2004 The Review of Metaphysics 58(2): 333-365. AQUINAS ON THE NATURE OF HUMAN ...

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by  Lacy, Meagan

© 2004 The Review of Metaphysics 58(2): 333-365.
AQUINAS ON THE NATURE OF HUMAN BEINGS
JASON T. EBERL
I
In this paper, I provide a formulation of Thomas Aquinas’s account of the nature of
human beings for the purpose of comparing it with other accounts in both the history of
philosophy and contemporary analytic philosophy.1 I discuss how his apparently dualistic
understanding of the relationship between soul and body yields the conclusion that a human
being exists as a unified substance composed of a rational soul informing, that is, serving as the
specific organizing principle of, a physical body. I further address the issue of Aquinas’s
contention that a human rational soul can exist without being united to a body and show how this
ability of a human soul2 does not contradict the thesis that a human being exists naturally as
embodied. I will also respond to two related questions. First, what accounts for the
individuation of human beings as distinct members of the human species? Second, what is the
principle of identity by which a human being persists through time and change?
II
According to Aquinas, a human being is a person. Aquinas adopts the definition of
person developed by Boethius: “an individual substance of a rational nature.”3 An example of
an individual substance4 is Bill Clinton. As an individual substance, Bill Clinton can be

1 I do not accomplish such a comparison in this paper, but plan to do so in future projects.
2 The terms “rational soul” and “human soul” will be used interchangeably throughout this paper.
3 See Boethius, Contra Eutychen et Nestorium, III; Aquinas, Summa theologiae [ST] I, q. 29, a. 1. All translations
are my own and, unless otherwise specified, are taken from the Leonine edition of Aquinas’s works: S. Thomae
Aquinatis Doctoris Angelici Opera Omnia (Rome: Commissio Leonina, 1882- ).
4 By this term, Aquinas intends the Greek term hypostasis (Latin: suppositum). The terms hypostasis and
suppositum are logically distinct in Aquinas’s mind, but they refer to the same thing in reality; hence, I make no
distinction between the terms here. See Aquinas, Summa contra Gentiles [SCG], bk. IV, ch. 38.
2
contrasted with humanity, which is not an individual substance, but is the nature in which many
individual substances—including Bill Clinton, William Shakespeare, and myself—share.
Being of a rational nature, that is, having an intellective mind, distinguishes human
beings from other material substances:
The form and species of a natural thing are known through their proper
operations. Now the proper operation of a human being, insofar as he is a human
being, is to understand and use reason. Hence the principle of this operation,
namely the intellect, must be that by which a human being is categorized by
species.5

In general, a person is a being that exists on its own with a specific nature, shared with
other beings of its kind, to be rational.6 A human being is not simply a person, however. In
addition to being rational, a human being is a sensitive, living, and corporeal substance. Human
beings have a material nature:
It belongs per se to a human being that there be found in him a rational soul and a
body composed of the four elements. So without these parts a human being
cannot be understood, and they must be placed in the definition of a human being;
so they are parts of the species and form.7

Aquinas further distinguishes human beings—from other types of persons—as rational
animals:
Animal indeed is predicated of a human being per se, and similarly rational of
animal. Hence this expression, rational animal, is the definition of a human
being.8

Aquinas refers to human beings as essentially animal, because, through their material bodies,
human beings share certain essential qualities with other members of the animal genus. The

5 Aquinas, Quaestio disputata de anima [QDA], q. un., a. 3. Cf. QDA, q. un., a. 1, sed contra; SCG, bk. II, ch. 60;
Aquinas, In decem libros ethicorum Aristotelis ad Nicomachum expositio [In NE], bk. I, lect.10; In NE, bk. X, lect.
10.
6 Aquinas recognizes different types of beings as persons. In addition to human beings, Aquinas claims that angels
are persons and that God exists as three distinct persons. Since my interest in this paper is solely with human
persons, I will not entertain any further discussion of such other types of persons.
7 Aquinas, In Boethii De trinitate expositio [In BDT], q. 5, a. 3.
8 Aquinas, In duodecim libros metaphysicorum Aristotelis expositio [In M], bk. VII, lect. 3, §1326. Ed. M. R.
Cathala and R. M. Spiazzi (Turin: Marietti, 1950).
3
primary exemplification of such similarity is the capacity for sense-perception. A human body,
though, is unique among other kinds of animal bodies in that it is organized not only to support
the capacity for sense-perception, but also the capacity for rational thought. The disposition of a
human body is determined by its having a rational soul as its substantial form. As a substantial
form, a human rational soul is responsible for (1) the esse (being) of a human being, (2) the
actualization of the matter composing a human being, and (3) the unity of existence and activity
in a human being.9
A human soul and the material body of which it is the substantial form are not two
separately existing substances. A substantial form is the actualization of a material body.
Aquinas asserts,
Body and soul are not two actually existing substances, but from these two is
made one actually existing substance. For a human being’s body is not actually
the same in the soul’s presence and absence; but the soul makes it exist actually.10

The intrinsic unity of matter and substantial form, body and soul, is responsible for the
unified existence of a human being. Against the Platonic conception of a human soul as a
substance which moves another substance—a human body—as an efficient causal agent,
Aquinas contends, “If you say that Socrates is not one simply, but one due to the aggregate of
mover and moved, many incoherencies follow.”11 The primary incoherency is that Socrates
would not be one being unqualifiedly (unum simpliciter).12 If Socrates is not unum simpliciter,
then he cannot count as a substance.
Aquinas notes various ways in which something may be considered a unity. For
example, a heap of stones is a unity in terms of the constituent stones being spatially continuous,

9 See SCG, bk. II, ch. 68; Aquinas, In Aristotelis librum De anima commentarium [In DA], bk. II, lect. 2.
10 SCG, bk. II, ch. 69.
11 Aquinas, De unitate intellectus contra Averroistas [DUI], ch. III.
12 See ST I, q. 76, a. 1.
4
a house is a unity in terms of its constituent parts being functionally organized in a certain
fashion, and a mover and that which it moves are a unity in terms of their agent/patient
relationship.13 None of these types of unity count as substantial unity, though, according to
Aquinas; that is, they are not unum simpliciter. Examples of things that are unum simpliciter,
according to Aquinas, are elemental substances, certain mixtures of elemental substances,
immaterial substances, and living organisms.14 The notion that Socrates is an aggregate of a
mover and that which it moves is analogous to the aggregate of a sailor and the ship he pilots.
One would not say that a sailor and his ship compose one substance; analogously, one would not
say that Socrates’s soul—the mover—and his body—the moved—compose one substance.15
One could say that a sailor and his ship, as well as Socrates’s body and soul on the Platonic
account, are unified in a certain respect or compose an aggregate sum, but such unity would not
be unum simpliciter.
A human being is not merely an aggregate of body and soul. A material human body and
its substantial form are metaphysically distinct, but neither of them alone is a substance. A
human being does not naturally exist without being composed of both a material body and a
rational soul. She is an individual substance brought about through a rational soul’s informing a
material body. Typically,16 when matter is informed by a rational soul, a new ontological
entity—a human being—comes into existence.

13 See QDA, q. un., a. 10; SCG, bk. II, ch. 57.
14 See Robert Pasnau, Thomas Aquinas on Human Nature (New York: Cambridge University Press, 2002), 88.
15 Aquinas states that two things that are joined by a contact of power (contactus virtutis), that is, one thing is the
efficient cause of change in the other, does not result in an unqualified unity. See SCG, bk. II, ch. 56; Norman
Kretzmann, The Metaphysics of Creation: Aquinas’s Natural Theology in Summa Contra Gentiles II (New York:
Oxford University Press, 1999), 278-90.
16 This qualifier is utilized here due to Aquinas’s understanding of the Incarnation of Christ, in which the unified
substantial existence of both human and divine natures precludes a new ontological entity having come into
existence when Christ’s soul assumed a human body. See Aquinas, Compendium theologiae [CT], ch. 211.
5
An analogous example is the case of salt. The elements sodium and chloride, which are
substances in themselves, come together to form a new substance—salt. When salt comes into
existence, the sodium and the chloride each cease to exist as distinct substances; though they
persist virtually as parts of a substance.17 Salt has a set of properties that is not merely the result
of combining the sets of properties had by sodium and chloride, respectively. The composition
of a human being by soul and body is not exactly the same as the case of salt, for soul and body
do not exist as distinct substances prior to composing a human being. However, soul and body,
like sodium and chloride, do not exist as distinct ontological kinds in a composite human being.
Neither is it the case that the set of properties had by a human being is merely the result of
combining the sets of properties had by her soul and body, respectively.
To summarize, neither a human soul nor the matter it informs alone is a substance.
Rather, the two together compose a substance—a human being. A human being is not identical
to her soul or her informed material body. Rather, a human being is composed of her informed
material body: “a human being is said to be from soul and body just as from two things a third is
constituted that is neither of the two, for a human being is neither soul nor body.”18
Aquinas holds that composition is not identity: something A may exist as composed of
something else B, but A is not identical with B.19 Aquinas’s adoption of this thesis is found in a

17 For his account of how elements combine to form a new substance, see Aquinas, De mixtione elementorum; SCG,
bk. IV, ch. 35; In M, bk. VII, lect. 17, §1680; ST I, q. 47, a. 2; ST I, q. 76, a. 4, ad 4; QDA, q. un., a. 9, ad 10.
18 Aquinas, De ente et essentia [DEE], ch. II. Cf. ST I, q. 75, a. 4.
19 Aquinas’s notion of composition without identity is similar to the contemporary notion of constitution without
identity. For elucidation and defense of the latter notion, see Lynne Rudder Baker, “Why Constitution Is Not
Identity,” The Journal of Philosophy 94 (1997): 599-621; Baker, “Unity Without Identity: A New Look at Material
Constitution,” in Midwest Studies in Philosophy 23, ed. Howard Wettstein (Oxford: Blackwell, 1999), 144-65;
Frederick Doepke, “Spatially Coinciding Objects,” Ratio 24 (1982): 45-60; Mark Johnston, “Constitution Is Not
Identity,” Mind 101 (1992): 89-105; E. J. Lowe, “Instantiation, Identity and Constitution,” Philosophical Studies 44
(1983): 45-59; J. M. Shorter, “On Coinciding in Space and Time,” Philosophy 52 (1977): 399-408; Judith Jarvis
Thomson, “Parthood and Identity Across Time,” The Journal of Philosophy 80 (1983): 201-20; Thomson, “The
Statue and the Clay” Nous 32 (1998): 149-73; David Wiggins, “On Being in the Same Place at the Same Time,” The
Philosophical Review 77 (1968): 90-5. For application of this notion to the issue of human nature, see Lynne
6
passage wherein he comments on different types of composition, including a type of composition
which results in two things forming another thing that is unum simpliciter, that is, a substance:
Since something is composed from another in this way “as a whole”—that is, the
whole is one—and not in the way as a heap of stones is, but as a syllable, which is
one unqualifiedly [unum simpliciter]; in all such instances, the composite itself
must not be that from which it is composed, as a syllable is not its elements. Just
as the syllable BA is not the same as the two letters B and A; nor is flesh the same
as fire and earth.20

That Aquinas applies the notion of composition without identity to a human being’s
relationship to her soul and body is evidenced by his discussion of the attribution of a human
being’s capacities and activities. Aquinas contends that the capacities of a human being must be
attributed to the human being herself, and not to any of her parts.21 A human being’s soul is the
source of her capacities; a human being’s body is the material support for such capacities.
However, a human being is that which has the capacities.
For example, I have the capacity to see. My capacity to see requires that I have eyes,
optic nerves, and a visual cortex. I have these organs due to my soul informing my material
body, hence both my soul and body are necessary for me to see; however, my soul does not see,
and neither does my body, eyes, or visual cortex. Rather, I see by utilizing the visual capacity I
have by virtue of being composed of my informed material body. That the capacity to see is
attributable to me, and not to my soul or body, demonstrates that I am not identical to either my
soul or my informed material body.

Rudder Baker, Persons and Bodies: A Constitution View (New York: Cambridge University Press, 2000); Kevin
Corcoran, “Persons, Bodies, and the Constitution Relation,” The Southern Journal of Philosophy 37 (1999): 1-20.
20 In M, bk. VII, lect. 17, §1674.
21 See Aquinas, Quaestio disputata de spiritualibus creaturis [QDSC], q. un., a. 11, ad 20. Referring to a human
being’s soul and body as parts requires an extended notion of “part” than the standard conception of parts as integral
to a substance, in the way a roof, walls, and floor are parts of a house. Such parts are composites of matter and form
that exist even when they do not compose something else. Soul and body, on the other hand, can be understood as
metaphysical parts that do not exist without composing a human being. For more on this distinction, see Eleonore
Stump, Aquinas (New York: Routledge, 2003), 42, 209-10; Aquinas, Scriptum super sententiis magistri Petri
Lombardi [In Sent], bk. II, dist. 3, q. 1, a. 4.
7
If capacities are properly attributed to the composite substance, then even more so are the
activities that follow upon such capacities:
The action of anything composed of matter and form is not of the form alone or
the matter alone, but of the composite . . . Therefore, if an intelligent substance is
composed of matter and form, understanding will be of the composite itself.22

Aquinas recognizes that certain capacities—rational thought and volition—can be had by a
human soul itself. They are capacities that a human soul would have even if it were separated
from a material body.23 Nevertheless, as Eleonore Stump points out, “Aquinas thinks that there
is something misleading about attributing cognitive functions just to the soul itself. Rather, even
such higher cognitive functions as understanding are to be attributed to the whole material
composite that is the human being.”24 Even if certain capacities belong to a soul itself, it is still
to a human being that their actual operation is attributable. Aquinas finds this to be important for
the sake of a human being’s being morally responsible for her actions.25
This way of construing Aquinas’s account of human nature allows for a human being to
exist post-mortem as composed of her soul alone. Interpretations differ as to whether Aquinas
explicitly holds a human being to persist between death and bodily resurrection. Robert Pasnau
contends that Aquinas denies a human being’s substantial existence between death and
resurrection; rather, a human being exists partially by virtue of her soul’s continued existence:
“so my separated soul is not anyone other than I, and in a sense it is I, but it is not fully I, not I in
the strictest sense.”26

22 SCG, bk. II, ch. 50. Cf. QDA, q. un., a. 6, ad 14.
23 See ST I, q. 77, aa. 5-8; QDA, q. un., a. 12, ad 16.
24 Eleonore Stump, “Non-Cartesian Substance Dualism and Materialism Without Reductionism,” Faith and
Philosophy 12 (1995): 512. See ST I, q. 75, a. 2, ad 2; ST I, q. 77, a. 5; DUI, ch. IV; In NE, bk. X, lect. 6; QDSC, q.
un., a. 2, ad 2; CT, ch. 85.
25 See DUI, ch. III.
26 Pasnau (2002), 389. See ST II-II, q. 83, a. 11, ad 5; Aquinas, In Aristotelis libros de caelo et mundo, bk. I, lect.
24.
8
Stump, on the other hand, argues that such an interpretation does not cohere with
Aquinas’s attribution of many “personal” qualities to a separated human soul:
Aquinas thinks that after death a human soul either enjoys the rewards of heaven
or suffers the pains of hell. He maintains that the separated soul is capable of
understanding and choosing. He also holds that after death a human being can
appear to the living; for example, speaking of the disembodied soul of a martyr
Felix, Aquinas says that Felix – not a simulacrum but the human being Felix –
appeared to the people of Nola. He thinks that the souls of the saints know the
prayers of the living and respond to them. He claims that the holy Fathers in hell
– who are separated souls – were waiting for Christ and were delivered by
Christ’s descent into hell. In these passages and many others, Aquinas attributes
to disembodied souls properties which he and we take to be most characteristic of
human persons, including intellectual understanding and love.27

Without adjudicating this dispute at the level of textual interpretation, I wish to point out that
Aquinas, by adopting the notion of composition without identity, can consistently hold that a
human being is not identical to her soul28 and yet persist between death and bodily resurrection
as composed of her soul alone. Before death, a human being “is” her soul, meaning that she is
composed of an informed material body, the existence and nature of which is provided by her
soul. A human being exists—is—as composed of her soul as a part, but she is not identical to
either it or the matter it informs. Since Aquinas conceives of a human being in this fashion, and
since being composed of a set of parts does not equal identity with such parts, taken individually
or as a set, a human being is not identical with her soul.
Furthermore, as Stump makes clear, that composition is not equal to identity entails that a
composed substance may lose some of its constituent parts without loss of its identity.29 Just as I
normally exist with two hands, as defined by my human nature, I could lose a hand and still exist
as the same human being. In the same way, just as a human being normally exists as composed
of soul and body, she can lose her body and still be identical to herself. In such a state, a human

27 Stump (2003), 52-3. See ST Supp., q. 69, aa. 2-5; ST Supp., q. 70, aa. 2-3; ST Supp., q. 72, a. 2.
28 See Aquinas, Super Primam Epistolam ad Corinthios lectura, ch. XV, lect. 2.
29 See Stump (2003), 51-2.
9
being exists composed of her soul alone, yet she is not identical to her soul. However, given that
Aquinas defines the substantial existence of a human being as composed of soul and body, a
human being who exists as a soul alone is deficient by not having all the parts proper to human
nature.
Such a deficient mode of existence does not entail that a human being ceases to be
“human,” that is, to exist as a rational animal, when she is composed of only her soul. A
separated human soul has all the capacities proper to existence as a rational animal, namely
intellective, sensitive, and vegetative capacities.30 Hence, though without her body a human
being is unable to actualize many of her capacities, she remains a rational animal by virtue of her
soul retaining all the capacities proper to such a nature.
I wish to note further that Pasnau’s interpretation rests upon taking Aquinas to hold that a
substance’s existence need not be an all-or-nothing affair. I am not convinced that this is a
position Aquinas holds. Stump’s conclusion, on the other hand, allows for a human being to
exist composed of only her separated soul; a human being exists if her soul exists, and does not
exist if her soul does not exist. Substantial existence remains, on Stump’s account, an all-or-
nothing affair.
A human being, then, is a substance that exists, has a set of specific capacities, acts, and
cannot be reduced to her parts taken individually or aggregately. While the existence and nature
of a human being is dependent upon her having a substantial form actualizing a sufficiently
complex organic material body, such parts are not acting substances in their own right. It is a
human being who exists and acts by means of her soul and body, which together compose her.

30 See ST I, q. 77, a. 8.
10
III
Aquinas argues that a human soul has a mode of being that distinguishes it from all other
substantial forms of material substances.31 This distinction is due to a human soul’s intellective
capacities, which are not dependent upon any material body for their functioning. Hence, such
capacities surpass the limits of matter in their ability to understand the universal forms of things;
such universal forms are the natures of things understood as abstracted from any particular
material conditions. Since intellective capacities surpass the limits of matter, no purely material
process can be responsible for the generation of substantial forms with such capacities. All other
substantial forms of material substances can be generated through purely material processes.
Aquinas thus argues that a human soul must receive its being (esse) directly from God:
It must be said that it is from the first cause [God] that such a soul has its esse,
also that it is intellectual and is a soul and consequently that it is impressed in a
body.32

The esse had by a human soul due to its intellective capacities creates a problem for
Aquinas. He asserts that a human soul is the substantial form of a material human body, and yet
it is separable in both esse and intellective operations. A tension thus exists between a human
soul’s being both naturally united to a material body as its substantial form and separable from
the body. I shall provide Aquinas’s assertions first for the soul’s separability, and then for its
being naturally united to a human body. Afterward, I will demonstrate how an important
distinction in Aquinas’s thought resolves this tension.
According to Aquinas, something can be corruptible in two ways: per se, that is, through
itself, and per accidens, that is, through another. No substantial form is corruptible per se,
because corruption per se involves the separation of a thing from its substantial form. A

31 See In Sent, bk. I, dist. 8, q. 5, a. 2, ad 1.
32 Aquinas, In librum de causis expositio [In LDC], prop. V. Ed. C. Pera (Turin, Marietti, 1955). Cf. ST I, q. 90, a.
2; ST I, q. 118, a. 2; SCG, bk. II, chs. 87-88.