## 1. Life and Works


### 1.1 Life



Thomas Aquinas was born near Aquino, halfway between Rome and Naples,
around the year 1225. He was the youngest of at least nine children,
and born into a wealthy family that presided over a prominent castle
in Roccasecca. As a teenage student in Naples, he fell under the sway
of the Dominicans, a newly founded order of priests devoted to
preaching and learning. Joining the order at the age of nineteen, he
was assigned to Paris for further study, but his plans were delayed by
the intransigency of his parents, who had hoped he would play a
leading role at the venerable local monastery, Monte Cassino, where he
had studied as a child. After confining him to Roccasecca for a year,
his parents yielded and Thomas went to Paris as a Dominican friar.



Thomas spent three years in Paris, studying philosophy, and then was
sent to Cologne, in 1248, under the supervision of
 Albert the Great.
 This older Dominican proved to be the ideal mentor. Albert was at the
time the leading figure in the newly prominent program of melding
Christian theology with Greek and Arabic philosophy. He possessed an
encyclopedic grasp of the sciences of the day, which had been
expanding at a dizzying pace thanks to the new availability of the
Aristotelian corpus in Latin translation. It was Albert's firm
conviction, which became Aquinas's own, that the Christian faith
could only benefit from a profound engagement with philosophy and
science.



Thomas evidently flourished under Albert's influence, for when
Albert was asked in 1252 to nominate a student to pursue an advanced
degree in theology at the University of Paris, he chose Thomas, even
though he was still two years younger than the minimum required age.
After four years as a bachelor of theology, lecturing on the Bible and
on Peter Lombard's *Sentences*, Aquinas received his
doctorate and was immediately appointed master of theology, once again
at an earlier age than the statutes officially allowed. From 1256
until 1259, he held the Dominican chair of theology in Paris,
preaching, lecturing on the Bible, and presiding over various
philosophical and theological disputations.



Although some masters at the University of Paris spent decades
teaching there, it was the custom of the Dominican order (as with the
Franciscans) to rotate scholars through these positions. Accordingly,
in 1259 Aquinas was sent back to Italy, where he spent most of the
following decade in several Dominican houses of study, first in
Orvieto (in Umbria) and then in Rome. During these years, while he
continued to preach, lecture on the Bible, and conduct academic
disputations, he found the time to develop his two most important
works, the *Summa contra gentiles* and the *Summa
theologiae*.



In 1268, Aquinas was asked to return to Paris for an unusual second
term as master of theology. Here he put his now venerable reputation
to work in attempting to steer the philosophical conversation away
from various extreme positions that were dividing scholars. He wrote a
brief treatise arguing for a middle ground on the vexed question of
whether the world could be proved to be eternal or created in time
(SS3), and a somewhat longer treatise against the view that all
human beings share a single intellect (SS6.2). These years were
dominated, though, by his efforts to complete the *Summa
theologiae* and at the same time write commentaries on all of
Aristotle's principal works. When his second four-year term in
Paris came to an end he returned to Italy, this time to Naples, in
1272. During these final years he nearly, but not quite, finished both
the *Summa theologiae* and his commentary series. Instead,
after a year and a half in Naples, he stopped writing, famously
explaining that "All that I have written seems to me like straw
compared with what has now been revealed to me." A few months
after that he died, in the Cistercian abbey of Fossanova, on March 7,
1274.



This chronology of Aquinas's life is fairly well attested. There
are also many colorful stories about his life, many of which stem from
testimony given during his canonization inquiry in 1319. The
authenticity of these stories is unknowable. Aquinas himself, in his
writings, reveals almost nothing about his personal life. (For
biography see Porro 2012 [2016]; Torrell 1993 [2022]; Legge 2022.)


### 1.2 Works



Over a mere two decades of literary activity, Aquinas left behind more
than eight million words (eight times more than has survived, for
instance, from Aristotle). It is a measure of Aquinas's
immediate and lasting influence that--quite unlike the situation
with other medieval philosophers--essentially everything he wrote
has survived and has been lovingly edited and translated into English
and many other modern languages. In outline, there are five categories
of works:


* encyclopedic theological treatises, such as the *Summa
theologiae* and the *Summa contra gentiles*;
* disputed questions, such as the *Questions on Truth*, often
more detailed than his *summae*;
* brief works (*opuscula*), such as *On Being and
Essence*;
* commentaries on Aristotle and on other philosophical texts;
* biblical commentaries.



The bibliography below contains further details about individual works
and their chronology.



The *Summa theologiae* (*ST*) generally represents
Aquinas's most considered thought on a given topic, and the work
is comprehensive enough that it contains at least some discussion of
almost all of Aquinas's intellectual concerns. It divides into
four parts:


* the *prima pars* (1a): the nature of God and the created
world, including human nature;
* the *prima secundae* (1a2ae): human action and what shapes
it (passion, virtue, vice, law, grace);
* the *secunda secundae* (2a2ae): the theological and
philosophical virtues, in detail;
* the *tertia pars* (3a): Christ and the sacraments (complete
through q. 90; his disciples drew on earlier texts to produce a
"supplement" that brings the work to its intended
end).



The treatments in *ST* are deliberately brief. Aquinas writes
in the preface that "our intent in this work is to develop those
issues that concern the Christian religion in a way that suits the
education of those who are just beginning." Today, no one thinks
of *ST* as a work for beginners, although its scope and
concision make it a useful place to start. A serious engagement with
Aquinas's work, however, requires comparing *ST* with
parallel treatments elsewhere, such as the earlier *Summa contra
gentiles* (SCG) and the still earlier commentary on
Lombard's *Sentences*. Moreover, since all three of these
long works are written as *theological* treatises, it is useful
also to compare these discussions with his accounts in the more
philosophically focused and often more detailed disputed questions. In
general, given how much Aquinas wrote and how often he reconsidered
the same issue from a slightly different perspective, a careful study
of his work requires comparing multiple treatments of the same issue.
And since he did not begin to circulate his work until he was 30, and
lived for only two decades more, those various treatments tend to
reinforce rather than conflict with each other. (Naturally, scholars
have long debated whether his thought evolves on one topic or another,
on which see Pini 2012.)



Most of Aquinas's central texts are written in the distinctive
scholastic style of the disputed question, in which the topic to be
discussed is posed at the start as a question. The dispute begins with
a series of arguments on one side. These, however, almost always
represent the opposite of Aquinas's own position, and so, after
the initial arguments, one or more arguments are posed to the contrary
(*sed contra*), and then Aquinas makes his own main reply in
the body (*corpus*) of the article, and finally he responds to
the initial arguments (*ad 1, ad 2, etc.*). This structure is
based on actual classroom procedures, even if Aquinas's
carefully composed works are never a literal record of an actual
classroom debate. It is a method that lends itself to argumentative
rigor, but often is best digested by reading the text in something
other than the order in which it appears on the page. (I myself like
to begin with the main reply.)



An important exception to the disputed-question format is
Aquinas's commentaries. These are often extremely useful for
understanding his own philosophical thought. This is true most
obviously of the philosophical commentaries, which cover most of
Aristotle's key works. The point holds true, however, for his
biblical commentaries as well, because they regularly contain
interesting discussions of wide-ranging philosophical topics (Stump
2010). One has to approach all of Aquinas's commentaries with
some caution, however, because his principal method of commentary is
to produce a word-by-word paraphrase. This means that, if one does not
read these works with one eye on the text being commented upon, a
passage that may seem to represent Aquinas's thought may instead
be a rephrased version of what Aristotle (or Isaiah or Paul) said.
Whether or not Aquinas's paraphrase of a passage from Aristotle
should be treated as a reflection of Aquinas's own thought is a
matter of some dispute (Jenkins 1996; Elders 2009), but readers should
at least be aware of how these commentaries are structured. The
passages most likely to shed light on Aquinas's own thought are
those where he breaks away from his line-by-line paraphrase to offer a
brief lecture on some issue germane to a proper understanding of the
underlying text.



It is tempting, for modern readers, to divide Aquinas's work
into the theological and philosophical, but lines here are difficult
to draw. The medieval university did formally distinguish between the
higher-level discipline of theology and the undergraduate arts
faculty, whose curriculum consisted largely in the works of Aristotle.
But many of the questions of the theologians--such as the
workings of intellect, the nature of freedom, the systematization of
ethical theory--are now seen as paradigmatically philosophical.
At the same time, much of Aristotle, and therefore much of the arts
faculty curriculum, looks to us more scientific than philosophical.
Aquinas, unlike his teacher Albert the Great, only occasionally
dwelled on what we now think of as scientific questions. Conversely,
as a professor of theology, he considered at great length various
matters that depend on
 divine revelation, such as the
nature of the Trinity and the character of Christ's humanity. A
complete understanding of Aquinas's thought thus requires
spending considerable time with such properly theological
questions. At the same time, it is appropriate today to number Aquinas
among the philosophers, because he was particularly interested in and
had particularly innovative things to say about what we now think of
as philosophical questions. The tremendous energy he put into
commenting on Aristotle's works is a testimony both to the
importance he gives philosophy for a proper understanding of theology,
and to his confidence that progress in philosophy will only benefit
the Christian faith. As he famously wrote, "if anything is found
in the words of the philosophers that is contrary to the faith, this
is not philosophy but rather an abuse of philosophy, due to a failure
of reason" (*Comm. Boethius De trinitate* 2.3c). 




For book-length general surveys of Aquinas's thought see Stump 2003,
Davies and Stump 2012, Shields and Pasnau 2016, Stump and White
2022.


## 2. God



Aquinas believes that natural reason can demonstratively prove
God's existence. The first step is to show that, for everything
in the changeable world around us, there is a first cause, or prime
mover, in virtue of which all other things have their existence, their
motion, their qualities and direction. This is the result he gets in
his much discussed "five ways" of proving God's
existence (*ST* 1a 2.3c). Aquinas denies that Anselm's
 ontological argument
 has any force as a proof (*ST* 1a 2.1 ad 2; *SCG*
I.10-11). Instead, the first three ways are
 cosmological arguments,
 based on the impossibility of an infinite series of causes.
Commentators generally agree that what Aquinas means to exclude is an
infinite series of simultaneous causes, a "vertical"
series as opposed to an infinite "horizontal" series of
causes going back in time, which he does not think can be proved to be
impossible (SS3). Each of the five brief arguments ends with a
variation on the formula "this is what everyone calls
God." He puts his claim cautiously at this point, because to
demonstrate the existence of a prime mover falls well short of
demonstrating the existence of a being worthy of being called God. For
all we have seen so far, for instance, this first cause might itself
be a body. But Aquinas thinks natural reason--what we now call
 natural theology--can
 go well beyond the five ways. *ST* 1a goes on to establish
that this first cause is not a body (3.1), nor composite in any way
and hence entirely
 simple
 (3.7), perfect in every respect (4.2), the highest good (6.2),
infinite (7.1),
 omnipresent
 (8.2), completely unchangeable (9.1), eternal (10.2), supremely one
(11.4), and possessing
 the most perfect knowledge
 (14.1). These results establish the existence of something worthy of
being called God. (On Aquinas's natural theology in general see
Kretzmann 1997; Wippel 2000 chs. 10-13; Stump 2003 pt. I. On the
five ways see Kenny 1969; MacDonald 1991a; Martin 1997; Pawl 2012;
Cohoe 2013.)



The successes of natural theology, for Aquinas, have their limit. For
although natural reason can establish the existence of a perfect
being, it is incapable of establishing many of the features that
distinctively characterize the Christian God, such as God's
triune nature and God's incarnation as a human being. Here is a
place where philosophy alone, unaided by revelation, fails to yield an
adequate theology. Since the ultimate goal of human life, salvation in
the life to come (SS8.1), requires some grasp of these Christian
truths, "it was necessary for the sake of human salvation that
certain truths that surpass human reason be made known to us through
divine revelation" (*ST* 1a 1.1). The study of revealed
theology is important, moreover, not only with respect to these
so-called mysteries of the faith, but also in cases where the results
of natural reason are too precarious to be counted on. Without direct
access through revelation, "the truth about God investigated by
reason would be available only to a few people, after a long time, and
with the admixture of many errors." Accordingly, "beyond
the philosophical disciplines investigated by human reason, it was
necessary to have a sacred teaching through revelation" (ibid.).
This sacred teaching (*sacra doctrina*) is the work of
theology.



One of the most difficult and most discussed issues in Aquinas's
natural theology is the limits he places on our ability to understand
God's nature. A sign of the difficulties to come emerges just
after he sets out his initial five-way proof, when he remarks,



> 
> Once it is grasped *that* something exists, it remains to be
> investigated *how* it exists, in order that what it is may be
> known. But because it is not possible for us to know what God
> *is*, but rather what God is *not*, we cannot consider
> how God exists, but rather how God does not exist. (*ST* 1a
> 3pr; see Davies 2018)
> 



That Aquinas in fact honors this constraint is far from obvious, given
the seemingly positive claims he makes regarding the divine nature. To
understand his approach one must come to grips with his theory of
 analogical predication.
 In a case like God's goodness, for instance, Aquinas does not
mean only to deny God's badness. It can be truly said, he
thinks, that God is good. The difficulty is that the word
'good,' when truly applied to God, has a different sense
from what it has in cases familiar to our own experience. The two
senses are thus not univocal, but nor are they purely equivocal, as if
God's goodness were wholly unrelated to the goodness we
experience. Instead, the usages are *analogical* (*ST*
1a 13.5). So, "when we say God is good, the sense is not that
God is the cause of goodness or that God is not bad; rather, the sense
is that what we call good in creatures preexists in God, albeit in a
higher mode" (*ST* 1a 13.2c). We can say with confidence,
on the basis of this analysis, that God is good, indeed preeminently
so. To the extent that we are good, it is so only by participation in
God's goodness. But our cognitive limitations preclude a more
adequate account of this "higher mode" in which God is
good. (On analogy in Aquinas see McInerny 1996; Montagnes 1963 [2004];
Dewan 2007; Ashworth 2014; Hochschild 2019.)


## 3. The Created World



The mainstream of philosophical tradition as Aquinas knew it, running
from Aristotle through the Greek commentators and into the Arabic
tradition, held that the material world, and all the kinds of living
things in it, had always existed. These earlier Aristotelians still
thought there was a place for God, as the first mover, but they
understood God to be the initial, remote source of motions that had
always existed, rather than an eternal being who created the universe
anew. Aquinas understood Christianity to be committed to the latter
view. He holds that the changeable universe around us has existed for
only a finite amount of time; that God, in contrast, has existed
eternally and unchangingly; and that God freely chose to create the
changeable universe from nothing (*ex nihilo*). To bring a
thing into existence *ex nihilo* is the defining feature of
 creation,
 in the strict sense of that term, and only God can do it (*ST*
1a 45). Everything else in the natural world causes new things to come
into existence by modifying what is here already. The proper
terminology for these natural changes is *generation* and
*corruption*, which refer to a substance's coming into or
going out of existence by natural means, and *alteration*,
which refers to a substance's undergoing accidental change:
continuing to exist through change to its shape, color, and other
non-essential properties.



Aristotle and many of his followers believed they could prove that the
world had always existed. Conversely, many of Aquinas's
contemporaries thought they could prove the opposite, that the world
had to have a beginning in time, and the debate between these rival
philosophical accounts was one of the most acrimonious issues in the
early medieval university (Dales 1990). Aquinas took up the question
in detail in his treatise *On the Eternity of the World*,
concluding that no demonstrative argument is possible either way. For
all we can tell, God could have created the world in such a way that
it had always existed. That we think otherwise is another deliverance
of the faith, not susceptible to philosophical proof. This measured
conclusion is characteristic of his thought in many domains. On the
one hand, he has complete confidence that philosophical reasoning,
properly pursued, will not yield results that are a threat to
Christianity. On the other hand, he thinks it discredits the faith to
defend it through dubious attempts at demonstration. The key is to see
where philosophy can be of service, and where it must give way to
revealed doctrine (Wippel 1995c; Kretzmann 1999 ch. 5).



It is not easy to think about God's relationship to the created
world, because without such a world there can be neither space nor
time (*ST* 1a 46.1, 46.3). Not space, because space is nothing
more than the existence of bodies, where bodies are beings that
possess parts outside of parts, and so constitute the
three-dimensional extension that we think of as space. Accordingly,
where there are no bodies there is no space. (Aquinas, following
Aristotle, denies that void space is coherent.) Not time, because
Aquinas accepts Aristotle's definition of time as the measure of
motion. In a world without motion, then, there is no time, which means
that the first moment of time was the first moment of the created
world. Although it is hard to resist thinking of God as having existed
"before" there was a material universe, God's mode
of existence, in a world without bodies, must be an existence outside
of space and time. This need not entail, however, that God even now
exists outside of space and time. God's
 omnipresence
 was commonly understood as a literal presence everywhere (Pasnau 2011
ch. 16), and Aquinas characterizes God's eternity not as a mode
of existence outside of time, but as "being present to every
time or instant of time" (*SCG* I.66.8). It is, however,
far from clear what "present" amounts to in such a claim
(Leftow 1990). These difficulties are of a piece with the general
difficulty we have in thinking about God using the concepts familiar
to us from the world around us.



However these metaphysical issues are resolved, it is clear that God
is constantly, everywhere engaged with the world. Because "a
thing's existence cannot remain after the cessation of the
action of the agent causing the effect" (*ST* 1a 104.1c),
the created world must have not just an initial creative cause but
also an ongoing cause that
 conserves
 it from moment to moment. Since God is necessarily perfect, the
created world cannot be beneficial for God, and since a universe with
a perfect being is itself already perfect, God's goodness does
not necessitate his creating anything at all. Moreover, since for
everything God creates he could create something better (*ST*
1a 25.6c), there is no sense to be made of the idea of a best of all
possible worlds. Accordingly, God is free to create otherwise than he
did, and free even not to create anything at all: "Since
God's goodness is perfect, and he can exist without other
things, ... it follows that there is no absolute necessity that
he will things other than himself" (*ST* 1a 19.3c).
Still, creation has a purpose: God "intends only to share his
perfection, which is his goodness" (*ST* 1a 44.4c). Among
creatures, those beings who possess intellects--angels and human
beings--are preeminent, in virtue of their capacity to love and
understand their creator. Accordingly, the rest of creation is
organized for their sake (*SCG* III.112). Ultimately, however,
the purpose of the created world is to reflect God's goodness
and glory: "the whole universe, with its individual parts, is
ordered to God as its end, inasmuch as, by a kind of imitation, the
universe represents divine goodness for the glory of God"
(*ST* 1a 65.2c). So it is that Aquinas takes everything in the
created world to be subject to God's providential order
(SS7). (For a broad study of Aquinas on creation see Kretzmann
1999.)


## 4. Form and Matter



Given the preeminence of intellectual beings in the created order, it
should be no surprise that most of God's creative efforts went
into their establishment. This is, primarily, the domain of the
angels, who "exceed in number, incomparably, material
substances" (*ST* 1a 50.3c). The angels are wholly
immaterial beings, which makes them unlike us (SS5) and unlike the
created world with which we are directly acquainted, which is a
material world. So, if we set aside the realm of immaterial
minds--God, angels, the human soul--we are left with a world
of material objects, suitable to be understood in terms of an
Aristotelian hylomorphic (matter-form) analysis.



Within this material domain, a material substratum underlies all
change, and real change typically involves the gain or loss of some
form. This can be made to look straightforward enough when examples
are carefully chosen. If a tree's limbs are bent under the
weight of an early fall snowstorm, then the tree undergoes alteration
(accidental change) in virtue of taking on a new shape (a new
accidental form), while the material substance (the tree itself)
endures. If the storm in fact kills the tree, then the substance is
corrupted (a substantial change), in virtue of losing its substantial
form, which is the internal principle that gives the substance its
nature. In cases like this, where the substance is destroyed, Aquinas
advances a metaphysical claim that is quite surprising: that all that
endures through the change is the most basic material substratum,
*prime matter*, which lacks all form. (Aquinas's early
*On the Principles of Nature* summarizes the basic story. For
general discussions of Aquinas's metaphysics of material bodies
see Wippel 2000 chs. 7-9, Brower 2014. On causation at the level
of creatures see Frost 2022.)



The resulting picture seems to postulate, as the constituents of
substances, three types of basic entities: prime matter, substantial
forms, and accidental forms. How exactly to understand each of these
has been subject to a great deal of discussion over the centuries.
With respect to accidental forms, one of the chief puzzles is to
define those cases in which we have a genuine change of form. Aquinas
clearly thinks, for instance, that colors and other proper sensibles
are accidental forms. But does he think, as the above example of the
tree implied, that shapes too are accidental forms? It is not entirely
clear, and depends on difficult questions about how he understands
Aristotle's theory of the
 categories
 (Wippel 2000 ch. 7; Pasnau 2011 ch. 12). With respect to substantial
form there is an analogous problem arising from unclarity over what
exactly the substances are (Pasnau 2002 ch. 3; Rota 2004; Marmodoro
and Page 2016). He clearly takes all living things to be substances,
although there are puzzles here about how to define the beginning and
end of life (Van Dyke 2012; Amerini 2009 [2013]). When it comes to
nonliving things the situation is still less clear. He clearly thinks
that artifacts like a house are not substances. Water is a substance,
but it is not clear whether a puddle of water is a single substance or
a collection of substances. Aquinas does not discuss these issues in
the sort of detail that later medieval authors would, and as a result
there is little scholarly consensus about his views. But one important
guide to his thinking about substances is that they have unity in a
very strong sense: "whatever is one in substance is one
absolutely (*simpliciter*)" (*ST* 1a2ae 17.4c). Of
course, no creature has the sort of unity that God has; God's
perfect simplicity precludes even the composition of essence and
existence, which is found in all created substances, even in
immaterial angels (Wippel 2000 ch. 5). Material substances, being
composites of form and matter, have less unity than the angels. Even
so, Aquinas thinks there can be complex material substances, unified
by their substantial form. How does a substantial form unify? An
important part of his answer is that a substantial form actualizes not
only the whole substance, but each part: "a form of the whole
that does not give existence to the individual parts of the body,
... such as the form of a house, is an accidental form"
(*ST* 1a 76.8c).



Substantial forms both unify a substance at a given time and
individuate a substance over time. To play this role, they must be
particulars rather than
 universals.
 In general, it seems that Aquinas thinks that no forms are universals
except inasmuch as we conceive of them in abstraction from
particulars. In external reality (*in re*), "no
commonness is found in Socrates; rather, whatever is in him has been
individuated" (*On Being and Essence* 3.80-2;
Leftow 2003; Brower 2016). The principle of individuation, for forms,
is matter (King 2000; Klima 2000). The account of individuation, then,
comes in two stages: matter individuates the form at the start, when
the form first inheres in that matter, and from that point forward the
form possesses a fixed identity of its own. If it is a substantial
form, it subsequently individuates the substance as a whole.



Fundamental to Aquinas's hylomorphism is his coordinate
distinction between potentiality and actuality. His brief treatise
*On the Principles of Nature*, perhaps his earliest work,
begins with this remark:



> 
> One should know that some things can be, although they are not, and
> some things are. That which can be is said to be in potentiality; that
> which is now is said to be in actuality.
> 



He goes on to associate potentiality with matter, and actuality with
form, and draws the distinctions we would expect between the various
kinds of matters and forms. He also remarks at this point that being
is of two kinds, and that the being of a substance is "something
different" from accidental being. This signals his commitment to
Aristotle's doctrine of the multivocity (or homonymy) of being.
What it means for a substance to exist is different from what it means
for an accidental aggregation of a substance with its accidents (say,
a pale man) to exist. In general, what properly exists is substances,
whereas "forms, accidents, and other things of this sort are
called beings not because they themselves exist, but because it is
*by* them that something exists" (*ST* 1a 45.4c).
Moreover, even among substances, God does not exist in the same way
that creatures do, but only in an analogous way (SS2). This denial
of the univocity of being was contentious among later medieval authors
(most notably
 John Duns Scotus)
 and its correct interpretation remains controversial today (Brower
2014; Pasnau 2018). Even more problematic is Aquinas's
conception of prime matter, which he characterizes as pure
potentiality: "being in actuality is incompatible with
matter's nature, since matter is a potential existent by its
very nature" (*Quodlibet* III.1.1). Although later
medieval Aristotelians accepted that substantial change requires a
material substratum, hardly anyone endorsed Aquinas's suggestion
that it is pure potentiality, because they were convinced that what
lacks actuality does not exist. Readers of Aquinas have disputed how
he himself could have evaded that outcome (Brower 2014 ch. 5), or
whether perhaps it is an outcome he intended (Pasnau 2002 ch. 1).


## 5. Soul and Body



For living things, their substantial form is their soul
(*anima*). In saying this, Aquinas should not be understood to
be ascribing some special sort of spirituality to plants and animals:
he thinks they are material objects just as much as rocks and streams
are. Rather, he is following the lead of Aristotle's *De
anima* in treating the soul as the first principle of life,
whatever that may be. Since Aquinas thinks that the primary internal
explanation for the existence of any substance is its substantial form
(SS4), it follows that every living substance has a soul that is
its substantial form.



As with any substantial form, the primary function of a soul is to
account for the substance's nature as a certain kind of thing,
possessed of the unity and persistence that characterizes substances.
This actualizing role is what Aquinas calls the soul's essence
(*ST* 1a 77.1c). The ongoing existence of living things also
requires that they carry out distinctive operations: taking
nourishment and reproducing, moving and perceiving (in the case of
animals), and reasoning (in the human case). Corresponding to these
operations are the powers of the soul. Medieval authors had long
puzzled over the relationship between the soul and its powers. Aquinas
takes a strong position that would be the subject of much later
dispute: that the soul's powers, being potentialities, are
really distinct from the soul's essence, which is an actuality
(*ST* 1a 77, *Quest. on the Soul* 12).



Yet even though Aquinas insists on a distinction between the soul and
its powers, he rejects the common medieval view that living things
have multiple souls or multiple substantial forms. Because of his
commitment to the absolute unity of a substance, he thinks that
nothing could count as a substance unless it possesses a unique
substantial form that provides unity, at a time and over time, to the
substance as a whole.



> 
> One thing *simpliciter* is produced out of many actually
> existing things only if there is something uniting and in some way
> tying them to each other. In this way, then, if Socrates were an
> animal and were rational in virtue of different forms, then these two,
> in order to be united *simpliciter*, would need something to
> make them one. Therefore, since nothing is available to do this, the
> result will be that a human being is one thing only as an aggregate,
> like a heap. (*Quest. on the Soul* 11c; also *ST* 1a
> 76.3-4, *Quest. on Spiritual Creatures* 3)
> 



Such remarks engendered a long dispute between those who favored this
sort of "unitarian" approach and those who took one or
another "pluralist" position. The pluralists argued that
living things contain one substantial form for their body (their
bodily form, *forma corporeitatis*) and at least one further
distinct form, their soul, in virtue of which they have life (Adams
1987 ch. 15; Cross 1998 ch. 4; Pasnau 2011 ch. 25). The principal
argument in favor of pluralism was that it explained how the body of a
living thing could persist after the departure of the living
thing's soul at death. Aquinas and other unitarians claimed that
such persistence was merely apparent, and that strictly speaking the
only thing that endures through a substance's corruption is
prime matter.



Given the tight connection that Aquinas describes between form and
matter, it is hard to characterize his theory of souls in general as
 dualistic.
 To be sure, in some sense, material substances are a composite of
substantial form and prime matter (with accidents on the outside, so
to speak, unified only accidentally with the substance). Still,
Aquinas takes pains to stress that it is the substance as a whole that
properly exists. And he definitely does not think that material
substances are a composite of form and *body*, since the
material substance *is* the body. His unitarian framework
deliberately makes it incoherent to speak of the body of a living
thing (or of any substance) as something distinct from that
substance.



The situation is much more complex, however, when we focus on the
human case. If a dualist is someone who thinks that human beings
consist in a material body and an immaterial, spiritual mind, then
Aquinas clearly qualifies, even if he works very hard to get the
result that these two aspects of human nature are unified as a single
substance (Bazan 1997; Klima 1997 [2002]; Stump 2003 ch. 6).
Although the human soul's essential role is to actualize a human
body, the human soul has a power--its intellect--that
operates independently of the body. Aquinas has a variety of much
discussed arguments aimed at showing that human thought, given its
universal scope and abstract content, cannot be carried out through
the brain or any other bodily instrument (*ST* 1a 75.2, 75.5;
*Quest. on the Soul* 2; Klima 2001; Wood 2020). As a result,
the human soul has a unique and puzzling status: it is both the form
of a body and "an incorporeal and subsistent principle"
(*ST* 1a 75.2c). From here, Aquinas further argues that the
human soul is incorruptible (*ST* 1a 75.6, *Quest. on the
Soul* 14). For although ordinary subsistent things--material
substances--routinely come into and go out of existence, with the
corruption of their bodies, a subsistent entity that is not a body has
no natural basis for its ceasing to exist. Hence, once God creates a
human soul (in coordination with the biological process of sexual
reproduction), that soul exists forever.



Given that the human body is corrupted at death, the soul's
incorruptibility entails that, after death, it will continue to exist
without the body. Aquinas devotes considerable attention to the
question of how it will function in that separated state (*ST*
1a 89; *Quest. on the Soul* 15-20). Prior to the delicate
but all-important question of what such lives will be like (a paradise
or a hell), there is a still more basic question of whether a human
person's separated soul continues to be that same person. Recent
commentators have been divided. Aquinas is very clear that, in this
life, a human being is the whole composite substance, form and matter
(*ST* 1a 75.4). Also clear is that Aquinas thinks our souls
will not be separated from their bodies forever; in the fullness of
time, on the Day of Judgment, they will be reunited with their
resurrected bodies (*SCG* IV.79-97). The situation
becomes less clear, however, when Aquinas argues, as he repeatedly
does, that a resurrection of bodies is required because otherwise
*we* would not survive. "Abraham's soul's
having life would not suffice for Abraham's being alive....
The life of the whole compound is required, soul and body"
(*Sentences* IV.43.1.1.1 ad 2). This *seems* to show
that Aquinas thinks a person's soul, surviving apart from its
body, is not sufficient for that person's survival, and some
scholars (the "corruptionists") read Aquinas in this way
(Toner 2009; Van Dyke 2012; Nevitt 2014). But to others (the
"survivalists") this looks like an incredible result, in
tension with claims that Aquinas makes elsewhere and with fundamental
Christian doctrine (Brown 2007; Brower 2014 ch. 12; Stump 2022).


## 6. Cognitive Theory


### 6.1 Perception



By definition, the living things that we call animals are those that
have the power of perception. Aquinas accepts the conventional list of
the five senses--sight, hearing, smell, taste, touch--and
argues that we arrive at this list of five because there are five
discrete kinds of qualities that make an impression on our bodies.
These objects are the *proper sensibles*--color, sound,
odor, flavor, hot-cold--so-called because they "make
an impression on the senses primarily and *per se*"
(*ST* 1a 78.3 ad 2). In contrast to these are the *common
sensibles*--size, shape, number, motion, rest--which
"do not move the senses primary and of themselves, but on
account of a sensible quality, as a surface does on account of its
color" (ibid.). Whereas each proper sensible is capable of being
perceived only by its corresponding sense, the common sensibles are
so-called because they can be grasped by multiple senses, as when we
both see and hear a thing move. To these two categories of sense
objects Aquinas adds a third category, the *accidental
sensibles*, as when I see that someone is alive (*Comm. de
anima* II.13.184-90). This is, however, not a purely sensory
operation, but an act of seeing that occurs in conjunction with a
conceptual judgment that the observed motions are a sign of life. (For
discussions of the perceptual process see Pasnau 2002 chs. 6, 9;
Lisska 2016; De Haan 2019; Cory 2022.)



Beyond these five external senses are the four *internal
senses* that use the brain as their organ. Following the lead of
 Ibn Sina
 (Avicenna), Aquinas puts considerable weight on these sensory powers
to explain the sophisticated behavior of animals. In contrast to the
consensus over the number of external senses, there had been a great
deal of historical disagreement over how many internal senses there
are. Aquinas settles on this list of four:


* the common sense;
* imagination (or *phantasia*);
* the estimative (or cogitative) power;
* memory.



The precise role of these four faculties is not easy to evaluate on
the basis of Aquinas's limited and scattered remarks, but
according to his most prominent account the common sense is what
allows us to make cross-modal sensory judgments (seeing something as
white and sweet) and to have second-order awareness, for instance
"when someone sees that he is seeing" (*ST* 1a 78.4
ad 2). The imagination--Aquinas uses both *imaginatio* and
*phantasia* to refer to this faculty--retains images
acquired through the external senses. The estimative power perceives
what the Arabic tradition referred to as the "intentions"
(*ma'ani*) that lie behind the sensible
qualities: for instance, that wolves are dangerous and that straw is
useful for nest building. (Aquinas refers to the more sophisticated
human capacity for grasping intentions as the cogitative power.)
Memory preserves these intentions. All of the internal senses are
richly engaged in human cognitive activity, which is to say that the
brain--despite not being the power of intellect--plays a
critical role in human cognition.



The senses are not wholly immaterial faculties in the way that the
intellect is, but the respect in which Aquinas thinks they are
material is a matter of some dispute. He remarks that the senses
operate "through a corporeal organ but not through any corporeal
quality" (*ST* 1a 78.1c). By this he intends a contrast
with ordinary physical causation, as when a fire, through the quality
of heat, brings water to a boil. In contrast, when we perceive heat,
even though the skin is heated in the same way that water is, still
the perception itself is not the skin's being made hot. Rather,
"the senses and intellect receive the forms of things
spiritually and immaterially, in virtue of a kind of intentional
existence" (*Comm. de sensu* 18.208-10). In
general, Aquinas treats the ability to receive forms intentionally as
a mark of the mental:



> 
> Things that cognize are distinguished from things that do not cognize
> in this: that the non-cognizers have nothing but their own form alone,
> whereas that which cognizes is naturally suited to have the form of
> another thing as well. (*ST* 1a 14.1c)
> 



Such forms, received intentionally in the external senses, are known
as sensible species. Received in the internal senses, they are known
as phantasms. Received in the intellect, they are known as
intelligible species. Such forms--for instance, the sensible
species of heat--are that in virtue of which we perceive, for
instance, the heat of external bodies. That such forms do not make
possessor actually hot (etc.) is perhaps just what it is for a form to
exist intentionally. But there is scholarly disagreement over what it
means to say that such forms are "spiritual" and
"immaterial" (Hoffman 2014).



As these passages make clear, perception (and cognition in general)
works through the reception of the very form of the thing that is
being perceived. This has led to much discussion over whether Aquinas
thereby subscribes to a strong form of direct realism, according to
which what we perceive (and what we think about) is the thing itself.
In favor of this reading is that Aquinas seems entirely unconcerned
about these intentionally existing species as intermediaries lying
between us and the things themselves. Why should he be concerned, the
thought goes, since the species just is the form itself of the thing
out in the world? (see Perler 2000; Baltuta 2013)
Against this reading is that the species (of heat, say) is not
numerically the same form as the one that inheres in the fire, and
that Aquinas does, at least sometimes, seem to think of these species
as representations that mediate our access to external things (Pasnau
1997 ch. 3; Panaccio 2001; see also Brower and Brower-Toland
2008).


### 6.2 Thought



Aquinas thinks that a great deal of complex cognition occurs within
the internal senses of the brain, but that those material powers are
incapable of abstract thought. To be more precise, he thinks that
material cognitive powers can represent things only as particulars,
and that universal concepts can be formed only within the immaterial
intellect. He writes,



> 
> If the intellective soul were composed of form and matter, then the
> forms of things would be received in it as individuals; then it would
> cognize only singular things, as happens in the sensory capacities,
> which receive the forms of things in a corporeal organ. (*ST*
> 1a 75.5c)
> 



The senses, both external and internal, can represent only this or
that particular cat. It takes the intellect to form the abstract
concept of cat. Conversely, Aquinas thinks that the human intellect is
incapable of grasping particulars (*ST* 1a 86.1). This means
that when we engage in a routine task like apprehending that a thing
is a cat we are simultaneously using both intellect and sense, using
the senses to apprehend the particular, and using the intellect to
conceptualize what it is. In our more theoretical reflections, we rely
on the internal senses to frame images that aid us in the process of
abstract reasoning.



Following Aristotle's lead in *De anima* III.5, Aquinas
distinguishes between two distinct intellectual powers, the possible
intellect and the agent intellect. The first starts out in
potentiality to all intelligibles, a *tabula rasa*, "like
a tablet on which nothing has been written" (*ST* 1a
79.2c), and gradually becomes actualized, taking in universal concepts
that are then stored there as dispositions. But because such concepts
are not immediately available through sensory experience--one
cannot grasp what it is to be a cat just by looking at a
cat--there must also be an active intellectual power, "a
power on the side of intellect to actualize intelligible things by
abstracting the species from material conditions" (*ST*
1a 79.3c). By Aquinas's time, generations of Aristotelians had
already argued over the correct understanding of these two powers,
with some treating the agent intellect as a single higher mind,
separate from human beings, and perhaps even to be identified with
God.
 Ibn Rushd
 (Averroes), the Spanish philosopher active a half century before
Aquinas, had even proposed treating the *possible* intellect as
a single higher power to which every human being has shared access.
Aquinas, in contrast, thinks that each human being has their own
intellect, agent and possible. That we all share in a single agent
intellect is, he thinks, wrong but not incredible--it has
affinities with the venerable Augustinian doctrine of
 divine illumination--but
 Aquinas regards the Averroistic doctrine of a separate possible
intellect as not just contrary to the faith but entirely preposterous,
since it would make it impossible to explain how we are each capable
of individual acts of thought. As he describes the absurd implication
in his treatise *On the Unity of the Intellect against the
Averroists*, "if I think about stone and you do likewise,
then it would be necessary that you and I have one and the same
intellectual operation" (4.101-3; see Ogden 2022).



Aquinas distinguishes between three principal stages of intellectual
operation: first, the formation of concepts; second, the composition
of judgments (propositions) built from concepts; third, inferential
reasoning from one judgment to another. These last two stages were the
subject of intensive study within medieval logic, but Aquinas has
relatively little to say about logic, and is most interested in the
initial stage of concept formation. Here, the possible intellect
passively receives concepts that the agent intellect has abstracted
from sensory impressions (phantasms). The possible intellect starts
out as a blank slate, which is to say that Aquinas rejects the theory
of innate ideas, at least as far as the possible intellect is
concerned. Moreover, he is very clear that all our concepts arise from
sensory experience: "the source of our cognition comes from the
senses" (*ST* 1a 84.6sc). From this vantagepoint, he
looks more like an empiricist than a rationalist. The role of agent
intellect, however, complicates the situation. Whereas the possible
intellect starts out as unformed potentiality, the agent intellect has
the actuality to abstract universal concepts from sensory experience.
It is not easy to say how abstraction occurs; indeed, for the whole of
the Aristotelian tradition, the agent intellect has been something of
a black box whose operation is taken as a brute fact rather than
analyzed. Sometimes Aquinas suggests that the transformation of
singular content into universal content occurs via the conversion of
material phantasms into immaterial intelligible species, but the link
between immateriality and universality is not entirely clear. (On
abstraction see King 1994, Cory 2015).


### 6.3 Knowledge and Science



Aquinas uses a variety of terms to talk about knowledge, including
*scientia*, *cognitio* and *notitia*. Often he
seems to use these terms interchangeably, and he shows no interest in
trying to define anything like knowledge in its loose and popular
sense. He is, however, very much interested in articulating what it
would be to have an ideal grasp of some subject, and he refers to such
a thing, speaking strictly, as *scientia*. This is what both
the theologian and the philosopher seek, which is to say that this is
what Aquinas himself seeks. Each of these fields, theology and
philosophy, can be described as a single complex
*scientia*--that is, as a science. We can also refer to
the myriad individual conclusions obtained in these fields as
*scientiae*--that is, as items of scientific knowledge.
Philosophy itself, moreover, can be divided into various discrete
fields of science: logic, physics, astronomy, biology, metaphysics,
ethics, and so on. Until the seventeenth century the physical sciences
were understood to be contained, in this way, within the field of
philosophy. Accordingly, the domains of *scientia*, conceived
most widely, were philosophy and theology.



The fundamentals of Aquinas's thinking about knowledge in this
ideal sense come from Aristotle's *Posterior Analytics*.
In particular, he takes the discussion in Book I chapter 2 to offer a
definition of *scientia* (in Greek,
*episteme*), which in his commentary he articulates
as follows (quotations are from *Comm. Post. An.* I.4):


* "to have *scientia* of something is to cognize it
perfectly";
* to have *scientia* requires grasping the cause of the thing
known;
* "*scientia* is a cognition with certainty of a
thing";
* the object of *scientia* is true and "cannot stand
otherwise";
* "to have *scientia* is the end or effect of a
demonstrative syllogism";
* the premises of such a syllogism must be "true, primary, and
immediate" and must be "better known, prior, and causes of
the conclusion";
* such premises must concern the universal natures of things.



Each of these bullet points raises a host of further questions, but,
in broad outline, Aquinas understands *scientia* as the
apprehension, with certainty, of an entity, event or proposition,
based on a demonstrative proof grounded in the essential nature of the
thing in question. As befits intellectual understanding (SS6.2),
*scientia* will concern universal truths. Because it concerns
the essences of things, it will concern necessary truths and will get
at the underlying cause of a thing's being so (MacDonald
1993).



Eventually, the demand that premises be "better known"
leads back to the first principles of the science, which are
self-evident (*per se nota*) in the sense that they are known
to be true simply in virtue of their terms (*Comm. Post.
Analytics* I.7.8). Nothing could be better known than such a
principle, but because these principles are self-evident there is also
no need for any further demonstration. Each science has its own
starting points--its own first principles--which are not
susceptible to demonstration within that science. But not all such
first principles are strictly self-evident. Within physics, for
instance, it is a first principle that there is motion, but that is
neither absolutely necessary nor strictly self-evident. Accordingly,
again following Aristotle, Aquinas orders the various sciences
according to relations of "subalternation", such that
biology is subalternate to physics, and physics to metaphysics. In
general, "the principles of a lower science are proved by the
principles of a higher science" (*Comm. Post. Analytics*
I.17.5). Among the natural speculative sciences--that is, setting
theology and ethics aside--the preeminent science is metaphysics,
or first philosophy, "from which all the other sciences follow,
taking their principles from it" (*Comm. Boethius De
trin.* 5.1c). (On the structure of the sciences see Maurer 1986;
Wippel 1995a; Dougherty 2004.)



This sort of foundationalist, infallibilist theory of knowledge is now
thought of as Cartesian, but
 Descartes
 is simply following the Aristotelian tradition for which Aquinas
became the preeminent spokesman. For Aquinas himself, however, the
account just described serves as an aspirational ideal rather than a
description of something we have actually attained. In practice, even
where we have demonstrative knowledge, we usually cannot trace the
principles all the way back to ultimate first principles like the law
of non-contradiction. Critical questions, like the non-eternity of the
world (SS3), are contingent on the free choice of God and so have
to be taken on faith. Even in places where certainty is possible in
principle, large gaps in our knowledge remain. In general, "our
cognition is so weak that no philosopher could have ever completely
investigated the nature of a single fly" (*On the
Apostles' Creed* [*Collatio in symbolum
Apostolorum*], proem). Accordingly, Aquinas distinguishes among
various non-ideal cases. A demonstration that grasps the cause of a
thing is a demonstration *propter quid*, but often--as in
proving God's existence--the best we can have is a
demonstration *quia*, which tells us only *that* a thing
is so. Much of what we know holds true only for the most part, and so
fails to be necessary, and often the premises of an argument cannot be
traced back to self-evident principles, in which case our argument is
*dialectical* rather than demonstrative, yielding a conclusion
that is merely plausible (*probabilis*) rather than
apodictic.



In all of this, Aquinas is following Aristotle and the later
Aristotelian tradition fairly closely, and thus the framework for
science and knowledge just sketched comes to be commonly accepted by
the later scholastic tradition all the way into the seventeenth
century.


## 7. Will and Freedom



The immaterial human mind, as Aquinas understands it, is not just the
intellect. The mind is an ensemble of three discrete powers, agent and
possible intellect working in conjunction with the will. The rise of
the will as a faculty of the soul is a distinctively medieval idea,
but has strong roots in Augustine and, before him, in late
 Stoicism.
 These late-ancient sources had ascribed a prominent role in human
agency to desire (*appetitus*) or volition (*voluntas*),
and particularly to those desires that we endorse as our own, and
which thus give rise to voluntary action. Among these earlier authors,
our will is conceived of as something that we need to control, if we
are to lead our lives as we should. By Aquinas's time, however,
the will (*voluntas*) had come to be identified not just as a
power of the soul but as the power that is principally in control of
human agency, directing both our thoughts and our actions. Thus,
whereas Augustine and
 Boethius
 had wondered about how we have free choice (*liberum
arbitrium*) over our wills, Aquinas identifies the two, concluding
that "the will and free choice are not two powers, but
one" (*ST* 1a 83.4c). He was by no means the first to
conceive of the power of will in this way, but the cogency and
influence of his account ensured that this framework would dominate
later discussions.



The will is one of three appetitive powers within us, distinguished
from the irascible and concupiscible powers in being not a sensual
appetite, the source of the passions (King 1998; Miner 2009), but a
*rational* appetite, the source of voluntary agency. Its
rationality consists in the way it is coordinate with intellect,
taking as its inputs the intellect's practical judgments
regarding what is to be done. Since the will's function is
strictly appetitive, it cannot render its own practical judgments, and
to that extent can act only on the basis of intellect: "every
choice and actual will within us is immediately caused by an
intelligible apprehension" (*SCG* III.85.3). But that is
by no means to say that the will is determined by the
intellect's judgment, because ultimately our will has the power
to control what we think about: "the will moves both itself and
all other capacities. For I think when I want to, and likewise I use
all my other capacities and dispositions because I want to"
(*Quest. on Evil* 6c). To be sure, by its very nature, the will
can desire only what is presented to it as a good (*ST* 1a2ae
8.1). Accordingly, since our ultimate happiness (*beatitudo*)
is good in every respect, "human beings have an appetite of
necessity for happiness" (*ST* 1a2ae 10.2c; see
SS8.1). But, with regard to any specific course of action, there
are always multiple perspectives from which the situation can be
judged, and the will can control how we think about
things--whether, for instance, we decide to focus on how
rewarding a thing would be or on how tiring it would be. Even in the
case of our ultimate happiness, moreover, the will can choose not to
think about it, and so even in that extreme case the will's
choices are not necessitated. On this basis Aquinas adamantly insists
on human freedom of choice, and treats claims that the will is
necessitated as "contrary to the faith and subverting all of the
principles of moral philosophy" (*Quest. on Evil*
6c).



Up to this point, Aquinas's position is fairly clear. There has
been considerable controversy over the centuries, however, regarding
whether Aquinas wants to go further and characterize the will's
agency as
 nondeterministic--as
 its being, in other words, a first and undetermined cause of free
action. Within a few years of his death, after the
 Condemnation of 1277,
 views of this sort came to dominate scholastic philosophy, yielding a
broad consensus that held until the rise of seventeenth-century
anti-scholasticism. Readers of Aquinas have been sharply divided,
however, over where he fits into that history. At first glance it
might seem obvious that Aquinas is an opponent of determinism, given
his adamant opposition to the will's being necessitated. Yet
Aquinas writes at a time at which the governing form of
Aristotelianism, drawn from the Arabic tradition of Ibn Sina
and Ibn Rushd, is explicitly deterministic. In general, Aquinas
strives where he can to harmonize the faith with the mainstream of
philosophical opinion, and here in particular he shows signs of
endorsing causal determinism as applied to the created world. Thus he
ascribes to Ibn Rushd ("the Commentator") the doctrine
that "from that which is open both ways (*ad utrumlibet*)
no action follows, unless it is inclined one way by something
else" (*ST* 1a 19.3 obj. 5). He hastens to reject this
principle when applied to God, who is able to determine freely
God's own choices. But outside the case of God he accepts the
principle: "a cause that is of itself contingent must be
determined to its effect by something external" (ibid., ad 5).
Accordingly, he explicitly insists that the will's freedom does
not require it to be its own first cause. Instead, he characterizes
the will's operations as entwined over time with the operation
of intellect, running backward through an agent's history, with
one cause preceding another, all the way back to God's initial
creation of the human soul (*Quest. on Evil* 6, *ST* 1a
83.1 ad 3, *ST* 1a2ae 9.4c). This insistence on determining
causes might seem to fit well with his strict conception of
 divine providence:



> 
> Everything that happens here, insofar as it is brought back to the
> first divine cause, is found to be ordered rather than to exist
> accidentally, even if in comparison to other causes it is found to
> exist accidentally. This is why the Catholic faith says that nothing
> in the world happens aimlessly or fortuitously, and that all things
> are subject to divine providence. (*Comm. Metaphys.* VI.3.26;
> see Goris 1997)
> 



On this reading of Aquinas, causal determinism fits with God's
preordained plan for the universe, and freedom is thus compatible with
both causal and theological determinism (Hause 1997; Kenny 1993 ch. 6;
Loughran 1999; Pasnau 2002 ch. 7; Shanley 2007).



Many scholars, however, have denied that Aquinas is any sort of
compatibilist. That there is room for doubt is suggested in a passage
that, at first, looks to support a compatibilist account:



> 
> Freedom does not necessarily require that what is free be the first
> cause of itself, just as one thing's being the cause of another
> does not require that it be the first cause of that other. God, then,
> is the first cause, moving both natural and voluntary causes. And just
> as his moving natural causes does not take away from their acts'
> being natural, so his moving voluntary causes does not take away from
> their actions' being voluntary. Instead, he makes this be so for
> them, because he works within each thing in accord with its own
> character. (*ST* 1a 83.1 ad 3)
> 



Although the passage begins in a way that looks friendly to causal
determinism, the passage goes on to suggest the situation is more
complicated. For although God is the first cause of our voluntary
actions, God's efficacy there is different from how it is with
natural causes: God somehow acts on the will in a way that "does
not take away" from its voluntariness. And Aquinas takes just
the same line in explaining how divine providence is consistent with
human freedom (*ST* 1a 23.6c; *SCG* III.72-73;
*Comm. Metaphys.* VI.3.30-32; *Comm. De interp.*
I.14.22).



Among those who think that Aquinas's God leaves room for
indeterministic human agency, there remains a fundamental difference
of opinion. Some (e.g., Stump 2003 ch. 9) think that such freedom, for
Aquinas, requires only that the will be an ultimate source, a first
creaturely cause. Others think that, in addition to this, Aquinas
further requires the leeway to choose among alternative possibilities
(e.g., Hoffmann and Michon 2017). Either way, a libertarian reading of
Aquinas is likely to stress his claim that the will is a self-mover:
"Just as the will moves other capacities, so it also moves
itself" (*Quest. on Evil* 6c; Gallagher 1994). This is an
idea, moreover, that Aquinas had urged from his very first discussion
of free choice in his *Sentences* commentary, where he argued
that free choice is possessed only by beings who "determine for
themselves their end and their action toward that end"
(*Sentences* II.25.1c). In some sense, then, the will is free
because it is self-determining. This distinguishes it from natural,
non-voluntary agents, whose determination comes from without.
Aquinas's focus on the will's role in such
self-determination--a tendency that becomes more prominent in his
later works--makes it reasonable to include him in the
thirteenth-century movement away from ancient intellectualism and
toward a voluntaristic conception of human nature. In emphasizing the
will's overarching control, Aquinas can appeal to the critical
importance of the will's various operations and dispositions.
The act of love, foundational to any Christian ethics, is an operation
of will. The dispositions of justice and charity--the central
virtues of philosophy and theology, respectively--are both
virtues of the will. But it is not clear that Aquinas goes so
far--as
 Peter John Olivi,
 Henry of Ghent, and Scotus, among many others, soon would--as to
exempt the will from the determinism that was agreed to characterize
the rest of the natural world. There remains a lively debate among
scholars over this issue.


## 8. Ethics



Aquinas's ethics is difficult to study because it is so vast and
multifaceted. Around half of the *Summa theologiae*, the
massive second part, is concerned with moral questions, suggesting
something of the importance Aquinas ascribes to ethics in theology. To
bring some order to this large topic, we can distinguish between three
parts of the theory:


* his conception of happiness as the goal of human life;
* his framework for the moral law, grounded in a theory of natural
law;
* his understanding of the virtues as essential to human
happiness.



(For wide-ranging treatments of Aquinas's ethics see McInerny
1997; MacDonald and Stump 1998; Pope 2002; Irwin 2007: chs
16-24; DeYoung, McCluskey, and Van Dyke 2009; McCluskey
2017.)


### 8.1 Happiness



Aquinas's worldview is thoroughly teleological, inasmuch as he
holds "it is necessary that all agents act for the sake of an
end" (*ST* 1a2ae 1.2c). There is a distinction to be
drawn, however, between those agents that are unable to understand
their end and rational agents, who "move themselves to their end
because they have control over their acts through free choice"
(ibid.). It is for rational agents that ethical questions arise.
Aquinas follows Aristotle in supposing that human choices must be
ordered toward a single ultimate end: "there must be some
ultimate end on account of which all other things are desired, whereas
this end itself is not desired on account of anything else"
(*Comm. Nic. Ethics* I.1.22). This ultimate end, whatever it
is, can be referred to as happiness (*beatitudo*). In the
Treatise on Happiness from the start of *ST* 1a2ae, after
working through various predictably unsatisfactory candidates for our
ultimate end--not riches, not honor, not pleasure, and so
on--Aquinas arrives at the view that happiness consists primarily
in intellectual contemplation. And since the highest truth and the
greatest good are of course God, our ultimate happiness must lie
there: "final and complete happiness can consist in nothing
other than the vision of the divine essence" (*ST* 1a2ae
3.8c). Human beings possess this most fully in the life to come (Brown
2021), but even in this life Aquinas's conception of happiness
is strongly intellectualist: "it is clear that people who give
themselves to the contemplation of truth are the happiest a person in
this life can be" (*Comm. Nic. Ethics* X.12.2110).



This eudaimonistic framework--to use the Greek term for ethical
theories aimed at happiness--shapes all of Aquinas's
ethical thinking. It is perhaps the most striking instance of his
confidence in Aristotelianism as the proper philosophical foundation
for a Christian worldview. Predictably, that confidence was met with
various sorts of challenges. One concerns his insistence that our
ultimate end is intellectual rather than volitional. For many
subsequent theorists, Christian and secular, the ultimate end of human
life should not be intellectual contemplation but instead should be
love. Aquinas agrees--indeed, he stresses--that love is an
essential complement to happiness, pulling us toward our end and
rejoicing in its grasp. Still, the goal of life properly speaking is
to understand God and God's creation (*ST* 1a2ae 3.4).
(For nuanced discussions see Stenberg 2016a and Stump 2022.)



A second concern, which troubles eudaimonism in all of its forms, is
that Aquinas rests the foundations of ethics on a self-directed
concern for our own happiness. That is liable to look like a doubtful
strategy for any ethical theory, and particularly for
Judeo-Christian ethics given its radically altruistic
orientation, as epitomized in the twin injunctions to "Love the
Lord your God with all your heart and with all your soul and with all
your mind" and to "Love your neighbor as yourself"
(Matthew 22:37-39, quoting Deuteronomy 6:5 and Leviticus 19:18).
On its face, the eudaimonistic approach might seem to conflict
directly with these injunctions, given the way that it grounds all
human agency in our ultimate pursuit of happiness:



> 
> The will naturally tends towards its ultimate end: for every human
> being naturally wills happiness. And this natural willing is the cause
> of all other willings, since whatever a human being wills, he wills
> for the sake of an end. (*ST* 1a 60.2c)
> 



Aquinas is unflinching in his insistence that our final end is our own
happiness. But it has to be remembered that 'happiness' is
just a generic label for the human activity that is ultimately best
for us. And what is ultimately best for us is not the selfish pursuit
of our own wellbeing, which is surely an ambition that would make
anyone miserable. What we should seek, as we have seen already, is an
understanding of the world and its Creator. This, again speaking
teleologically, is what we were created to do. By pursuing this
substantive goal, and actualizing our intellectual capacities to the
utmost, we make ourselves as good as we can be, and we contribute to
the universe as a whole's being as good as it can be. How does
such intellectual activity ground a theory of ethics? It does so
because when we understand God and the created order, we understand
why we should love God preeminently and why we should love our
neighbor. But to see how these ideas get developed, we need to turn to
his theory of the moral law. (For general discussions of
Aquinas's eudaimonistic framework see MacDonald 1991b; Bradley
1997; Kenny 1998; Osborne 2005; Stenberg 2016b.)


### 8.2 Natural Law



Aquinas's moral theory does not offer the sort of brief and
comprehensive criterion for rightness and wrongness that draws
students toward
 consequentialism
 or
 Kant's moral philosophy.
 What he offers instead is a deep and systematic explanation of where
morality comes from, yielding a moral code that is impressively
responsive to the changing circumstances of human life.



The key text for Aquinas's thinking about the moral law is his
Treatise on Law (*ST* 1a2ae 90-108). There he
distinguishes between four kinds of law that play a role in guiding
right human action:


* eternal law: God's plan of governance for the world (q.
93);
* natural law: the distinctive way rational beings participate in
the eternal law (q. 94);
* human law: particular developments of natural law worked out by
human reason (qq. 95-97);
* divine law: divinely revealed laws directing human beings to their
end (qq. 98-108).



The eternal law governs everything, but can serve to guide us only
when it is somehow transmitted to us. One way in which it is
transmitted is through divine law, preeminently through the Bible, and
here Aquinas distinguishes between the old law of the Hebrew Bible
(qq. 98-105) and the new law described in the Gospel (qq.
106-8). The other form of transmission, philosophically the most
interesting part of his account, is the
 natural law.
 Whereas human law is the contingent result of social and political
organization, the natural law is innate within us. Since Aquinas
thinks that God orders everything to its proper end (SS8.1), there
is a sense in which all things follow a natural law by which they
participate in the eternal law. But when Aquinas refers to natural law
in a moral context, he means the distinctive way in which rational
agents have been ordered to achieve their proper end; hence he has in
mind a law that governs the mind. Thus, "the law of nature is
nothing other than the light of intellect, placed within us by God,
through which we grasp what is to be done and what is to be
avoided" (*On the Ten Commandments* [*Collationes in
decem praeceptis*] proem).



So described, natural law might be nothing more than an innate
inclination to accept a fixed set of moral laws prescribed by God. In
fact, however, Aquinas's theory offers something more complex
and nuanced, because he thinks the moral law arises not out of brutely
innate inclination but out of rational reflection on the good (Finnis
1998 ch. 3; Rhonheimer 1987 [2000]). Mirroring the way he develops
theoretical science out of first principles (SS6.3), he thinks
moral reasoning also rises out of self-evident first principles, the
grasp of which is the defining task of the intellect's power of
synderesis (*ST* 1a 79.12). The most fundamental such precept
is this:



> 
> The good should be done and pursued, and the bad should be avoided.
> (*ST* 1a2ae 94.2c)
> 



Aquinas takes this to be self-evident, because he accepts
Aristotle's claim at the start of the *Nicomachean
Ethics* that the good is what all things desire. By beginning
here, Aquinas builds into the ground floor of his account a
substantive story about
 moral motivation.
 The story requires tying the ensuing development of his theory to
facts about what human beings actually desire; without that tie, the
theory would equivocate on what it means by the good. Aquinas
explicitly recognizes as much, remarking that "the order of the
precepts of natural law accords with the order of natural
inclinations" (*ST* 1a2ae 94.2c). At this point, Aquinas
could appeal to eudaimonism: that all human beings desire happiness as
their ultimate end (SS8.1). A very thin conception of natural law
might suppose that human beings are given only this much innate
direction, and thereafter left to sort out for themselves what will
make them happy. But Aquinas's conception of natural law is much
thicker, both in the sense that he articulates a rich and substantive
notion of the happiness that is our ultimate end (as above), and in
the sense that he thinks we have been given various further innate
inclinations, intended to provide us with specific guidance toward
that ultimate end. Among these are inclinations to preserve our own
lives, toward sexual activity, toward educating the young, toward
knowing the truth about God, and toward living in society (*ST*
1a2ae 94.2c; see also *SCG* III.129). These innate
inclinations, combined with the first practical principle ("The
good should be done...") and Aquinas's substantive
conception of happiness, are the foundations from which arises a
comprehensive account of the moral law, relying on
 conscience
 as the rational activity of working out what ought to be done from a
moral point of view (*ST* 1a 79.13).



Inasmuch as the moral point of view is no different from the point of
view that seeks our own happiness, Aquinas shares with ancient ethics
the conviction that rational self-interest provides an adequate
foundation for morality (Irwin 2007: ch. 19). But the weight Aquinas
puts on our innate inclinations in shaping the moral law gives his
account a distinctive character. Among its champions, this approach
has been celebrated for the way it grounds traditional values in facts
about human nature. To its critics, the view looks intellectually
dubious twice over: on scientific grounds inasmuch as it makes false
claims about the universality of various inclinations (Massey 1999);
and philosophically inasmuch as it grounds
 normativity in metaethics
 on descriptive facts about human nature. Such criticisms, however,
miss the philosophical sophistication of the overarching framework. To
be sure, we now understand the diversity of human
inclinations--for instance, regarding gender and
sexuality--much better than we did even a century ago.
Aquinas's theory can survive when we update these assumptions,
and indeed may generate surprisingly modern results (Oliva 2015). As
for the complaint that the theory conflates normativity with nature,
that would have real force only supposing that we had some better
account of the basis of normative value. As things are, if there is a
God, then it seems plausible to suppose that God would want us to be
happy and would create our natures to guide us in seeking such
happiness. On the other hand, if there is no God, then it is not clear
what foundation for ethics there might be other than facts about the
nature of human beings and how we best thrive in the world we live in.
(The literature on the natural law in Aquinas is large and
contentious. For a sample see Grisez 1965; Lisska 1996; Murphy 2001;
Jensen 2015; Porter 2018. For an introduction to the variety of recent
perspectives see Angier 2021. For a sense of the complexity of the
contested issues see
 Aquinas' moral, political, and legal philosophy.)


### 8.3 Virtue Theory



The theory of natural law gives us Aquinas's moral theory: it
identifies the foundations of ethics and frames the general contours
of the moral law. For us to adhere to that law, however, it is not
enough to trust to the light of intellect and our natural
inclinations. Reliably moral agents must, in addition, cultivate the
right sorts of virtuous dispositions. Accordingly, Aquinas develops a
complex
 virtue ethics
 to frame his account of our moral psychology.



The virtues are a certain kind of psychological disposition
(*habitus*): they are the perfections of those powers of the
soul that are under our voluntary control. Aquinas devotes roughly a
quarter of the *Summa theologiae--*mainly the
2a2ae--to his theory of the virtues, and also dedicates a series
of disputed questions to the subject. He takes from Aristotle the
familiar idea that it is through the repetition of certain kinds of
actions that we acquire the virtues (as well, of course, as the
vices): by acting honestly we acquire, over time, the virtue of
honesty. Less familiarly, he imposes a very high standard on what
counts as a virtue: it must be incapable of giving rise to an immoral
act.



> 
> Two things are required for an act's perfection: first, the act
> must be right; second, the [underlying] disposition must be incapable
> of being the source of a contrary act. For that which is the source of
> good and bad acts cannot, of itself, be the perfect source of a good
> act. (*Quest. on the Virtues in General* 2c)
> 



This is not to say that a person with the virtue of honesty cannot be
dishonest. Just as virtues can be acquired through practice, so they
can be lost through the wrong kind of practice, and that loss must
begin somewhere. Still, in such a case, the dishonesty will not arise
through the virtue. Any characterization of a virtue that would make
its possession a mixed blessing would have failed to delimit a true
virtue.



The reason that the virtues are so important to Aquinas's ethics
is that he thinks that we cannot, over time, act morally without them.
He highlights three reasons why moral activity requires the virtues
(*Quest. on the Virtues in General* 1c):


* for uniformity of action, avoiding unreliable vicissitudes;
* to be prompt in that action, rather than needing constantly to
deliberate;
* to take pleasure in the action, as a kind of second nature.



These are not meant to be necessary conditions on an act's
goodness, but Aquinas thinks they are required for a person's
actions to count as wholly praiseworthy. In contrast to the effortful
calculations of a utilitarian striving to maximize the beneficial
consequences of every act, he thinks moral goodness requires a
dispositional stability in our underlying psychology. And given
morality's ultimate basis in Aquinas's overarching theory
of happiness, it should be no surprise that he thinks happiness, in
this life, consists in our acting through these virtues (*ST*
1a2ae 5.5-6).



Among the many fine-grained distinctions that Aquinas makes between
various virtues and vices, he treats as preeminent the traditional
four cardinal virtues and the three theological virtues. The cardinal
virtues are


* prudence: the perfection of the intellect's
 practical reasoning
 (*ST* 2a2ae 47-51);
* justice:
 the will's stable disposition to give to each individual what
is due to him (*ST* 2a2ae 58);
* bravery: the strength of the irascible appetite in pursuing what
is in accord with reason (*ST* 2a2ae 143);
* temperance: the moderation of the concupiscible appetite with
respect to its desires that conflict with reason (*ST* 2a2ae
141).



Unlike some of his contemporaries, who argued that all the virtues
should be dispositions of the will (Kent 1995), Aquinas thinks that
each of these four virtues perfects a different power of the soul.
Bravery and temperance, indeed, are not perfections of rational powers
at all. They are, however, defined in terms of right reason, and this
both connects his virtue theory to the theory of natural law and helps
to explain why the virtues cannot be the cause of immoral actions. The
person who is "brave" in a foolhardy way would not
strictly count as brave. Deepening the connection between these
virtues and right reason is that Aquinas treats the intellectual
virtue of prudence (*prudentia*, or *phronesis* in
Greek) as foundational to all the other cardinal virtues:
"rightness and full goodness in all the other virtues comes from
prudence" (*Quest. on the Virtues in General* 6c; see
Westberg 1994). Without prudence, none of the other virtues are
possible. Conversely, prudence itself requires the proper disposition
of our various appetitive powers. Accordingly, Aquinas endorses the
traditional doctrine of the unity of the virtues: one cannot have any
of them without having all of them (*ST* 1a2ae 65.1).



Although the four cardinal virtues, as their name suggests, have
historically been given pride of place within philosophy, Aquinas
gives greater preeminence to the theological virtues that Paul
describes in 1 Corinthians 13:


* faith: the intellectual disposition to assent to what is not
apparent (*ST* 2a2ae 1-7);
* hope: the will's dispositional confidence in achieving a
good that is difficult to obtain (*ST* 2a2ae 17-18);
* charity: the will's disposition for perfect love
(*ST* 2a2ae 23-27).



For these to count as *theological virtues*, rather than as
generic dispositions, they must each be further defined as having God
as their object. They must also be understood to be infused in us by
God, because the humanly attainable versions of these virtues are not
sufficient for our moral perfection (*ST* 1a2ae 62.1). Indeed,
for similar reasons, moral perfection requires that even the cardinal
virtues be infused by God (*ST* 1a2ae 63.3). To receive the
virtues from God requires grace, which is something we can never fully
merit, and which God freely chooses to give or to withhold: "no
matter how much someone prepares himself, he does not receive grace
from God necessarily" (*ST* 1a2ae 112.3sc). Only through
this grace, and the infused virtues it brings, can we achieve the
perfect happiness that is our ultimate end. (On the cardinal virtues
see Hause 2007; Stump 2011. On the theological virtues see Wawrykow
2012; Porter 2019. On the infused cardinal virtues see Mattison 2011;
Goris and Schoot 2017; Knobel 2021. On grace see Torrell 1996 [2003]
chs. 6-9; Stump 2018 ch. 7; Hoffmann 2022.)


## 9. Influence



During his lifetime, Aquinas was recognized as an extraordinary figure
both for his intellectual achievements and for his personal sanctity.
The immense number of surviving manuscripts and the wide range of his
influence testifies to the first. As for the second, his death in
Fossanova immediately gave rise to the sort of veneration reserved for
the saints: claims of miracles at once arose, and a long-running
dispute began over where the holy relic of his body would be kept.
This process culminated in his canonization on July 18, 1323.



These developments notwithstanding, many of Aquinas's specific
views--particularly in philosophy--were extremely
controversial. Among the 219 articles condemned in Paris in 1277 are a
significant number that seem to implicate Aquinas's teachings
(Hissette 1977; Wippel 1995b). It would have been lost on no one at
the time that these articles were promulgated three years, to the day,
after his death. It seems likely that the authorities in Paris were
planning an explicit censure of certain of Aquinas's core
theses--especially his unitarian theory of substantial form
(SS5) and his conception of prime matter as pure potentiality
(SS4)--until higher authorities cut this process short
(Torrell 1993 [2022] ch. 16). Later in March 1277, in Oxford,
Archbishop
 Robert Kilwardby
 condemned a series of theses that explicitly includes those two
distinctively Aquinian doctrines. Also around this time, the
Franciscan William de la Mare published a lengthy *Correctorium
fratris Thomae*, with verbatim quotations from Aquinas's
corpus paired with putative "corrections". Soon after,
various disciples of Aquinas rallied to his defense, in a series of
works entitled the *Correctorium Corruptorii Thomae* (Glorieux
1927; Jordan 1982). For the quarter century after Aquinas's
death, scholasticism took shape around his Dominican supporters and
his Franciscan critics.



In the early fourteenth century, Aquinas's influence had to
compete against the emergence of two brilliantly original Franciscans:
first Scotus and his expansive realism; then
 William Ockham
 and his parsimonious nominalism. In the face of these and many other
complex crosscurrents, Aquinas's views came to seem both less
urgent and less dangerous. Two years after his canonization, the
bishop of Paris clarified that the 1277 condemnations should not be
held to apply to Aquinas's teaching, though the bishop made
clear that this should not be taken as an endorsement. Among the many
emerging rival views, Aquinas continued to have his vociferous
supporters, particularly among the Dominicans, who were statutorily
required to promote his teaching. In the early fifteenth century, John
Capreolus emerged as a champion of Thomism as a systematic philosophy,
and a century later Cajetan (Thomas de Vio) made further systematic
developments. In 1567, Pope Pius V numbered Aquinas among the doctors
of the Church, a title previously reserved for the ancient Church
fathers. When the Jesuit order came to prominence, around this same
time, its members--including
 Francisco Suarez--were
 enjoined to adhere to those views that were safest and best
established, and especially to those of Aquinas, "because of his
authority and his more secure and approved teachings" (Pasnau
2011, 436).



Even as Thomism gradually became ascendant within scholastic
Aristotelianism, that whole tradition started to look more and more
hidebound, as it moved into its increasingly mannerist and then
baroque stages.
 Martin Luther's
 hostility toward the Roman Catholic Church mirrored his hostility
toward scholastic philosophy and theology: "soon no Thomist,
Albertist, Scotist, or Ockhamist will be left in the world, but all
will be simple children of God and true Christians" (Oberman
1966, 18-19). The Italian humanists attacked from a different
direction, prizing historical scholarship over intricate metaphysics.
By the seventeenth century,
 Thomas Hobbes
 and his contemporaries flaunted their scorn for the scholastics, who
"converse in questions of matters incomprehensible"
(*Leviathan* viii.27). It would not be until the eighteenth
century, however, that these "moderns" began to drive the
Thomists and other scholastics out of the universities. (On the
reception of Aquinas over the centuries see Levering and Plested
2021.)



In 1879, Pope Leo XIII called for a revival in the study and teaching
of Aquinas. One fruit of this encyclical was the founding of the
definitive Leonine edition of Aquinas's work. Another was a
renewal of the old Thomistic picture of Aquinas as standing at the
summit of philosophical and theological achievement, leaving
subsequent developments--from Scotus to Descartes, Kant and
beyond--to be dismissed as just so much decline and decay. Such
zeal was hardly likely to be sustained for long, however, and the
twentieth century witnessed a flourishing of Thomisms of all kinds, in
scholars like
 Jacques Maritain,
 Etienne Gilson,
 Elizabeth Anscombe,
 and Alasdair MacIntyre. As more recent scholarship has deepened our
understanding both of Aquinas and of his successors, we now have a
much fuller picture of the richness of later medieval philosophy.
Aquinas today, viewed as a philosopher, looks not so much like the
culmination of an era, but rather like the brilliant beginning of
Europe's philosophical renaissance.