No one denies that the sensitive mean (mesotˆes) is central to Aristotle’s theory of
sense-perception. Commentators both ancient and modern at the very least acknowledge its
importance in passing. J.L. Beare set the modern tone by noting: “For Aristotle this doctrine
ofmesotˆesis of cardinal importance in the theory of sense-perception. Without understanding
it we must fail to grasp his explanation of howaisthˆesisapprehends form without matter.”3
What is baffling is that Beare’s comment notwithstanding, scant attention is paid to this principal aspect of Aristotle’s theory of sensory cognition.4 Further, while affirming the
centrality of the sensitive mean to Aristotle’s theory of sense-perception, Beare’s insight ends
there. The sensitive mean does not figure in any further explanation of either Aristotle’s
general theory of perception, or in the mechanics of sensory cognition. If we are to take
Beare’s claim seriously, which we should given that Aristotle is explicit about the central role
of the mean in perception, then any adequate interpretation of Aristotle’s theory of
sense-perception must do more than merely pay lip service to the sensitive mean in general
terms. Needed are detailed analyses ofwhatthe mean is, ofhowit works, or what it does that
gives it its special status in Aristotle’s theory.
This chapter is a detailed analysis of Aristotle’s treatment of the mean5: the nature of the
mean (what it is), how it works, and the role it plays in Aristotle’s systematic treatment of
sense-perception. It will become clear that the mean serves as Aristotle’s condition of
sentience—that which separates animals from plants and inanimate objects. Each of the
3Beare 1906, 232.
4There have been only two scholarly articles treating the topic specifically: Slackey (1961) and Brennan
(1973). More striking is that manuscript length treatments rarely tend to give the mean more than merely a cursory treatment. Hicks (e.g.) spends only two paragraphs discussing the mean (1907, 414). The influential works of Ross and Hamlyn are guilty of the same (their respective takes on the mean is discussed below), which is symptomatic of the historical trend to merely mention the mean in passing. Given that the mean is central to Aristotle’s theory of sense-perception, one would expect the literature to be riddled with varying accounts of it. Moreover, since understanding the mean is critical to understandingaisthˆesisvis-`a-vis the relation between matter and form, one would think that the mean would be a key battleground for the entrenched debates and controversy that stem from this relation.
5I make no claim to be giving an exhaustive account of the mean as it appears in its various guises throughout
the corpus. My focus here is the sensitive mean and any possible connections between the mean as we see it in Aristotle’s psychological works and that of theEthics, for instance, is orthogonal to this inquiry.
special organs of sense is so constituted that its objects fall on a range between contraries such
as white and black that sandwich the discriminating mean. So much is obvious from
straightforward exegesis. What is not clear iswhythe mean works as Aristotle’s condition for
sentience, andwhatit is about the mean that moves Aristotle to takes pains to set a place for it
on the sensitive range.
Prior to delving into analyses of the nature and function of the mean, it will serve us well
to reflect on Aristotle’s motivation for using the mean in his theory of perception. This
motivation stems from Aristotle’s fascination with a crucial difference between plants (and inanimate objects) and animals. In many places throughout the corpus Aristotle is very much
concerned with how it is that warm-blooded animals maintain equilibrium in temperature.
More specifically, he takes great pains to explain how living things, which are also things that
produce heat, do not overheat. Thus we get much explanation of how refrigeration works in
animals.6 From these various passages it becomes evident that what Aristotle is trying to
explain is the homeostatic nature of animals with respect to temperature. That is, he works to
give an account of how animals maintain roughly a static temperature despite being
continuously heated.
Be it due to metabolic processes or basking under the mid-day sun, being alive provides
animals with plenty of sources of heat. Thus the animal body needs a mechanism of
temperature control, otherwise animals, humans included, would suffer ever-increasing body temperatures. The corpus contains a number of accounts of the various processes that bring
about heat in the animal body. Accordingly, there is much explication of how bodies are able
to refrigerate or cool off.7 Aristotle spends much time in various works discussing respiration,
which is directly connected to regulating body heat (420b23). Similarly, in theParts of
Animals, for instance, Aristotle elaborates the physiology of the brain to explain that it
functions primarily as a radiator. He writes:
So far then this much is plain, that all animals must necessarily have a certain
6Seede Respiratione470a7ff, 477a11-31 (chapter 19), 480b18ff. 7Seede Sensu5 444a23, andMeteorologica11 389a26ff.
amount of heat. But as all influences require to be counterbalanced, so that they
may be reduced to moderation and brought to the mean (for in the mean, and not
in either extreme, lies their substance and account) (Parts of Animals652b15ff)8
For our purposes the exact physiology of how brains work as radiators is not important. What
is interesting, however, is that the purpose of the brain is to dissipate accrued heat in order to
return body temperature back to the mean. The mean in this case is the average body
temperature, or the not-too-hot and not-too-cold state that animals tend to find themselves in.
What is also important is that this mean state is not merely the state in which animals find
themselves more often than not. It is the state in which theystrive to be.
The mean is central in the brain’s activity of thermal regulation, for it is its end state—that
to which its function aims. From this a crucial difference between plants and animals becomes evident. It takes brains to regulate heat. It does so, however, only insofar as brains function as
radiators striving to maintain a mean body temperature. Although plants lack brains, the
reason why this precludes them from the ability to regulate their temperatures is due to their
lack of a mean. This should be no surprise, for Aristotle makes this claim explicitly:
This explains also why plants cannot perceive, in spite of their having a portion of
soul in them and being affected by tangible objects themselves; for their
temperature can be lowered or raised. The explanation is that they have no mean,
and so no principle in them capable of taking on the forms of sensible objects but
are affected together with their matter. (de Anima424a32-b4)9
Plants are ensouled, living beings, yet they fall into the same category as inanimate objects
in terms of how they are affected by heat. What Aristotle calls an explanation here—that plants cannot perceive because they lack the mean—seems to be no explanation at all, for it
8 VΟτι μὲν οὖν θερμότητος τὰ ζῷα μετέχειν ἀναγκαῖον, δῆλον ἐκ τούτων. >Επεὶ δ` ἅπαντα δεῖται τῆς ἐναντίας ῥοπῆς, ἵνα τυγχάνῃ τοῦ μετρίου καὶ τοῦ μέσου (τὴν γὰρ οὐσίαν ἔχει τοῦτο καὶ τὸν λόγον, τῶν δ` ἄκρων ἑκάτερον οὐκ ἔχει χωρίς). 9 καὶ διὰ τί ποτε τὰ φυτὰ οὐκ αἰσθάνεται, ἔχοντά τι μόριον ψυχικὸν καὶ πάσχοντά τι ὑπὸ τῶν ἁπτῶν (καὶ γὰρ ψύχεται καὶ θερμαίνεται)· αἴτιον γὰρ τὸ μὴ ἔχειν μεσότητα, μηδὲ τοιαύτην ἀρχὴν οἵαν τὰ εἴδη δέχεσθαι τῶν αἰσθητῶν, ἀλλὰ πάσχειν μετὰ τῆς ὕλης.
remains unsaid what it is about the mean that allows for perception. This has been received as
a cryptic remark since antiquity, and the reason, I contend, is that it reveals ignorance about
just what this mean is and what work it does for Aristotle in his theory of sense-perception.
Understanding the nature of the mean, then, is crucial to understanding Aristotle’s
explanation. And because Aristotle’s explanation works as his criterion to distinguish the
sentient from the merely sensitive, coming to grips with the mean is integral to an
interpretation of Aristotle’s theory of sensory cognition. What follows is an attempt at just
that: understanding the nature of the sensitive mean and its role in Aristotle’s psychology. In
the next section, we develop an argument showing that the textual evidence favours a richer
understanding of the nature of the mean than has been appreciated by commentators both
ancient and modern. The sensitive mean is a product of sense-organs properly constituted so
as to be able to perceive their respective objects or sensibles. Further, the mean enables the
homeostasis required for organs to perceive veridically. That is, the mean enables the organs
to return to their neutral mean states prior to and after each act of sensation thereby allowing
subsequent perceptions.