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How Can We Reach the True Definition of Something

δι
άν

οι
α

HOW CAN WE REACH THE TRUE 
DEFINITION OF SOMETHING?

Essence, Definition, and Teleology in Aristole's 
Metaphysics    

MINJUN LEE

“!e ultimate value of life depends upon awareness and the power of 
contemplation rather than upon mere survival.”

-Aristotle

INTRODUCTION 

What is wisdom (sophia)? In Metaphysics, Aristotle says, “Clearly, wisdom is 
knowledge of certain principles and causes (ἡ σοφία περί τινας ἀρχὰς 

καὶ αἰτίας ἐστὶν ἐπιστήμη, δῆλον)” (982a2). 1 For him, the task of Metaphysics 
is investigating being qua being, the principles and causes of being.2 !e inquiry 
into being qua being signi"es the question of what a being is. What a being is and 
what a substance is are the same: “Namely, what is being? is just the question, What 
is substance?” (1028b3).3 Aristotle believes that a substance is equal to an essence: 
“the essence, the account of which is a de"nition, is said to be each thing’s substance” 
(1017b22). !e essence and the de"nition (horismos) are one in some sense: “the 
de"nition is the account of the essence” (1031a11). If the essence of a thing X is p, 
then the de"nition of X is the statement “X is p.” !us, reaching the true de"nition 

! "is citation from Metaphysics refers to the Loeb translation.
# Metaphysics 1003a20-27
$ All citations here and throughout this paper refer to the Reeve translation; see appended bibliography.



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of something is identical to knowing what something is and can be an answer to the 
question of Metaphysics.

Nevertheless, there is a puzzle with Aristotle’s method of reaching the true de"nition 
of something in Metaphysics. He approaches the de"nition of something by 
dividing genera and "nding di#erentiae. However, if we follow Aristotle’s system 
to reach the de"nition of something, we face the moment when it is questionable 
whether the de"nition by means of genera and di#erentia is the true de"nition. For 
instance, Aristotle gives a de"nition of a human: “A human is a featherless, two-
footed animal.”4 Since the de"nition of something is the account of its essence, if 
the account “a human is a featherless, two-footed animal” is the true de"nition of 
a human, the essence of a human is nothing but ‘featherless and two-footed.’ !is 
is because the essence means “what something is” (1030a3). Does such a de"nition 
truly capture the essence of a human being what a human is? Although a human is 
the only featherless and two-footed living creature, this does not seem to be the true 
de"nition of a human. We know that what a human is goes beyond this de"nition by 
virtue of being a human. Living a human life is more valuable than just surviving as 
one of the animals. !erefore, this de"nition is the mere taxonomic de"nition. !is 
de"nition does not express a vivid human life. 

Hence, we should know how to "nd the true de"nition of something in order for 
us not to remain as a merely featherless, two-footed animal. Unfortunately, Aristotle 
does not give us any suggestion of how we can overcome this issue. Unless we know 
the solution to surmount this problem with Aristotle’s method, we may not reach the 
true de"nition of something. If we cannot reach the true de"nition of something, 
then we cannot know its true essence. !is is a signi"cant problem for Aristotle, who 
seeks knowledge of what it is in Metaphysics.

However, there is a way of reconciling the method of reaching a taxonomic de"nition 
and a true de"nition. !is is the aim of this paper. We will not dismiss Aristotle’s 
method of dividing genera and "nding di#erentia. Instead, we will interpret the true 
meaning of the ultimate di#erentia in this paper. !is interpretation of the meaning 
of the ultimate di#erentia will guide us towards the true de"nition of something. 
Hence, the ultimate goal of this project is to "nd the true meaning of the ultimate 
di#erentia of something, supplement Aristotle’s method of division, complete his 
un"nished project, and reach its true de"nition.

OVERVIEW
In order to achieve this aim, we will proceed in three parts. In Part 1, we will investigate 
what a de"nition is and how Aristotle approaches the de"nition by means of genera 
and di#erentiae. Using this method, Aristotle de"nes a human as ‘a featherless, two-
footed animal.’ However, it is questionable whether this is the true de"nition of a 
human due to two reasons. !erefore, we will articulate why this is not the true 
de"nition of a human and begin to look for a solution. In Part 2, we will attempt 

% Metaphysics 1037b34



29Issue X ◆ Spring 2023

How Can We Reach the True Definition of Something

to rectify this issue by showing that the ultimate di#erentia (teleutaia!diaphora) of 
something, is, in fact, the same as the end (telos), which lies at the heart Aristotle’s 
theory. In Part 3, we will attempt to reach the true de"nition of something by 
reconciling Aristotle’s method with its ultimate di#erentia (telos).

PART !: DEFINITION OF GENERA AND DIFFERENTIA 
What is a de"nition? !e de"nition of something is the account of its essence. 
Aristotle says, “For the essence is just what something is (ὅπερ γάρ τί ἐστι τὸ τί ἦν 
εἶναι)” (1030a3). !us, if we ask the question “what is a human?” then the answer to 
this question would require the essence of a human. Nevertheless, what does he mean 
by “what something is?” Let us look at another explanation for the essence to have a 
better grasp of it. Aristotle describes the essence as what it is said to be intrinsically:

!e essence of each thing is what it is said to be intrinsically. For the 
being for you is not the being for musical. For you are not intrinsically 
musical. [Your essence], therefore, is what you are [said to be] intrinsically. 
(1029b14-15)

You can be musical. However, you do not need to be musical. You can still be you 
without being musical. Being musical is not necessary for being you. !erefore, it 
is accidental. However, let us assume that there is some characteristic p essential 
to being X. Since to be p is necessary for X, X cannot be X without it. Hence, the 
essence of each thing is that which is necessary to being it. !us, it appears that the 
de"nition of a thing is the account of what is necessary to being it.

!en, what is the account (logos)? !e Greek word ‘λόγος (logos)’ means speech or 
thought. !us, the de"nition of something is the speech or thought of its essence. 
However, what we want to know is the essence of something (or what something 
is), since the main task of Metaphysics is the investigation of what something is. 
In that case, why do we need speech? !e reason may be that the essence of a thing 
does not belong to us; it belongs to the thing itself. !us, if we want to relate to and 
understand a thing, we need some mediator between it and us. It is the speech (logos) 
that belongs to us and expresses our understanding of it.5 !us, the de"nition belongs 
to us since a de"nition is a kind of an account. We come to the essence by means of 
a logical procedure of our language. And since an account consists of a subject and 
a predicate that states an attribute of the subject, the de"nition also has a linguistic 
structure composed of a subject and a predicate. Hence, since the de"nition of a 

$ In On the Soul, Aristotle explains in detail how we cognize and understand a sensible object logico-linguistically. When we perceive 
(aisthanesthai) a sensible object, imagination (phantasia) works. Because of imagination, a mental image (phantasma), the sensible form 
of a sensible object, is presented to us (427b20). And then, our thinking capacity, the mind (nous), thinks this mental image. !inking 
(noe") is the process in which the mind in the active sense makes the mind in the passive sense actually identical with the objects of 
thought (430a14-17). When we think a thing, we make the notion of it. When we try to understand an object, we combine notions 
and make an account such as “S is P.” !erefore, our understanding of the world is intertwined with our account. !e account of 
something expresses our understanding of it. (!is footnote refers to the Loeb translation.)



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thing is the account of its essence, the de"nition is the linguistic expression of that 
which is necessary to its being.

If the de"nition consists of a subject and a predicate, what kind of thing can be 
the subject of a de"nition? In other words, what can we de"ne? In Metaphysics, 
Aristotle deals only with substance (ousia) for the de"nition of something: “It is 
clear, therefore, that only of substance is there a de"nition” (1031a1). !erefore, only 
substance can be situated in the subject of the de"nition. What, then, is a substance?6

In his early book Categories, Aristotle divides a substance into two groups: particular 
objects and species.7 Individual things (tode ti; a “this something”), such as an 
individual man, are substances. And species (a human) and the genera of these 
species (an animal) are also substances. !is shows us that Aristotle thinks that both 
a particular substance (say, Socrates) and a species (say, a human) or the genus of a 
species (say, an animal) can be a substance in a broad sense. 

Nevertheless, Aristotle thinks that not all substances are proper candidates for the 
subject of their de"nitions. Of substances, we can only de"ne a species. !e de"nition 
is the account of the essence. Only things that are species of a genus have an essence: 
“Hence the essence will belong to things that are species of a genus and to nothing 
else” (1030a11-12). !erefore, only the species of a genus have their de"nitions.8 

How, then, does Aristotle express the essence of something in the predicate of its 
de"nition? !e subject of the de"nition of something is a species, so the predicate of 
its de"nition must be its genus and its di#erentiae. !is is made clear by the fact that 
he says a species is a genus plus di#erentiae: “the species are composed of the genus 
and the di#erentiae” (1057b7). Undoubtedly, he expresses the account of its essence 
with its genus and its di#erentia:

% What is a substance? !is is an extremely complicated question to answer since his explanations of a primary substance and a second-
ary substance in Categories and Metaphysics are di#erent. First, concerning a primary substance, in Categories, Aristotle explains that 
primary substances are particular objects (tode ti; a this something). In contrast, Aristotle says that the what-it-is (to ti esti) is primary in 
Book Z1 of Metaphysics (1027a14). Why are they di#erent? !e simplest hypothesis I can make is that, in Categories, he thinks that 
tode ti is primary, but in Metaphysics, he divides tode ti into matter (hyl#) and form (eidos). By dividing tode ti into matter (hyl#) and 
form (eidos), he thinks the what-it-is (to ti esti) is primary. Second, pertaining to a secondary substance, in Categories, Aristotle says that 
a species or the genera of a species are secondary. But in Metaphysics, species are regarded as form (eidos). However, he thinks that form 
is primary in Metaphysics. !ere would be more complexities related to what a substance is. Nevertheless, this is not our main project in 
this paper. What we want to know here is what can have a de"nition. !erefore, with the question of what a substance is, we will merely 
think that a substance is that which exists (being) such as 1) movable and perishable things in the sub-lunar level (animals or cups), 2) 
movable and eternal things (planets), and 3) something immovable and eternal (the prime mover). (1069a29-33, Metaphysics)
& Categories 2a13-18.
' Why does Aristotle think that there is no de"nition of a particular and perceptible substance? !ere are two reasons. First, an account 
does not admit the generation or destruction of something. (1039b23-30, Metaphysics) Perceptible and particular substances have 
matter. Substances having matter can come to be and pass away. However, an account only outlines whether something is or is not. 
!erefore, there is no de"nition of perceptible and particular substances. For instance, Socrates is a particular perceptible substance 
because Socrates has his body, which is matter. !erefore, Socrates is coming to be and passing away. We cannot de"ne Socrates qua 
Socrates. Although we can de"ne Socrates qua man, this is not a particular de"nition but his species de"nition. Second, a subject itself 
should not be present in the predicate of its de"nition. (1029b18-20, Metaphysics) !e predicate of a de"nition should explain its sub-
ject without using the subject in the predicate. For example, we should not say that the de"nition of a cup is “a cup is a cup that is . . .” 
However, if we attempt to de"ne a particular object, we will violate this rule. For instance, let us try to reach the de"nition of this cup. 
Its de"nition would be something like “this cup is a cup which is here.” !is disobeys the rule that a subject itself should not be present 
in the predicate of its de"nition. In addition, Aristotle also excludes the possibility of the de"nition of the genus of a species since the 
genus of a species does not have the essence. Nonetheless, we can see that we de"ne an animal as a perceptual living thing, for instance. 
It seems that, for Aristotle, we can give a de"nition of genus, but when we do this, we do not truly work on the question of what it is.



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We should "rst investigate de"nitions that are by division. For there is 
nothing else in the de"nition except the genus that is mentioned "rst and 
the di#erentiae; the other genera are in fact the "rst one along with the 
di#erentiae combined with it. (1037b28-30)

By dividing genera, Aristotle "nds the de"nition of something. First, he discovers 
the genus of something and then "nds the di#erentiae combined with the genus. For 
instance, Aristotle o#ers “a human is a featherless, two-footed animal” as a de"nition.9 
In this de"nition, a species (a human) is the subject, and the genus (animal) and the 
di#erentiae ‘two-footed’ and ‘featherless’ are predicated of it.

Nonetheless, before we try to follow how Aristotle reaches this de"nition, we should 
understand what a genus and a di#erentia are. !e genus of something is the common 
thing of what are distinct in species:

What is distinct in species is distinct from something, in something, and 
this latter thing must belong to both—for example, if an animal is distinct 
in species [from another], then, both are animals. Hence [two] things that 
are distinct in species must be in the same genus. For this is the sort of thing 
I call a genus, that by reference to which both things are said to be one and 
the same, and which is not coincidentally di#erentiated, whether as matter 
or otherwise. (1057b34-1058a1)

If we interpret the "rst sentence as “what is distinct in species (A) is distinct from 
something (B), in something (C),” we know that A and B are distinct in species, but 
both A and B belong to C. !en, C is the genus of A and B. For example, a human 
(A) and a horse (B) are distinct in species. However, the common thing they share 
is their genus (C). Both a human and a horse belong to the genus of animals (C). 
!erefore, animal (C) is the genus of a human (A) and a horse (B). 

In addition, a di#erentia is what makes the di#erence between A and B:

!is di#erentia, therefore, will be a contrariety (as is also clear from 
induction). For all things are divided by opposites, and it has been shown 
that contraries are in the same genus. For contrariety was seen to be complete 
di#erence, and all di#erence in species is di#erence from something, in 
something, so that this latter thing is the same for both and is their genus . 
. . Hence the di#erentia is a contrariety. (1058a8-16)

If we consider the di#erentia ‘two-footed,’ then between a human and a horse, a 
human is two-footed, but a horse is not. !erefore, this di#erentia makes a distinction 
between them within the genus “animal.” 

( Metaphysics 1037b12



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So far, we have investigated what a de"nition is and what we can de"ne in 
Metaphysics. !e de"nition of something is the account of its essence. !e subject 
of a de"nition is a species, and the predicate expresses its essence. Aristotle thinks 
that he can logico-linguistically express the essence of a species with its genus and 
its di#erentia and reach its de"nition. He does this by the method of division and 
classi"cation.10 He proceeds from genus to species by dividing genera and "nding 
di#erentiae. Accordingly, let us attempt to "nd the de"nition of a human using his 
method in order to understand the problem with his method.

!e initial task is to "nd its genus. Of things that exist in the universe, some are 
natural, and others are arti"cial. A human certainly belongs to the natural things 
category. Aristotle says that natural things are simple bodies, such as earth, "re, water, 
and air, and living bodies, such as plants and animals.11 Among natural things, a 
human is an animal. !erefore, a human is placed under the genus of animals. Since 
the genus of animals is the proximate genus of a human, we know the genus of a 
human. However, under the genus of animals, there are innumerable species (say, a 
horse, a bird, a dog, a whale, etc.). !us, to proceed from genus to species, we should 
"nd the di#erentiae of a human. Among animals, some are ‘footed,’ and others are 
not. A human belongs to footed animals. Of footed animals, some are ‘two-footed,’ 
and others are not (say, ‘four-footed’). A human has two feet. !erefore, a human is 
a two-footed animal. Nonetheless, there is still an abundance of two-footed animals: 
all types of birds, as well as humans, qualify as two-footed animals. We require more 
di#erentiae. How about ‘feathered? !e contrariety of ‘feathered’ is ‘featherless.’ 
A human does not have feathers. !erefore, a human is a featherless, two-footed 
animal. Is there another featherless, two-footed animal? If so, we should "nd another 
di#erentia. However, a human is the only featherless, two-footed animal. Hence, 
this is the de"nition of a human by means of genera. !e diagram below shows the 
journey of this division.

10 1037b29
11 Physics 192b9-10 



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How Can We Reach the True Definition of Something

We have followed the way in which Aristotle expresses the essence of a species 
by "nding the most speci"c genus and di#erentiae. As a result, we have reached 
the de"nition of a human: A human is a featherless, two-footed animal. !is 
de"nition sounds plausible. No one would deny it. !e di#erentiae ‘featherless’12 
and ‘two-footed’ are unquestionably essential characteristics of a human. A human 
is intrinsically ‘two-footed’ and ‘featherless.’ A human is the only animal having 
the di#erentiae ‘featherless’ and ‘two-footed’ together. !erefore, this de"nition is 
exclusive to a human being.13 However, is this de"nition satisfactory? !e de"nition 
of something is the account of its essence. Does this de"nition capture the essence 
of a human quite well? We can be satis"ed with this de"nition if we regard ourselves 
nothing but featherless, two-footed animals! However, we are not satis"ed with the 
de"nition “a human is a featherless, two-footed animal” for two reasons.

First, the combination of the di#erentiae ‘featherless’ and ‘two-footed’ are not what 
makes us humans in a positive sense even though these di#erentiae may set us apart 
from all other animals. In other words, these qualities express not what we ourselves 
are intrinsically but what other animals are not. Aristotle says that the essence (to ti 
#n einai; ‘the-what-it-was-to-be’) is the cause: “It is evident, accordingly, that we are 
inquiring into the cause. !is is the essence, logico-linguistically (φανερὸν τοίνυν 
ὅτι ζητεῖ τὸ αἴτιον (τοῦτο δ᾿ ἐστὶ τὸ τί ἦν εἶναι, ὡς εἰπεῖν λογικῶς))” (1041a27). 
What is the cause of a thing X? !is question is the same question as “what causes 
X to be X?” !e answer is the essence of X. Since the essence of X belongs to X, X is 
X. If the account “a human is a featherless, two-footed animal” is the true de"nition 
of a human, then the essence of a human is the combination of the di#erentiae 
‘featherless’ and ‘two-footed.’ It means that because of these di#erentiae ‘featherless’ 
and ‘two-footed,’ we are humans. Is it because “featherlessness” and “two-footedness” 
belong to me that I am a human being? De"nitely not; we know that what a human 
is goes beyond this de"nition. What is it to be a human? To live a human life is 
not just to exist as a creature which has no feathers and has two legs. Although 
the di#erentiae ‘featherless’ and ‘two-footed’ seem to be essential components for a 
human because a human can move owing to two feet, these two essential qualities do 
not seem to make a human a human. How can we capture what it is to be a human 
in the de"nition of a human that Aristotle suggests?

Without capturing the essence of something, it is hard to say that we know what 
it is. Knowing what it is means that we know its essence and have reached its true 
de"nition. !erefore, if the di#erentiae of something do not capture its essence, it is 
unlikely that we know what it is and have reached its true de"nition. 

12  It is strange to say that ‘featherless’ is essential for a human being. How can we talk about some non-existent 
qualities? We should say what a human has instead of saying what a human does not have. We will look into this 
problem in a few paragraphs.

13 In fact, there are some other ‘two-footed, featherless animals,’ such as kangaroos and Tyrannosaurus Rex, although 
I assume that Aristotle is not aware of them in his life, unfortunately.



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!e second reason why the de"nition reached by genus and di#erentiae is insu)cient 
is that the essence of something cannot be changeable, but the combination of 
essential di#erentiae seems subject to change. !ere are still copious di#erentiae that 
are essential characteristics of a human. A human is a ‘vertebrate.’ A human is ‘warm-
blooded.’ A human is ‘two-handed.’ If we begin the task of de"ning a human with 
another di#erentia, we will reach another de"nition of a human being. For instance, 
let us try to "nd the de"nition of a human with the di#erentia ‘vertebrate.’ We know 
the genus of a human is an animal. We can divide animals into ‘vertebrates’ and 
‘invertebrates.’ A human is a ‘vertebrate.’ !ere are innumerable vertebrate animals. 
We can break them down into ‘warm-blooded’ and ‘cold-blooded.’ Since a human is 
‘warm-blooded,’ the tentative de"nition of a human is a warm-blooded, vertebrate 
animal. However, we know that there are a lot of warm-blooded, vertebrate animals. 
From this, we can add the previous example—‘featherless’ and ‘two-footed’—to the 
de"nition we have reached in this paragraph. !en, at the end of this process, the 
de"nition of a human is a featherless, two-footed, warm-blooded, vertebrate animal. 

!is shows that we will sometimes de"ne a human as a featherless, two-footed 
animal, and at other times we will de"ne a human as a featherless, two-footed, warm-
blooded, vertebrate animal. Hence, the de"nition of something changes whenever we 
"nd another di#erentia. Indeed, Aristotle himself is aware of the problem with his 
dichotomous method in his book Parts of Animals: 

Now if man was nothing more than a cleft-footed animal, this single 
di#erentia would duly represent his essence. But seeing that this is not the 
case, more di#erentiae than this one will necessarily be required to de"ne 
him; and these cannot come under one division; for each single branch of a 
dichotomy ends in a single di#erentia, and cannot possibly include several 
di#erentiae belonging to one and the same animal. It is impossible then to 
reach any of the ultimate animal forms by dichotomous division. (644a7-
13, Parts of Animals)

!e di#erentiae can be changed because one and the same genus can be dichotomously 
divided in many di#erent ways. He thinks that there is no single de"nition of an 
animal species. How can the essence of something change depending on the method 
of di#erentiation? How can sometimes ‘featherless and two-footed’ make a human be 
a human and other times ‘featherless, two-footed, warm-blooded, vertebrate’ make a 
human be a human? 

If the essence is unchangeable, the essential di#erentia should also be unchangeable. 
However, the combination of essential di#erentiae seems to be changeable insofar as 
we can swap out one di#erentia for another. Why does this problem occur? We do 
not have a way to guide us among these decisions. We do not have a basis from which 
to decide because they both capture essential qualities. !erefore, even if we follow 



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the methodical dichotomous division of the genus, we might run into problems. 
For example, consider that we want to de"ne a cup. !e genus of a cup is ‘vessel.’ 
However, both a house and a cup can be considered to be vessels. Now, we need a 
di#erentia to distinguish them. A house is ‘doored.’ In contrast, a cup is ‘doorless.’ 
According to Aristotle’s method, we "nd ourselves in a position of saying that a cup is 
a doorless vessel. We know that a door never has anything to do with a cup. Similarly, 
though it is supposedly a “featherless, two-footed animal,” a feather does not have 
anything to do with a human at all. Why do we consider some feature that does not 
apply to us as essential? How can something that we do not have make us what we 
are? By following the method of di#erentiation, we "nd ourselves including in the 
de"nition essential qualities that are lacking in the species and that have no relevance 
to a human and a cup. Hence, even in the most careful application of Aristotle’s 
method, we still reach a de"nition that includes arbitrarily chosen di#erentiae.

So far, we have seen the reasons why the account “a human is a featherless, two-
footed animal” is not the true de"nition of a human. !e combination of the 
essential di#erentiae of X might not make X be X. It is true that the combination of 
the essential di#erentiae of X may tell us that X is di#erent from any others due to 
the combination of the essential di#erentiae of X. However, it does not mean that 
these qualities signify what X itself is intrinsically. It only tells us what others are not. 
In addition, the arbitrariness of this method makes it vulnerable for us to reach the 
true de"nition.

Unless we know the essence of something, it is unlikely that we reach the true 
de"nition of something. We might have to be satis"ed with a mere taxonomic 
de"nition: “what de"nitions are like” (1038a35) while we pretend to think we know 
the essence of something. !erefore, in Part 2, we will "nd a solution to overcome 
this problem.

PART ": THE ULTIMATE DIFFERENTIA #TELEUTAIA 
DIAPHORA$ IS THE END #TELOS$
In Part 1, we attempted to understand what a de"nition is, what we can de"ne, and 
how Aristotle reaches the de"nition of something by the method of division. However, 
this method is not enough to capture the essence of something. To overcome this 
problem and reach the true de"nition of something, we need to consider the ultimate 
di#erentia (teleutaia diaphora). As Aristotle presents it, the ultimate di#erentia is 
the one that cannot be further divided. For instance, when we de"ne a human as 
a featherless, two-footed animal, we divide animals with the "rst di#erentia ‘two-
footed’ and the second one ‘featherless.’ !en, we reach a point in the procedure at 
which it becomes impossible. Nevertheless, it seems to me that the more signi"cant 
feature of the ultimate di#erentia is its aptitude for capturing the essence of a thing. 



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Dianoia: The Undergraduate Philosophy Journal of Boston College

!erefore, the goal of Part 2 is to investigate how to arrive at an ultimate di#erentia 
that is not arbitrary but that reliably captures the essence of the thing.

If we know the ultimate di#erentia of something, we will reach its satisfactory 
de"nition. Aristotle thinks this is the correct procedure to follow: “!us it is evident 
that the de"nition is the account composed of the di#erentiae, or, if it is in accord 
with the correct procedure, the ultimate one” (1038a28-30). Without knowing the 
ultimate di#erentia of something, we might get lost in the labyrinth of the di#erentiae 
and not be able to "nd its true de"nition.

Aristotle suggests that the ultimate di#erentia will be the form and the substance: 
“If, then, we take a di#erentia of a di#erentia, one di#erentia—the ultimate one 
(teleutaia diaphora)—will be the form (eidos) and the substance (ousia)” (1038a24-
25). !is is the only clue we can use in order for us to reach the true de"nition of 
something. Hence, the aim of Part 2 is to understand that the ultimate di#erentia 
of something is its end (telos) by utilizing this hint. Because Aristotle says that the 
ultimate di#erentia is the form and the substance, the exploration of the form (eidos) 
and substance (ousia) is our primary task.14

 !en, what are the form and the substance?15 Aristotle thinks that form and 
the substance are the same: “And by form I mean the essence of each thing and the 
primary substance” (1032b1). And we have seen that the primary substance is the 
what-it-is.16 !e essence of each thing is what it is. !erefore, the form is the same as 
the substance. In addition, he says that the form and the substance are the activity: 
“So it is evident that the substance and the form are activity (ὥστε φανερὸν ὅτι 
ἡ οὐσία καὶ τὸ εἶδος ἐνέργειά ἐστιν)” (1050b1). !erefore, since the ultimate 
di#erentia is the same as the form and the substance, and the form and the substance 
are the same as the activity, the ultimate di#erentia will express the activity of the 
thing de"ned.

 !en, what is the activity (energeia) of something? Aristotle says, “the 
activity is the end (τέλος δ᾿ ἡ ἐνέργεια)” (1050a9). And the end is the characteristic 
activity of a thing. It is that activity for-the-sake-of-which something is, without 
which it would not be what it is. !en what is the end (telos), or that for-the-sake-of-
which?

14 Indeed, this direction is reasonable. !e de"nition is the account of the essence. In fact, Aristotle says that the 
essence belongs to the form and the activation: “Since the essence belongs to the form and the activation (τὸ 
γὰρ τί ἦν εἶναι τῷ εἴδει καὶ τῇ ἐνεργείᾳ ὑπάρχει)» (1043b1). !us, if we know the form and the activation 
of something, we immediately come to know its essence. Since the ultimate di#erentia of something is its form 
and its substance, and since our project aims to know what the ultimate di#erentia of something is, this is the 
correct path toward its true de"nition to investigate the form, the activation, and the substance.

15 We do not undertake a deep investigation of the meanings of the form (eidos) and the substance (ousia). 
Remember that Part 2 of this paper aims to understand that the ultimate di#erentia (teleutaia diaphora) and the 
end (telos) are the same. !e deep exploration of these terms will distract the big picture.

16 1028a14.



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!e end, and this is the for-the-sake-of-which—for example, of taking walks health is 
the end. For why does [he] take walks? “In order that he may be healthy,” we say. And 
in speaking that way we think we have presented the cause. Also, anything, then, that 
comes to be as an intermediate means to the end, when something else has started 
the movement: for example, in the case of health, making thin, purging, drugs, or 
instruments, since all these are for the sake of the end, although they di#er from each 
other in that some are instruments and others works. (1013a33-1013b3)

!e end of something or some action is that for-the-sake-of-which. Why does he 
take a walk? Or what is the function of him taking a walk? !e end of taking a walk 
is health. Why does he make himself thin? Or what is the purpose of making himself 
thin? To be healthy. Why does he take drugs? To be healthy. All the instruments and 
actions here have an end, which is health. !is is because when we ask a question 
about instruments or works with the interrogative ‘why,’ one type of answers uses 
“for.”17 Why do you eat? For health. Why do you work? For making money or for 
serving the society. Hence, since the ultimate di#erentia is the activity, and since the 
activity and the end are the same, the ultimate di#erentia is the end. For instance, let 
us consider a cup. !e end of a cup is to contain liquid. !en, the ultimate di#erentia 
of a cup is ‘containing liquid’ and ‘containing non-liquid (say, solids).’ We know that 
a cup is for containing liquid. !erefore, the ultimate di#erentia of a cup is its end.

So far, we have demonstrated that the ultimate di#erentia (teleutaia diaphora) of 
something expresses its end (telos). In order to grasp the entire demonstration in the 
simplest way, let us look at its summary in a Euclidean way:

“!e ultimate di#erentia (teleutaia diaphora) is the form (eidos) and the substance 
(ousia)” (1038a25).

“!e substance and the form are activity (energeia)” (1050b1).

“!e activity (energeia) is the end (telos)” (1050a9).

 !e ultimate di#erentia (teleutaia diaphora) is the end (telos).

!is shows us that Aristotle’s terms ‘the ultimate di#erentia (teleutaia diaphora)’, ‘the 
form (eidos)’, ‘the substance (ousia)’, ‘the activity (energeia)’, ‘the end (telos)’ are the 
same. !erefore, the ultimate di#erentia of something is the same as its end.

At the end of Part 1 of this paper, we have seen the necessity of overcoming the 
problem with Aristotle’s method. Hence, we have looked for the possibility to 
surmount this issue in Part 2. Finding a solution begins with the fact that the ultimate 
di#erentia of something is the same as its form and its substance (or essence).18 We 

17 Physics 198a15-2
18 It is safe to say that the essence and the substance are the same in that both signify “what something is.”



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Dianoia: The Undergraduate Philosophy Journal of Boston College

have "gured out that the ultimate di#erentia of something is eventually the same as 
its end (telos). Indeed, Aristotle claims that the essence of something and its end are 
the same: “What a thing is and its purpose are the same (τὸ μὲν γὰρ τί ἐστι καὶ τὸ 
οὗ ἕνεκα ἕν ἐστι; the literal translation is “the essence and for-the-sake-of-which are 
the same”)” (198a25, Physics). 

Now, the procedure by genus and di#erentia that leads to the de"nition “a human is a 
featherless, two-footed animal” is like the process that we would use. However, this is 
not the complete version of this process. Indeed, Aristotle seems to acknowledge that 
this is not the perfect procedure for the true de"nition of something at the end of Z 
12: “Where de"nitions by division are concerned, then, let this much su)ce as a "rst 
statement as to what they are like” (1038a35). Here, Aristotle seems to be suggesting 
that though this de"nition serves as an example as to how one might proceed by 
division, he is not necessarily asserting the given de"nition to be complete.

Nevertheless, we will not dismiss this procedure entirely. Instead, since we now 
know that the ultimate di#erentia of something is its end, we will rely on a method 
like genus and di#erentiae, but we will involve a teleological ultimate di#erentia.19 
!erefore, let us continue on to see whether we can fully capture the essence by 
"nding the ultimate di#erentia: that is, telos.

PART %: THE DEFINITION OF MEANS OF GENERA AND 
THE END #TELOS$
In Part 1, we "gured out the reason why we cannot reach the true de"nition of 
something. !e cause of the problem is that we bifurcate the genera and species 
without knowing the ultimate di#erentia of something. !erefore, in Part 2, we 
have made an e#ort to know what the ultimate di#erentia of something is. !e 
answer is that the ultimate di#erentia of something is its end. !erefore, "nding out 
the ultimate di#erentia involves not just "nding a feature that obeys contraries, but 
"nding a purposive division under the genus. Now, we should assess whether we can 
reach the true de"nition of something when we know its end. !is is the aim of Part 
3.

Let us imagine an example. Your friend asks, “What is the purpose of a house?” We 
may say that the purpose of a house is to shelter property and bodies. !is is the 
activity of a house:

!at is why of [1] those who give de"nitions, . . . [2] !ose, on the other 
hand, who propose that it is a receptacle to shelter property and bodies, or 
something else of that sort, are speaking of the activation (energeia). . . For 

19 See Page 18 or 1038a28-30.



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How Can We Reach the True Definition of Something

it seems that [2] the account that it is given in terms of the di#erentiae is of 
the form (eidos) and the activation (energeia). (1043a13-19)

!e activity of a house is the same as its purpose. Since we know the purpose of a 
house, we know what a house is, and we can reach its true de"nition: “A house is a 
receptacle (or vessel) to shelter property and bodies.”

!erefore, if we know a thing’s end, we can reach its true de"nition. However, as 
stated above, this does not reject Aristotle’s method of division and classi"cation. 

!e only di#erence is that we do not divide the genus with arbitrary bifurcations. 
We add the ultimate teleological di#erentia to his method. For example, let us de"ne 
a doctor with Aristotle’s method and the ultimate di#erentia. !e end of a doctor is 
to cure patients. With the method of genus and di#erentia, the genus of a doctor is 
a profession. !e ultimate di#erentia of a doctor is to cure or not to cure since this 
is the end of a doctor. !erefore, the true de"nition of a doctor is that a doctor is a 
professional who cures patients. Likewise, let us de"ne a cup. !e genus of a cup is 
a vessel. !is is because there are many types of vessels, so that vessel is not a species 
but the genus of a cup. Since we know the genus of a cup, if we know its ultimate 
di#erentia, we can reach its true de"nition. !e ultimate di#erentia (the end) of a 
cup is to contain liquid. !us, its true de"nition is a vessel that contains liquid. 

So far, all the examples we have examined are artefacts. Since we create them with 
our craft, we certainly know the end of them. We are the e)cient cause of arti"cial 
things. We are builders who build a house or a cup. If we do not know the purpose 
of a house or a cup, it is impossible to make them. Hence, we can reach the true 
de"nition of arti"cial things since we know their purposes.

Nevertheless, let us "nally attempt to reach the de"nition of a human. If we know the 
end of a human, then we can reach its true de"nition. What is the end of a human? 
Let us attempt to "nd it in a two-fold manner if we can.

On the one hand, Aristotle suggests that knowledge is our end: “All humans by 
nature desire to know” (980a21). !is is possible because a human has reason (logos). 
!is is the most fundamental di#erence between humans and other living creatures: 
“Man alone among the animals has speech (λόγον δὲ μόνον ἄνθρωπος ἔχει τῶν 
ζῴων)” (1253a10, Politics). A human can think (noe"), understand (phrone"), and 
judge (krin"). !anks to this faculty, we naturally pursue knowledge. What do we 
want to know? We experience the world and understand it. However, a human’s 
desire to know does not merely signify knowledge of phenomena–that is, knowledge 
that “the ball I grab falls down to the ground when I unfold my hand” or “the sky 
is blue.” We want to know “why?” What causes these things? What is the cause of 
phenomena? !e primary starting-points and causes are what we want to know. !is 
is wisdom, as stated above. Hence, all human beings think–no#sis–to know. !is 
is our activity (energeia). !is is our goal (telos). !erefore, a human being is for 
thinking.

On the other hand, in Nicomachean Ethics, Aristotle suggests that happiness is the 
"nal end of a human: “So happiness appears to be something complete and self-
su)cient, it being an end of our actions” (1097b20). In Book 1 of Nicomachean 
Ethics, Aristotle thinks that happiness is an activity of soul in accord with virtue: 



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Dianoia: The Undergraduate Philosophy Journal of Boston College

…if this is so, then the human good becomes an activity of soul in 
accord with virtue, and if there are several virtues, then in accord with 
the best and most complete one (εἰ δὴ οὕτως, ἀνθρώπου δὲ τίθεμεν 
ἔργον ζωήν τινα, ταύτην δὲ ψυχῆς ἐνέργειαν καὶ πράξεις μετὰ 
λόγου, σπουδαίου δ᾿ ἀνδρὸς εὖ ταῦτα καὶ καλῶς, ἕκαστον δ᾿εὖ 
κατὰ τὴν οἰκείαν ἀρετὴν ἀποτελεῖται). (1098a16-18, Nicomachean 
Ethics)

Happiness is an activity of soul in accord with virtue. To achieve happiness, we 
utilize our logos (praxeis meta logou). By using our logos, the end of human beings is 
happiness. !is shows us that the end (telos) of a human and the activity (energeia) 
work together. It seems that our happiness does not have to do with the divine being. 
However, Aristotle thinks that this happiness ultimately occurs with the divine being:

If happiness is an activity in accord with virtue, it is reasonable that it would accord 
with the most excellent virtue, and this would be the virtue belonging to what is 
best. So whether this is the intellect or something else that seems naturally to rule, to 
command, and to possess intelligence concerning what is noble and divine, whether 
it itself is in fact divine or the most divine of the things in us—the activity of this, 
in accord with the virtue proper to it, would be complete happiness. And that this 
activity is contemplative has been said. (1177a13-18, Nicomachean Ethics)

Our happiness ultimately belongs to the divine being. !is is complete happiness. 
!is is contemplation. We think and contemplate. !is is our activity (energeia). We 
want happiness. !is is our goal (telos). !is happiness is contemplation. !erefore, a 
human being is for contemplating.

Both thinking and contemplating–our activity (energeia)–are possible because of our 
logos. Both thought and contemplation–(telos)–occur with the divine being.20 Since 
the genus of a human is animal, if we accept Aristotelian teleological view, we can 
accurately de"ne a human as a thinking—or rational— animal.21 

Aristotle at various times refers to human as a rational animal, at other times as a 
featherless two-footed animal. We have seen two distinct de"nitions of a human 
that appear throughout Aristotle’s corpus. !ey are not just two interchangeable 
de"nitions with equal utility. One is taxonomic, an example of what de"nition is 

20 Metaphysics 1026a15-18, 1026a28-31
21 One might point out that if we de"ne a human as a political animal, it is also arbitrary. Sometimes, we de"ne a 

human as a political animal. At other times, we de"ne a human as a rational animal. It is a reasonable argument. 
However, it seems that for Aristotle, political activity is subordinate to rational activity. A human being takes 
pleasure in society and lives an ethical life because of reason. !en, one might ask again how a political life 
and a contemplative life can be described under the realm of the de"nition of a human: “A human is a rational 
animal.” A politician is a practitioner who pursues practical wisdom. A philosopher, by contrast, pursues 
speculative wisdom. Insofar as both a politician and a philosopher seek wisdom with reason, both of them are 
rational.



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How Can We Reach the True Definition of Something

like. !e other one represents the achievement that can be made when we know the 
true end of a thing—that is, true de"nition. Now it gives us a model to follow in 
other cases and an appreciation for the di)culty of reaching de"nition in cases where 
the telos is hidden from us. 

CONCLUSION 
For Aristotle, the question “What is being?” is signi"cant. !is question is the same 
as the question “What is a substance?” !is question leads us to what the essence is. 
Since the de"nition and the essence are one in some sense, if we know the essence 
of something, we can reach its de"nition. However, without knowing the ultimate 
di#erentia of something, it is unlikely for us to reach its true de"nition. For artifacts, 
we have access to the true purpose of anything that we make as humans. In addition, 
we are for thinking.22 So, we can reach the true de"nitions of artifacts and ourselves. 
However, for natural things other than humans, it is unclear whether we can know 
their purposes or not. We do not know whether our interpretation of phenomena 
is the same as the true end of a thing. While we experience the world, we observe 
the phenomena of things. We see an acorn growing up and becoming an oak tree. 
We think the "nal cause of an acorn is to become an oak tree. Even if Aristotle says 
that our perception of proper objects is always true,23 does Aristotle think that we 
can know the ends of all the natural things? For instance, what is the unique end of 
the mature oak tree? It is di)cult to answer. All things are ordered and related to 
each other, contributing to the good of the world. !ey contribute to the order, the 
beauty, and the good of the world in their own ways. 24 !erefore, unless we know the 
order of the world, it is unlikely that we know the true end of things.

Hence, for natural things, taxonomic de"nitions might have to be enough. If we 
know their true ends, we will be happy. However, we may or may not know their 
purposes. So, we may know only what the de"nition is like. A horse, for instance, 
is approximately a four-legged hoofed animal, but this de"nition misses its essence. 
However, this does not mean that Aristotle’s investigation of being and pursuit of 
wisdom in Metaphysics has failed. !e mere taxonomic de"nition can be useful 
for scientists as Aristotle was a scientist who tried to understand the physical world 
scienti"cally. Furthermore, if the goal of humans is thinking and contemplating 
using our logos, then his task cannot be considered a failure. !inking and knowing 
are not the same. We are not sure whether we can know, but we can continue to seek 
wisdom about, for each thing that exists, the end.

22 !ere is a possibility that Aristotle might think that not every human being is for thinking. For example, while 
natural slaves are humans, are they indeed for thinking? In addition, poets compose not by using reason but 
by inspiration. !is problem led us to the question of whether we can even de"ne a human. However, this is 
a wholly di#erent matter. Although signi"cant, we will follow Aristotle’s account of the goal of a human in 
Metaphysics and Ethics in this essay.

23 De Anima 427b13
24 Metaphysics 1075a15-25



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REFERENCES

Aristotle, Metaphysics. Loeb Classic Library edition. Trans. Hugh Tredennick. 
Cambridge, Massachusetts: Harvard University Press, 1933.

Aristotle, Metaphysics. Trans. C.D.C. Reeve. Hackett Publishing Company, Inc, 
2016.

Aristotle, On the Soul. Loeb Classic Library edition. Trans. W.S. Hett. Harvard 
University Press, 1936.

Aristotle, Categories. Trans. J. L. Ackrill. Oxford: Clarendon Press, 1963.

Aristotle, Parts of Animals. Trans. William Ogle. New York: Garland Publishing, 
1987

Aristotle, Physics. Trans. Robin Water"eld. New York: Oxford University Press, 1996.

Aristotle, Politics. Trans. Carnes Lord. !e University of Chicago Press, 2013.

Aristotle, Nicomachean Ethics. Trans. Robert C. Bartlett and Susan D. Collins. 
Chicago and London: !e University of Chicago Press, 2011.


