore be ascribed to that which belongs to the senses properly and "per se." Now, sense is a passive power, and is naturally immuted by the exterior sensible. Wherefore the exterior cause of such immutation is what is "per se" perceived by the sense, and according to the diversity of that exterior cause are the sensitive powers diversified.

Now, immutation is of two kinds, one natural, the other spiritual. Natural immutation takes place by the form of the immuter being received according to its natural existence, into the thing immuted, as heat is received into the thing heated. Whereas spiritual immutation takes place by the form of the immuter being received, according to a spiritual mode of existence, into the thing immuted, as the form of color is received into the pupil which does not thereby become colored. Now, for the operation of the senses, a spiritual immutation is required, whereby an intention of the sensible form is effected in the sensile organ. Otherwise, if a natural immutation alone sufficed for the sense's action, all natural bodies would feel when they undergo alteration.

But in some senses we find spiritual immutation only, as in "sight" while in others we find not only spiritual but also a natural immutation; either on the part of the object only, or likewise on the part of the organ. On the part of the object we find natural immutation, as to place, in sound which is the object of "hearing"; for sound is caused by percussion and commotion of air: and we find natural immutation by alteration, in odor which is the object of "smelling"; for in order to exhale an odor, a body must be in a measure affected by heat. On the part of an organ, natural immutation takes place in "touch" and "taste"; for the hand that touches something hot becomes hot, while the tongue is moistened by the humidity of the flavored morsel. But the organs of smelling and hearing are not affected in their respective operations by any natural immutation unless indirectly.

Now, the sight, which is without natural immutation either in its organ or in its object, is the most spiritual, the most perfect, and the most universal of all the senses. After this comes the hearing and then the smell, which require a natural immutation on the part of the object; while local motion is more perfect than, and naturally prior to, the motion of alteration, as the Philosopher proves (Phys. viii, 7). Touch and taste are the most material of all: of the distinction of which we shall speak later on (ad 3,4). Hence it is that the three other senses are not exercised through a medium united to them, to obviate any natural immutation in their organ; as happens as regards these two senses.

Reply to Objection 1: Not every accident has in itself a power of immutation but only qualities of the third species, which are the principles of alteration: therefore only suchlike qualities are the objects of the senses; because "the senses are affected by the same things whereby inanimate bodies are affected," as stated in Phys. vii, 2.

Reply to Objection 2: Size, shape, and the like, which are called "common sensibles," are midway between "accidental sensibles" and "proper sensibles," which are the objects of the senses. For the proper sensibles first, and of their very nature, affect the senses; since they are qualities that cause alteration. But the common sensibles are all reducible to quantity. As to size and number, it is clear that they are species of quantity. Shape is a quality about quantity. Shape is a quality about quantity, since the notion of shape consists of fixing the bounds of magnitude. Movement and rest are sensed according as the subject is affected in one or more ways in the magnitude of the subject or of its local distance, as in the movement of growth or of locomotion, or again, according as it is affected in some sensible qualities, as in the movement of alteration; and thus to sense movement and rest is, in a way, to sense one thing and many. Now quantity is the proximate subject of the qualities that cause alteration, as surface is of color. Therefore the common sensibles do not move the senses first and of their own nature, but by reason of the sensible quality; as the surface by reason of color. Yet they are not accidental sensibles, for they produce a certain variety in the immutation of the senses. For sense is immuted differently by a large and by a small surface: since whiteness itself is said to be great or small, and therefore it is divided according to its proper subject.

Reply to Objection 3: As the Philosopher seems to say (De Anima ii, 11), the sense of touch is generically one, but is divided into several specific senses, and for this reason it extends to various contrarieties; which senses, however, are not separate from one another in their organ, but are spread throughout the whole body, so that their distinction is not evident. But taste, which perceives the sweet and the bitter, accompanies touch in the tongue, but not in the whole body; so it is easily distinguished from touch. We might also say that all those contrarieties agree, each in some proximate genus, and all in a common genus, which is the common and formal object of touch. Such common genus is, however, unnamed, just as the proximate genus of hot and cold is unnamed.

Reply to Objection 4: The sense of taste, according to a saying of the Philosopher (De Anima ii, 9), is a kind of touch existing in the tongue only. It is not distinct from touch in general, but only from the species of touch distributed in the body. But if touch is one sense only, on account of the common formality of its object: we must say that taste is distinguished from touch by reason of a different formality of immutation. For touch involves a natural, and not only a spiritual, immutation in its organ, by reason of the quality which is its proper object. But the organ of taste is not necessarily immuted by a natural immutation by reason of the quality which is its proper object, so that the tongue itself becomes sweet and bitter: but by reason of a quality which is a preamble to, and on which is based, the flavor, which quality is moisture, the object of touch.

Articulus 3

Argumentum 1

Ad tertium sic proceditur. Videtur quod inconvenienter distinguantur quinque sensus exteriores. Sensus enim est cognoscitivus accidentium. Sunt autem multa genera accidentium.

Cum ergo potentiae distinguantur per obiecta, videtur quod sensus multiplicentur secundum numerum qui est in generibus accidentium.

Argumentum 2

Praeterea, magnitudo et figura, et alia quae dicuntur sensibilia communia, non sunt sensibilia per accidens, sed contra ea dividuntur in II de anima.

Diversitas autem per se obiectorum diversificat potentias. Cum ergo plus differant magnitudo et figura a colore quam sonus; videtur quod multo magis debeat esse alia potentia sensitiva cognoscitiva magnitudinis aut figurae, quam coloris et soni.

Argumentum 3

Praeterea, unus sensus est unius contrarietatis; sicut visus albi et nigri. Sed tactus est cognoscitivus plurium contrarietatum, scilicet calidi et frigidi, humidi et sicci, et huiusmodi. Ergo non est sensus unus, sed plures.

Ergo plures sensus sunt quam quinque.

Argumentum 4

Praeterea, species non dividitur contra genus. Sed gustus est tactus quidam. Ergo non debet poni alter sensus praeter tactum.

Sed Contra

Sed contra est quod philosophus dicit, in III de anima, quod non est alter sensus praeter quinque.

Corpus

Respondeo dicendum quod rationem distinctionis et numeri sensuum exteriorum quidam accipere voluerunt ex parte organorum, in quibus aliquod elementorum dominatur, vel aqua, vel aer, vel aliquid huiusmodi.

Quidam autem ex parte medii, quod est vel coniunctum, vel extrinsecum; et hoc vel aer, vel aqua, vel aliquid huiusmodi.

Quidam autem ex diversa natura sensibilium qualitatum, secundum quod est qualitas simplicis corporis, vel sequens complexionem.

Sed nihil istorum conveniens est. Non enim potentiae sunt propter organa, sed organa propter potentias, unde non propter hoc sunt diversae potentiae, quia sunt diversa organa; sed ideo natura instituit diversitatem in organis, ut congruerent diversitati potentiarum. Et similiter diversa media diversis sensibus attribuit, secundum quod erat conveniens ad actus potentiarum.

Naturas autem sensibilium qualitatum cognoscere non est sensus, sed intellectus. Accipienda est ergo ratio numeri et distinctionis exteriorum sensuum, secundum illud quod proprie et per se ad sensum pertinet. Est autem sensus quaedam potentia passiva, quae nata est immutari ab exteriori sensibili. Exterius ergo immutativum est quod per se a sensu percipitur, et secundum cuius diversitatem sensitivae potentiae distinguuntur.

Est autem duplex immutatio, una naturalis, et alia spiritualis.

Naturalis quidem, secundum quod forma immutantis recipitur in immutato secundum esse naturale, sicut calor in calefacto.

Spiritualis autem, secundum quod forma immutantis recipitur in immutato secundum esse spirituale; ut forma coloris in pupilla, quae non fit per hoc colorata. Ad operationem autem sensus requiritur immutatio spiritualis, per quam intentio formae sensibilis fiat in organo sensus. Alioquin, si sola immutatio naturalis sufficeret ad sentiendum, omnia corpora naturalia sentirent dum alterantur. Sed in quibusdam sensibus invenitur immutatio spiritualis tantum, sicut in visu. In quibusdam autem, cum immutatione spirituali, etiam naturalis; vel ex parte obiecti tantum, vel etiam ex parte organi. Ex parte autem obiecti, invenitur transmutatio naturalis, secundum locum quidem, in sono, qui est obiectum auditus, nam sonus ex percussione causatur et aeris commotione. Secundum alterationem vero, in odore, qui est obiectum olfactus, oportet enim per calidum alterari aliquo modo corpus, ad hoc quod spiret odorem. Ex parte autem organi, est immutatio naturalis in tactu et gustu, nam et manus tangens calida calefit, et lingua humectatur per humiditatem saporum. Organum vero olfactus aut auditus nulla naturali immutatione immutatur in sentiendo, nisi per accidens. Visus autem, quia est absque immutatione naturali et organi et obiecti, est maxime spiritualis, et perfectior inter omnes sensus, et communior.

Et post hoc auditus, et deinde olfactus, qui habent immutationem naturalem ex parte obiecti. Motus tamen localis est perfectior et naturaliter prior quam motus alterationis, ut probatur in VIII physic..

Tactus autem et gustus sunt maxime materiales, de quorum distinctione post dicetur.

Et inde est quod alii tres sensus non fiunt per medium coniunctum, ne aliqua naturalis transmutatio pertingat ad organum, ut accidit in his duobus sensibus.

Ad 1

Ad primum ergo dicendum quod non omnia accidentia habent vim immutativam secundum se; sed solae qualitates tertiae speciei, secundum quas contingit alteratio.

Et ideo solae huiusmodi qualitates sunt obiecta sensuum, quia, ut dicitur in VII physic., secundum eadem alteratur sensus, secundum quae alterantur corpora inanimata.

Ad 2

Ad secundum dicendum quod magnitudo et figura et huiusmodi, quae dicuntur communia sensibilia, sunt media inter sensibilia per accidens et sensibilia propria, quae sunt obiecta sensuum.

Nam sensibilia propria primo et per se immutant sensum; cum sint qualitates alterantes. Sensibilia vero communia omnia reducuntur ad quantitatem. Et de magnitudine quidem et numero, patet quod sunt species quantitatis. Figura autem est qualitas circa quantitatem; cum consistat ratio figurae in terminatione magnitudinis.

Motus autem et quies sentiuntur, secundum quod subiectum uno modo vel pluribus modis se habet secundum magnitudinem subiecti vel localis distantiae, quantum ad motum augmenti et motum localem; vel etiam secundum sensibiles qualitates, ut in motu alterationis, et sic sentire motum et quietem est quodammodo sentire unum et multa. Quantitas autem est proximum subiectum qualitatis alterativae, ut superficies coloris. Et ideo sensibilia communia non movent sensum primo et per se, sed ratione sensibilis qualitatis; ut superficies ratione coloris. Nec tamen sunt sensibilia per accidens, quia huiusmodi sensibilia aliquam diversitatem faciunt in immutatione sensus. Alio enim modo immutatur sensus a magna superficie, et a parva, quia etiam ipsa albedo dicitur magna vel parva, et ideo dividitur secundum proprium subiectum.

Ad 3

Ad tertium dicendum quod, sicut philosophus videtur dicere in II de anima, sensus tactus est unus genere, sed dividitur in multos sensus secundum speciem; et propter hoc est diversarum contrarietatum. Qui tamen non separantur ab invicem secundum organum, sed per totum corpus se concomitantur; et ideo eorum distinctio non apparet. Gustus autem, qui est perceptivus dulcis et amari, concomitatur tactum in lingua, non autem per totum corpus; et ideo de facili a tactu distinguitur. Posset tamen dici quod omnes illae contrarietates, et singulae conveniunt in uno genere proximo, et omnes in uno genere communi, quod est obiectum tactus secundum rationem communem. Sed illud genus commune est innominatum; sicut etiam genus proximum calidi et frigidi est innominatum.

Ad 4

Ad quartum dicendum quod sensus gustus, secundum dictum philosophi, est quaedam species tactus quae est in lingua tantum. Non autem distinguitur a tactu in genere, sed a tactu quantum ad illas species quae per totum corpus diffunduntur. Si vero tactus sit unus sensus tantum, propter unam rationem communem obiecti, dicendum erit quod secundum rationem diversam immutationis, distinguitur gustus a tactu. Nam tactus immutatur naturali immutatione, et non solum spirituali, quantum ad organum suum, secundum qualitatem quae ei proprie obiicitur. Gustus autem organum non immutatur de necessitate naturali immutatione secundum qualitatem quae ei proprie obiicitur, ut scilicet lingua fiat dulcis vel amara; sed secundum praeambulam qualitatem, in qua fundatur sapor, scilicet secundum humorem, qui est obiectum tactus.