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ARISTOTLE ON THE UNITY OF SENSES
by
Melsen Tunca İşeri
January ‘12
Boğaziçi University Philosophy Department
TABLE OF CONTENTS
I. INTRODUCTION
II. OBJECTS OF SENSE (AISTHETON)
III. COMMON SENSIBLES
IV. IN SEARCH OF A MISSING SENSE
V. SENSATION AS SYNTHESIS
VI. COMMON SENSE (SENSUS COMMUNIS)
VII. HOW COMMON SENSIBLES ARE SENSED?
VIII. PERCIEVING OF PERCEPTION
IX. CONCLUSION

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ARISTOTLE ON THE UNITY OF SENSES
I. INTRODUCTION
In De Anima Aristotle asks a big question ‘What is life?’ and tries to answer it by dwelling in an
inquiry which apparently retains natural philosophical concerns mostly. Throughout the book
beyond other investigations, he develops a theory of aesthesis (perception or sensation), since he
observes that all animated creatures have sensations to some extent.
In Book III, Aristotle discusses the possibility of a central sense which is common to all
five senses. [DA, 424b
22-30] He is aware that accurate perception requires the collaboration of
senses in order to combine a manifold of received stimulus to each other. If a white thing is sweet
at the same time, how the distinct sensations of sight and taste are connected to be perceived as the
qualities of the same single object? If a separate sixth sense exists for this purpose, then where
does its sense organ reside? Does it also responsible for the sensation of common sensibles? [DA,
418a
11] Is there a reliable way without yielding into infinite regress [DA, 425b
15-18] to explain the
grounds for our own perceiving that we perceive? In other words, in the perception of an object,
the subject who has those sensations is immediately aware of her perception. Furthermore, this
subject who senses both white and sweet (or asserts the difference between black and white) must
be self-identical throughout the sensation. In contrast, this numerically one and indivisible subject
must actualize different potentialities simultaneously in order to have such complex sensations.
In this paper I would like to analyze Aristotle’s accounts in De Anima about the common or
central sense. I find that he refers to common sense with different features which seem to be
alternative approaches at first sight, but in fact are all together unavoidable parts of his definition.
Additionally, I would like to summarize what he understands from “perceiving that we perceive.”

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II. OBJECTS OF SENSE (AISTHETON)
Aristotle does not question the ontological status of objects of sensation rather he simply takes
them as to be out there. He mentions that our sensation is independent from us. We cannot sense
whenever and whatever we want in contrast to thinking which mostly depends on our will. He
states that there must be the presence of externally sensible objects for us to have sensations. [DA,
417b
24-27] There must be something out there that affects the sense organ in order to have
sensation. Animals can also sense the objects of sense.
In De Anima II-6, Aristotle mentions three sorts of ‘sensible objects’ (aistheton) which are all
sensed in different ways.
i. Special (or proper) objects (ideon): A special object can be perceived only by one
sense. Each sense has such kind of an object which it discerns and never errs in
reporting that what is before it is color or sound although it may err as to what is
colored or what is sounding. [DA, 418a
11]
ii. Common Sensibles (koinon) : What Aristotle calls as common sensibles are perceived
by more than one sense, e.g. sight and touch. These are movement, rest, magnitude,
figure, number, unity and probably some more others which he did not exemplified in
De Anima. [DA, 418a
18-19] Some of them sensed by all the senses.
iii. Incidentals (or accidentals): These are things, like the “Son of Diares,” a piece of
marble or a letter paper. Their being white or round or perfumed are merely accidental
because they may be black or triangular or bad smelling as well. Incidental objects are
always particulars.
The special objects and common sensibles are in themselves (kathauta), but the incidental objects
are accidental. In other words the first two sorts are perceived in virtue of themselves while the
incidentals only accompany these and consequently they can be accepted as accidents of these.

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III. COMMON SENSIBLES
Aristotle thinks that in addition to colors, sounds, odors, flavors and tactile qualities sensed by the
five senses, there exist common sensibles. His examples for this sort of sensible objects in De
Anima are; movement, rest, magnitude, figure, number, unity. In elsewhere1
he mentions other
common sensible like roughness, smoothness, sharpness, bluntness, and, surprisingly, time2
.
Common sensible are perceived in virtue of themselves because they necessarily
accompany the special objects that are perceived in virtue of themselves. [DA, 425b
5-9] Special
objects are sensed directly, thus common sensible are perceived directly as well. But they cannot
be reduced to special sensibles like color or sound because common sensible are sensed by more
than one sense. For example, figure is perceived along with color but it cannot be simply the color
because it is also tangible.3
IV. IN SEARCH OF A MISSING SENSE
Aristotle argues about a missing or extra sense. He thinks, if it exists then there are two candidates
for it: It will be on the same level with all other five senses or it will be a common sense.
At the outset of De Anima Book III, Aristotle excludes explicitly the possibility of a sixth
sense which we lack or happen not to know. As he proceeds he verifies that there exists no sixth
sense in addition to the five because five senses have a definite arrangement that makes them a full
complete set. He confirms this completeness with explanations relying on the limited types of
fundamental elements; air, water, fire and earth and their combinations. If there was a sixth sense,
there will be extra sensory qualities and it will not perceive sensibles directly because there is no
1
De Sensu, 442b
5-7 and De Memoria, 450a
9-10, 451a
16-17
2
Time is a kind of number or magnitude of motion and number presupposes unity.
3
Ronald Polansky, Aristotle’s De Anima (New York: Cambridge University Press, 2007), p. 258.

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extra material that cannot be sensed by the present proper senses. He discloses that there cannot be
more senses but these five in the sub-lunar realm.
Aristotle believes that nature fulfills itself in the best possible ways and leaves nothing out
that is possible to it. It has given humans and most of the animals an arrangement of senses and
sense organs that ensures sensing whatever can be perceived regarding the four fundamental
elements. He also talks about incomplete creatures where he calls incompleteness as atelos. He
observes that there exist animals which have only sense of touch or lack some of the senses, like
animals merely having touch. [DA, 433b
31] They can survive with their limited capabilities, thus
they have what they need. With incomplete creatures he means for example three legged tiger.
Aristotle believes that the nature would not make such incomplete creatures. He gives an example
of the mole which has rudimentary eyes under its skin. Thus animals may have rudimentary sense
organs. On the other hand he does not accept the existence of a sixth sense which is rudimentary in
humans. Aristotle ensures that we have all the required senses and corresponding sense organs for
perceiving all types of sensible objects.
The sensible objects execute in a medium are also fitted according to corresponding sense
organs. Therefore, having an appropriate sense organ gives access to all its special sense objects,
because it cannot be the case that color, sound or odor differs depending upon whether it is in air
or water. Aristotle’s argument proceeds by asserting that if a sense organ’s compositional material
is not lacking then the senses competent to perceive all the related sensibles through that material
medium, i.e. air, water; will not be missing either. He has proven that the sensation of the special
objects require no additional sense. In the next steps he carries on plating the elements of his
theory of sensation with questioning whether there is a special sense organ for the sensation of the
common sensibles.

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V. SENSATION AS SYNTHESIS
Aristotle uses the word aesthesis for both sensation and perception. Sensation is a cognitive power
which enables a relation between things in the world and sense organs of the subject. Aesthesis’s
direction is towards the sensible objects (aistheton). Aesthesis also has connotations of synthesis.
The scattered stimulants are synthesized in order to distinguish an individual object of sensation
when received by the sense organs. The performance of such a synthesis is an activity but
aesthesis generally appears as a passive faculty of the soul throughout De Anima. Similarly, his
stress on the relation between alteration and aesthesis does not imply sensation to be active but
rather as a passive potential. He takes the fact as a support: The sense organs do not sense
themselves but only external objects. [DA, 417a
2-5] The eye does not see its own parts, nor does the
nose smells inside of the nostril. Aristotle believes that they should be ready for the upcoming
sensory stimulus because senses themselves hold potentiality. They require an actual alteration
trigger for sensing. They should be affected in order for us to have sensations.
Aristotle prefers to call the status of being affected as paschein although he reminds that it
is not affection in real terms but instead implies ‘as if’ being affected so he correlates it with
‘being moved’. [DA, 417a
1-21] During the process of sensation kinesis starts in our sense organ by
the stimulus sourcing from the sense object. According to him passive powers are moveable by
that which they are related while active powers move that to which they are related. The organ of
sense is the potentiality not the actuality but after it has been affected it becomes alike with the
perceived object. Aristotle states that sensory organ becomes like the sensory object through the
medium. [DA, 418a
1-6]
At first sight the whole sensation process as described by Aristotle appears to be a passive
alteration of the sense organs but here the alteration should not be understood literally. Since the
only effect an odor can produce is smelling, it follows that things which cannot smell cannot be

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effected by odors. Aristotle says “What more is smelling than being affected by something?” [DA,
424b
18] On the other hand aesthesis, according to Aristotle is the power of receiving into itself the
sensible forms without matter. A sense organ is potentially the seat or proper base of such power
similar to a piece of wax which takes on the impress of a signet-ring without the metal. Except for
the cases where there is the presence of excess amount of sensory stimulus to such an extent that
may destroy the corresponding sense organ, the passive power of sensation is not affected in
reality. They are ‘flexible or have plasticity’ similar to a wax. The sense is affected only by the
properties intrinsic to the object of sense i.e. color, odor, flavor. [DA, 424a
16-24] For instance, the
sense of sight becomes alike with the color red, soon after with blue or green successively without
having any permanent alteration on its organic matter.
VI. COMMON SENSE (SENSUS COMMUNIS)
Aristotle starts his inquiry in order to realize whether there are more than five senses and this
inquiry has led him to appreciate that these five senses are united indeed.
Each sense can perceive exclusively it own special sensibles. Every sensation is infallible
for its proper object but the five senses are sensed incidentally by each other. Cross sensation is
incidentally sensed. In other words the five senses perceive each other’s proper sensible objects
merely accidentally. If the yellow thing which I see in front of me is also sour it is by accident.
That yellow thing also happens to be a thing, i.e. a lemon accidentally. There is difference between
yellowness, sourness and that particular lemon? What enables us to bring distinct representations
to our mind at the same time? What is the ground of the combination of these different aestheon
on that particular lemon? According to Aristotle we sense across the senses only because senses
are joint to each other. Most sensation is passive and this unifying process is not achieved by an
outer agency. Each sense perceives unity or one. Unity is a common sensible.

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The senses perceive each other's special objects incidentally; not because the percipient sense is
this or that special sense, but because all form a unity: this incidental perception takes place
whenever sense is directed at one and the same moment to two disparate qualities in one and the
same object, e.g. to the bitterness and the yellowness of bile, the assertion of the identity of both
cannot be the act of either of the senses; hence the illusion of sense, e.g. the belief that if a thing is
yellow it is bile. [DA, 424b
30-425a
5]
Aristotle continues to plot his theory of sensation along with a surprising path and declares
of the view that the senses are unified in a single sensibility. [DA, 424a
31-425b
2] From his accounts
it can be interpreted that the five senses appear to be sub-faculties of a combined faculty of sense.4
The theory on the unification of senses clarifies his opinions that the common sensibles are all
perceived incidentally together with the special sensible objects. The senses are conjoined and
different special sensibles belong to a single manifold of representations and it follows from this
model that the senses can perceive the objects of the other senses accidentally.
VII. HOW COMMON SENSIBLES ARE SENSED?
Aristotle first asks whether there is an extra sense for perceiving common and incidental sensibles.
If a separate sense exists for this purpose, then where does its sense organ reside? [DA, 418a
11]
…[I]n the case of the common sensibles there is already in us a general sensibility which
enables us to perceive them directly; there is therefore no special sense required for their
perception. [DA, 424b
26-30]5
4
Polansky, Aristotle’s De Anima, p. 376.
5
All references to De Anima Books I, II, III are given in accordance with the standard pagination used in generally
accepted Corpus Aristotelicum and are all from J.A. Smith’s translation.

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In the first section of Book III, he exhibits that accurate perception requires the collaboration of
senses in order to combine a manifold of received stimulus to each other. If a white thing is sweet
at the same time, how the distinct sensations of sight and taste are connected to be perceived as the
qualities of the same single object? We can differentiate the common sensibles because there is a
unified sensing achieved by all the five others. Since the common sensible are each and all
perceived as they are by each five senses, it is not plausible that there is a proper sense for any of
the common sensibles. Distinct senses are unified insofar that their combined function turns out to
have further meaning in terms of a common sense and perception of the common sensibles is only
one of the responsibilities of this connected sensation faculty.
The model about the unity of senses also makes clear how we are able to sense the
incidentals. The son of Diares is perceived as a white thing is incidental unlike the accidental
perception of the patch of white color which has necessarily has a magnitude and figure. Common
sensibles can be accidents of the special sensible while being basically conjoined with them and
perceived in virtue of themselves.
VIII. PERCIEVING OF PERCEPTION
According to Aristotle’s theory of sensation as he presents in De Anima, the common sense is not
another sense away from the proper five senses rather the unification of all senses. This common
sense also has the task for perception that goes beyond the sensation of proper, common and
accidental sensibles and the differentiation of various special objects. Aristotle attributes this
central faculty as to be the ground of self-awareness of the subject who has these sensations.
If self-awareness will require another separate sense, there will be more than five senses.
Aristotle has already discussed this possibility of extra sense and negated it. Since the senses are

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combined and working together, Aristotle suggests that the common sense enables to be aware of
sensing. He also discusses the possibility that a sense may perceive itself as well.
According to Aristotle, we are aware that we are i.e. seeing or hearing through the sense. If
another separate sense was responsible for the awareness, then there would be the need of another
third sense to be aware of the first awareness. Subsequently he easily displays that the process
would yield into an infinite regress. Thus he proposes that we must in advance presume a sense
which is aware of itself.
Since it is through sense that we are aware that we are seeing or hearing, it must be either
by sight that we are aware of seeing, or by some sense other than sight. But the sense that
gives us this new sensation must perceive both sight and its object, viz. colour: so that
either there will be two senses both percipient of the same sensible object, or the sense
must be percipient of itself. Further, even if the sense which perceives sight were different
from sight, we must either fall into an infinite regress, or we must somewhere assume a
sense which is aware of itself. If so, we ought to do this in the first case. [DA, 425b
12-17]
It seems that, for Aristotle awareness and self-awareness are closely related. Both perception of
sensibles and awareness of perceiving belong to perceptual experience without being quite the
same or too different from each other.6
Perception and self awareness have deep and certain
connections in Aristotle’s understanding of the soul. As he figures out after dealing with difficult
arguments that perceiving constantly and continuously comprise perceiving of perceiving.
IX. CONCLUSION
It is an undeniable fact that most of the thinkers in various philosophical periods and domains -
even the contemporaries - are deeply influenced by Aristotle, especially by his systematic methods
in the inquiry of philosophical problems. However, in the literature I find that there is not much
6
Polansky, Aristotle’s De Anima, p. 380.

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emphasis on the relation between Aristotle and Kant for the specific argument of transcendental
apperception. Conversely, for the case of apperception, I am fairly convinced that Aristotle has an
important influence on Kant. I believe that revealing such a close link between the ancestor and
his possible follower might be helpful for having a better insight to understand this sophisticated
notion of Kant’s.
Immanuel Kant deals with united manifold of sense representations and self-awareness in
the “Transcendental Deduction” chapter of Critique of Pure Reason, while his motives are
probably quite different from Aristotle’s. In the second edition of the “Transcendental Deduction”,
Kant attempts to establish a correlation between objectively valid representations of objects and
the united self-consciousness of the subject who perceives them. Although his main concern is
exhibiting the legitimacy of the natural sciences, he first focuses on the nature of self-
consciousness which he calls apperception by adapting Leibnitz’s term.
I suggest that Aristotle’s common or central sense corresponds to empirical apperception
in Kant’s model. However empirical apperception at the end yields to a loose bundle of senses just
like in Hume’s notorious portrayal of self. Thus Kant assures the necessary relation of the
empirical apperception with the transcendental apperception in order to eliminate this difficulty.
Kant describes the transcendental unity of apperception as a special kind of synthesis.
Aristotle’s aesthesis also has connotations of synthesis but with an essential problematic. The
performance of such a synthesis seems to be an activity although sensation generally appears as a
passive faculty of the pscyhe in De Anima. Aristotle prefers to call the status of being affected as
paschein, although he is cautious that this does not mean affection in real terms. [DA, 417a
1-21]
Kant spends effort to solve this problem by introducing the notion of active synthesis.
Additionally, I believe he uses some other Aristotelian features in his apperception model. I hold
the opinion that the similarities and differences between Aristotle’s and Kant’s theories on the
unity of senses and self awareness should be studied in further details.