Even among contemporary philosophers who are otherwise unfamiliar with his work, it is fairly well known that Aquinas held that the existence of God, the immortality of the soul, and the content and binding force of the natural moral law could be established through purely philosophical arguments (as opposed to an appeal to divine revelation).
But those arguments themselves are in general very badly misunderstood by those who are not experts on Aquinas.
The reason is that most contemporary philosophers have little or no awareness of just how radically different the fundamental metaphysical assumptions of ancient and medieval philosophers are, in general, from the assumptions typically made by the early modern philosophers and their successors.
A distinctive conception of causation, essence, form, matter, substance, attribute, and other basic metaphysical notions underlies all of Aquinas’s arguments in philosophy of religion, philosophy of mind, and ethics; and it is a conception very much at odds with the sorts of views one finds in Descartes, Locke, Hume, Kant, and the other founders of modern philosophy.
While most contemporary philosophers would probably not identify themselves as Cartesians, Lockeans, Humeans, Kantians, or the like, their thinking about the metaphysical concepts just noted nevertheless tends, however unconsciously, to be confined within the narrow boundaries set by these early modern thinkers.
Hence when they come across a philosopher like Aquinas, they unthinkingly read into his arguments modern philosophical presuppositions he would have rejected. The result is that the arguments are not only misinterpreted, but come across as far less interesting, plausible, and defensible than they really are.
In rejecting them, as contemporary philosophers tend to do, they do not realize that what they are rejecting is a mere distortion or caricature of Aquinas’s position rather than the real McCoy.
An overview of Aquinas’s general metaphysics is therefore a necessary preamble to a consideration of his views in these other areas of philosophy. Such an overview would be of value in any case, for Aquinas’s metaphysical ideas are important and interesting in their own right. We shall also see that they are as defensible today as they ever were, and (ironically enough) that some work by contemporary philosophers, quite outside the camp of Thomists and otherwise unsympathetic to Aquinas’s overall project, tends to support this judgment.
A distinctive conception of causation , essence , form , matter , substance , attribute , and other basic metaphysical notions underlies all of Aquinas’s arguments in philosophy of religion , philosophy of mind , and ethics ; and it is a conception very much at odds with the sorts of views one finds in Descartes , Locke , Hume , Kant , and the other founders of modern philosophy .
Hence when they come across a philosopher like Aquinas , they unthinkingly read into his arguments modern philosophical presuppositions he would have rejected . The result is that the arguments are not only misinterpreted , but come across as far less interesting , plausible , and defensible than they really are .
In rejecting them , as contemporary philosophers tend to do , they do not realize that what they are rejecting is a mere distortion or caricature of Aquinas’s position rather than the real McCoy .
Act and potency The Greek philosopher Parmenides ( c . 515 – 450 B.C . ) notoriously held that change is impossible . For a being could change only if caused to do so by something other than it . But the only thing other than being is non - being , and non - being , since it is just nothing , cannot cause anything . Hence , though the senses and common sense tell us that change occurs all the time , the intellect , in Parmenides ’ view , reveals to us that they are flatly mistaken .
At the same time , Aristotle was loath simply to dismiss a theory like Parmenides ’ on the grounds that it was odd or counterintuitive ; it was important to understand exactly why such a theory was mistaken .
Take any object of our experience : a red rubber ball , for example . Among its features are the ways it actually is : solid , round , red , and bouncy . These are different aspects of its “ being . ” There are also the ways it is not ; for example , it is not a dog , or a car , or a computer . The ball’s “ dogginess ” and so on , since they don’t exist , are different kinds of “ non - being . ” But in addition to these features , we can distinguish the various ways the ball potentially is : blue ( if you paint it ) , soft and gooey ( if you melt it ) , and so forth . So , being and non - being are not the only relevant factors here ; there are also a thing’s potentialities .
Here lies the key to understanding how change is possible .
a potential or potency for gooeyness does exist in the ball ,
Change just is the realization of some potentiality ; or as Aquinas puts it , “ motion is the actuality of a being in potency ”
Potential gooeyness ( for example ) , precisely because it is merely potential , cannot actualize itself ; only something else that is already actual ( like heat ) could do the job .
Why did this potential gooeyness become actual at precisely that point ? The obvious answer is that the heat was needed to actualize it . If the potency for gooeyness could have actualized itself , it would have happened already , since the potential was there already .
So , as Aquinas says , “ potency does not raise itself to act ; it must be raised to act by something that is in act ” ( SCG I . 16.3 ) .
This is the foundation of the famous Aristotelian – Thomistic principle that “ whatever is moved is moved by another ” ( In Phys VII . 2.891 ) .
“ absolutely speaking act is prior to potency ”
But it is not incoherent to speak of something as being purely actual , with no potentiality at all .
So , while for us to understand act and potency we need to contrast them with one another , in the real world outside the mind actuality can exist on its own while potentiality cannot .
the distinction between act and potency forms the basis of Aquinas’s entire metaphysical system ;
Aquinas , following Aristotle , concludes that “ in everything which is moved , there is some kind of composition to be found ”
a related Aristotelian doctrine to the effect that the ordinary objects of our experience are composites of form and matter – a doctrine known as hylemorphism ( sometimes spelled “ hylomorphism ” ) after the Greek words hyle ( “ matter ” ) and morphe ( “ form ” ) .
It is only the form and matter together that constitute the ball .
Anything compounded of form and matter is also compounded of act and potency , but there are compounds of act and potency that have no matter ( namely angels , as we shall see later on ) .
“ What makes something exist substantially is called substantial form , and what makes something exist accidentally is called accidental form ”
Aquinas tells us that “ what is in potency to exist substantially is called prime matter ”
The notion of prime matter is just the notion of something in pure potentiality with respect to having any kind of form , and thus with respect to being any kind of thing at all . And as noted above , what is purely potential has no actuality at all , and thus does not exist at all .
On the hylemorphic analysis , considered apart from the substances that have them , form and matter are mere abstractions ;
Aristotle and Aquinas are , like Plato , realists about universals : when we grasp “ humanity , ” “ triangularity , ” and the like , what we grasp are not mere inventions of the human mind , but are grounded in the natures of real human beings , triangles , or what have you .
not every form exists in a material substance .
just as act can exist without potency even though potency cannot exist without act , so too form can exist without matter even though matter cannot exist without form
they are not themselves the sorts of things that come to be and pass away .
we should note that prime matter , and even form , are neither generated nor corrupted , inasmuch as every generation is from something to something . That from which generation arises is matter ; that to which it proceeds is form .
Hence , properly speaking , only composites are generated .
The material cause or underlying stuff the ball is made out of is rubber ; its formal cause , or the form , pattern , or structure it exhibits , comprises such features as its sphericity , solidity , and bounciness . In other words , the material and formal causes of a thing are just its matter and form , considered as two aspects of a complete explanation of it .
For instance , to understand what a heart is , you need to know its material cause , namely that it is made out of muscle tissue of a certain sort . But there are many muscles in the body that aren’t hearts , so you also need to know its formal cause , and thus such things as that the muscle tissue is organized into ventricles , atria , and the like . Then there is the efficient cause , which in this case would be the biological processes that determined that certain embryonic cells would form into a heart rather than , say , a kidney or a brain . Finally there is the heart’s final cause , namely that it serves the function of pumping blood .
All functions are instances of final causality , but not all final causality involves the having of a function , if by “ function ” we mean the sort of role a bodily organ plays in the life of an animal or the role a mechanical part plays in the operation of a machine . For the Aristotelian , final causality or teleology ( to use a more modern expression ) is evident wherever some natural object or process has a tendency to produce some particular effect or range of effects .
in that way manifests just the sort of end - or goal - directedness characteristic of final causality , even though the match does not ( unlike a heart or a carburetor ) function as an organic part of a larger system .
The same directedness towards a certain specific effect or range of effects is evident in all causes operative in the natural world . When Aristotelians say that final causality pervades the natural order , then , they are not making the implausible claim that everything has a function of the sort biological organs have , including piles of dirt , iron filings , and balls of lint . Rather , they are saying that goal - directedness exists wherever regular cause and effect patterns do .
“ every agent acts for an end : otherwise one thing would not follow more than another from the action of the agent , unless it were by chance ”
In other words , we cannot make sense of efficient causality without final causality . They go hand in hand , just as a thing’s material and formal causes go hand in hand in the sense that matter cannot exist without form and form , in the ordinary case anyway , does not exist without matter .
“ an ordering or tendency to an act belongs to a thing existing with a potency to that act ”
Hence the existence of final causes also entails the act / potency distinction . Implicit within the notion of final causality , then , is the entire Aristotelian metaphysical apparatus .
As Aquinas writes , “ although every agent , be it natural or voluntary , intends an end , we should realize nevertheless that it does not follow that every agent knows or deliberates about the end ”
To “ intend an end ” in the sense Aquinas has in mind in the passage just quoted is not necessarily to make a conscious decision to pursue some goal , but rather just “ to have a natural inclination toward something ”
the Aristotelian , efficient causes cannot be understood apart from final causes ,
David Hume ( 1711 – 1776 ) , tended to think it “ conceivable ” that any cause might produce any effect or none .
For Aristotle and Aquinas , it is things that are causes , not events ; and the immediate efficient cause of an effect is simultaneous with it , not temporally prior to it . “ It should be understood in speaking of actual causes that what causes and what is caused must exist simultaneously , such that if the one exists , the other does also ”
Hume also claims that something could in principle come into being without any efficient cause whatsoever .
“ everything whose act of existing is other than its nature [ must ] have its act of existing from another ”
In other words , whatever is contingent , not having its existence by virtue of its own nature , must be caused to exist by something else .
“ effects must needs be proportionate to their causes and principles ”
“ whatever perfection exists in an effect must be found in the effective cause ” ( ST I . 4.2 ) . For a thing cannot give what it does not have .
the effect is not always contained in the cause “ formally , ” it will yet be contained in it “ eminently ” or “ virtually . ”
This last principle came to be known within the Scholastic tradition as the principle of proportionate causality . That whatever comes into existence , and more generally that any contingent thing , must have a cause , came to be known as the principle of causality . Aquinas’s dictum that “ every agent acts for an end ” is known as the principle of finality . These three principles are central to Aquinas’s general metaphysics , and , as we shall see in the next chapter , to his arguments concerning the existence and nature of God in particular .
let us complete our survey of Aquinas’s metaphysical framework by examining some of its components that most clearly constitute developments of Aristotelian ideas beyond the point at which Aristotle himself left them .
Aquinas’s famous theory of essence and its relationship to existence .
The essence of a thing is just that which makes it the sort of thing it is , “ that through which something is a certain kind of being ”
Hence to grasp humanity is to grasp the essence of human beings – that which makes them human – and thus to understand what a human being is ; to grasp triangularity is to grasp the essence of triangles – that which makes them triangles – and thus to understand what a triangle is ; and so forth . A thing’s essence is also sometimes called its “ nature , ” “ quiddity , ” or “ form ” ( though as we shall see , “ form ” sometimes has a narrower sense in which it refers to only a part of a thing’s essence ) . The doctrine that ( at least some ) things have real ( as opposed to merely conventional ) essences is called essentialism .
The reason is that his ability to learn languages derives from his rationality ; its necessity , though real , is therefore a derived necessity . It is only those features of a thing that are not derived in this way that can , from the Aristotelian point of view , count as part of the essence of a thing . Those features deriving from the essence , such as Socrates ’ ability to learn languages , are instead referred to as “ properties , ” since they are proper or necessary to a thing in a way that its purely contingent features ( like Socrates ’ being in Athens or having been a soldier ) are not .
Thus considered , however , humanity exists , not in the world outside the mind , but as a concept . “ The character species is included among the accidents which follow upon [ an essence or nature ] according as it exists in the intellect . The characters genus and difference also belong to nature so considered ”
Though humanity and the like qua universals exist only in the intellect , “ such conceptions have an immediate basis in reality ”
But that doesn’t entail that humanity does not exist at all in Socrates , George Bush , and other human beings , only that it does not exist in them in the abstract way in which it exists in the intellect , that is , divorced from all individualizing features . Aquinas is thus a realist , albeit of the Aristotelian or “ moderate ” sort ( as opposed to the “ extreme ” realism represented by Plato’s Theory of Forms ) . “ The nature is said to be in the thing inasmuch as there is something in the thing outside the soul that corresponds to the conception of the soul ”
But humanity as such is neither particular nor universal , neither one nor many , and could not be either , for “ each is extrinsic to the notion of humanity , and either can happen to it ” ( DEE 3 ) . If universality or “ manyness ” was part of humanity as such , then humanity could never exist in a particular thing , as it obviously does in ( for example ) Socrates . If particularity or “ oneness ” was part of humanity as such , then humanity could never be shared by multiple distinct individuals , as it obviously is shared by ( for example ) Socrates and George Bush . Hence , “ universals as such exist only in the soul ; but the natures themselves , which are conceivable universally , exist in things ”
With respect to material things , “ the term ‘ essence ’ signifies the composite of matter and form ” ( DEE 2 ) , and not just the form alone ; “ otherwise , ” Aquinas says , “ there would be no difference between definitions in physics and in mathematics ”
Hence matter is part of the essence of objects of the latter sort.
matter is for Aquinas the “ principle of individuation ” between members of a species of material things , that which makes them distinct things of the same type
we must make a distinction between matter in general , and this or that particular parcel of matter . It is the former , or “ common matter , ” that is part of the essence of trees , and the latter , or “ designated matter , ” that individuates one tree from another .
An angel , says Aquinas , is a form without matter , and thus its essence corresponds to its form alone ( DEE 4 ) . But precisely because there is no matter to distinguish one angel in a species from another , “ among these substances there cannot be many individuals of the same species . Rather , there are as many species as there are individuals ” ( DEE 4 ) .
Even an angel has to be created , and thus pass from potency to act . But since angels are immaterial , this cannot involve matter taking on a certain form . What it does involve is the form or essence being conjoined to what Aquinas calls an actus essendi or “ act of existing . ” Matter is “ in potency ” or only potential relative to form , which is what actualizes matter . But relative to the act of existing , both pure form ( as in an angel ) and a composite of form and matter ( as in a material object ) are themselves in potency or only potential .
Here we come at last to Aquinas’s famous doctrine of the distinction between essence and existence . To return again to our example of humanity , “ it is … evident that the nature of man considered absolutely abstracts from every act of existing , but in such a way , however , that no act of existing is excluded by way of precision ”
But in that case , “ it is evident that the act of existing is other than essence or quiddity ” for “ whatever is extraneous to the concept of an essence or quiddity is adventitious , and forms a composition with the essence ”
Or in other words , if it is possible to understand the essence of a thing without knowing whether it exists , its act of existing ( if it has one ) must be distinct from its essence , as a metaphysically separate component of the thing .
That is to say , something whose essence is its existence would depend on nothing else ( e.g . matter ) for its existence , since it would just be existence or being . But there could only possibly be one such thing , for there would be no way in principle to distinguish more than one .
exist . But all of this shows that in everything other than God , essence and existence must be distinct .
For now we can note that his conception of God as that in which essence and existence are identical dovetails nicely with the older Aristotelian notion of God as pure act .