Sumario

1. The problem in Aristotle’s De Anima

The problem of the soul’s immortality has haunted westerns thinkers since the birth of philosophy in Ancient Greece. Aristotle is probably responsible for laying down the epistemological basis that provided the first systematic and scientific approach to this issue. However, his work is not free from problems, especially internal logic dilemmas. Some may even say that, within the frame of the De Anima, the problem has no answer, and that it offers more problems than solutions, becoming an aporia more than anything else. In this first part, we will go over the main passages of the De Anima that consider the possibility for the individual human soul to be immortal. It is important to consider that the number of passages grows considerably when considering those that refer to the issue indirectly, or that the idea involved can shed a light on the main problem but does not address it directly. Nonetheless, since our main goal is to discuss Thomas Aquinas’ interpretation, only the main passages relevant to the medieval philosopher as part of his own argumentative structure will be studied. Our thesis is that Aquinas made an effort to maintain his case for the immortality of the individual soul truthful to Aristotle’s psychology, avoiding adding Neoplatonic elements when dealing with this aporia. Also, that he kept his reasoning within the rational realm, not needing the Revelation to demonstrate the immortality of the human soul. Finally, we will also show that, within the De Anima, Aristotle expresses his view on the soul-body relationship in a strong form of hylomorphism as well as in a soft one,¹ which is the one taken by Aquinas to express his own theory.

First, let us see the main passages from the De Anima which can be used to discuss the individual soul’s immortality.² In the same order as they appear in the text, they are the following:³

[T1] there seems to be no case in which the soul can act or be acted upon without involving the body; e.g. anger, courage, appetite, and sensation generally. Thinking seems the most probable exception; but if this too proves to be a form of imagination or to be impossible without imagination, it too requires a body as a condition of its existence. If there is any way of acting or being acted upon proper to the soul, the soul will be capable of separate existence; if there is none, its separate existence is impossible. (1.1, 403a 3-13)

[T2] Yet to say that it is the soul which is angry is as if we were to say that it is the soul that weaves or builds houses. It is doubtless better to avoid saying that the soul pities or learns or thinks, and rather to say that it is the man who does this with his soul. (1.4, 408b 12-14)

[T3] the body corresponds to what is in potentiality; as the pupil plus the power of sight constitutes the eye, so the soul plus the body constitutes the animal. From this it is clear that the soul is inseparable from its body, or at any rate that certain parts of it are (if it has parts) (2.1, 413a 1-4)

[T4] We have no evidence as yet about thought or the power of reflexion; it seems to be a different kind of soul, differing as what is eternal from what is perishable; it alone is capable of being separated. (2.1, 413b 25-27)

[T5] Hence the rightness of the view that the soul cannot be without a body, while it cannot be a body; it is not a body but something relative to a body. That is why it is in a body, and a body of a definite kind. (2.2, 414a 18-21)

[T6] That perceiving and understanding are not identical is therefore obvious; for the former is universal in the animal world, the latter is found in only a small division of it. (3.3, 427b 7-8)

[T7] For imagination is different from either perceiving or discursive thinking, though it is not found without sensation, or judgement without it. That this activity is not the same kind of thinking as judgement is obvious. For imagining lies within our own power whenever we wish (e.g. we can call up a picture, as in the practice of mnemonics by the use of mental images), but in forming opinions we are not free: we cannot escape the alternative of falsehood or truth. (3.3, 427b 14-22)

[T8] That imagination is not sense is clear from the following considerations: Sense is either a faculty or an activity, e.g. sight or seeing: imagination takes place in the absence of both, as e.g. in dreams. Again, sense is always present, imagination not. (3.3, 428a 5-9)

Let us point out a few ideas about these passages.⁴ In [T1] Aristotle seems to be reinforcing his hylomorphic theory by raising the question whether there is an operation that can be carried out by the soul alone. At first, it looks like this is not the case and that all operations require the complex of body and soul. However, when it comes to the intellect and imagination, he evaluates the idea but discards it quickly, because imagination requires the body “as a condition of its existence”. Right after this, he leaves a question open: “If there is any way of acting or being acted upon proper to the soul, the soul will be capable of separate existence; if there is none, its separate existence is impossible”. This means that if it is possible to find an operation that belongs solely to the soul, it can exist separately, i.e., be immortal. As we will see later, this idea is key to understand Aquinas’ theory of immortality. In [T2], however, this question seems to be answered in a very clear way: it is not the soul who thinks, but rather the human being “with his soul”. Hence, the possibility of finding an operation that belongs only to the soul seems to be rather impossible, which is reinforced by [T3]: since the soul is “inseparable” from the body, that particular operation does not exist.

Nonetheless, [T4] opens the question again: the intellect, and intellection itself, are of a different kind, as different as “what is eternal from what is perishable”. If thinking is related to what is eternal, it must be immortal.⁵ The philosopher even adds: if something can be separable, it must be the intellect. Therefore, the solution to the aporia of the soul’s immortality seems to travel through the conceptualization of the intellect. In [T5], the philosopher negates one more time what he has said earlier and returns to the idea that “the soul cannot be without a body”, rejecting his previous considerations. However, in [T6] he draws a difference between perception and thinking, opening the discussion to what it is said in [T7], that the imagination is voluntary, which means that human beings can use it at will. Finally, in [T8], he states that “imagination takes place in the absence” of both a faculty and an activity; even though “sense is always present”, imagination is not. This last remark seems to enable imagination (and thinking) to operate with independence of the body.

As it is presented in these passages, Aristotle’s vision on the immortality of the soul is somewhat aporetic. In some chapters, he strongly defends the idea that both body and soul cannot exist or operate independently, but later he suggests the idea that some operations (imagination and thinking) could function with no body. Which Aristotle do we trust? The options seem to pick between a strong hylomorphism and a soft one. In the second part of this article, we will go over Aquinas’ remarks on the problem at hand to see how he addresses this aporia.

2. Thomas Aquinas’ Summa Theologiae

Aquinas’ clearest defense of the immortality of the soul can be found in his opera magna, the Summa Theologiae.⁶ At the beginning of the Treatise on Man (I q.75-102), he sets the basis for his anthropological proposal: body and soul, the powers of the soul, intellectual and appetitive powers, intellect and will, and so on. In q.75, a.2 he approaches the topic of the soul’s subsistence, which, in his mind, is crucial to argue for its immortality. It is very interesting how he considers, in the objections, some of the problems mentioned above: obj.1 it seems that the soul is not subsistent because is not a hoc aliquid; obj.2 the soul has no independent operation, rather the human being operates as a whole; and obj.3 thinking is impossible without the body, for it needs phantasms (images). As it can be seen, objections 2 and 3 make a direct allusion to the problems Aristotle suggests in [T2] and [T1] respectively.⁷ Now, let us see what solution Aquinas offers in the respondeo:

I answer that, It must necessarily be allowed that the principle of intellectual operation which we call the soul, is a principle both incorporeal and subsistent. For it is clear that by means of the intellect man can have knowledge of all corporeal things. Now whatever knows certain things cannot have any of them in its own nature; because that which is in it naturally would impede the knowledge of anything else. Thus, we observe that a sick man's tongue being vitiated by a feverish and bitter humor, is insensible to anything sweet, and everything seems bitter to it. Therefore, if the intellectual principle contained the nature of a body it would be unable to know all bodies. Now, everybody has its own determinate nature. Therefore, it is impossible for the intellectual principle to be a body. It is likewise impossible for it to understand by means of a bodily organ; since the determinate nature of that organ would impede knowledge of all bodies; as when a certain determinate color is not only in the pupil of the eye, but also in a glass vase, the liquid in the vase seems to be of that same color.

Therefore, the intellectual principle which we call the mind or the intellect has an operation "per se" apart from the body. Now only that which subsists can have an operation "per se". For nothing can operate but what is actual: for which reason we do not say that heat imparts heat, but that what is hot gives heat. We must conclude, therefore, that the human soul, which is called the intellect or the mind, is something incorporeal and subsistent.

Aquinas is trying to assert two key ideas: first, that the soul is incorporeal, and second, that the soul is subsistent. For the first idea, he offers the following reasoning: if the soul were somehow material (and dependent from a bodily organ to exist or operate) it would be “unable to know all bodies”. But, since the intellect is capable of knowing all bodies, it must not have a material dimension in its nature, which makes it incorporeal. From this, he reaches to his second idea: if the intellect is not dependent from and can transcend the limitations of a bodily organ (to know all things), it is necessary that its particular operation (to think based on phantasms) can take place with independence of the body. In saying this, he is being truthful to [T1]: the body is necessary to inform the soul, and from there, the soul can operate based on this information without having an immediate need of the body. This is corroborated by Aristotle himself in [T6]: perceiving and understanding “are not identical” which, for the philosopher, is an “obvious” observation. The difference, in Aquinas argumentative line, is that perception requires the body and understanding does not. Moreover, in [T7] draws a distinction between imagination, perception, and thinking, with emphasis in the first. Imagination “is not the same kind”; its main characteristic is that it is voluntary, i.e., humans can imagine whatever they want, not depending on sensation, based on previous information acquired through it. This means that imagination does not need perception to happen at the same time as it performs its operation. Hence, it is possible to form and combine images based on what has been previously perceived. This is ratified in [T8]: “imagination takes place in the absence of both” faculty and activity. If imagination requires no body to operate, it can be concluded that the intellect (being even more praised in the De Anima as an independent and separated faculty)⁸ can do the same with similar conditions, i.e., think based on what has been perceived and imagined without needing those operations to work at the same time. In other words, once information has been stored, the imagination can produce phantasms based on that information, and the intellect can produce the verbum mentis⁹ based in those phantasms. None of these processes require sensation to occur simultaneously.

“If there is any way of acting or being acted upon proper to soul, soul will be capable of separate existence”, says Aristotle in [T1]. Therefore, Aquinas’ effort to find that activity proper to the soul would allow the soul to be separated, i.e., to exist without the body. This is precisely the conclusion he offers in the second part of the respondeo. Because the soul is immaterial, it is capable of having an operation per se, which makes it subsistent and capable of operating and exist without the body. Furthermore, this is exactly what Aristotle says in [T4] about the power of reflexion: “it alone is capable of being separated”.

However, there are still a few Aristotelian assertions not solved yet, specifically in [T1] (thinking requires a body), [T2] (the human being thinks, not the soul), [T3] (the soul is inseparable from its body), and [T5] (the soul cannot be without a body). If Aquinas wants to be thorough, he must explain why Aristotle says those things in order to solve the aporia; explaining his own ideas is not enough. He approaches[T1] and [T2] in the replies to the objections. In reply 2 he tries to explain why the Greek philosopher says that it is the unity of the entire human which carries out certain operations, specifically to think. He starts by making a methodological distinction: “Aristotle wrote those words as expressing not his own opinion, but the opinion of those who said that to understand is to be moved, as is clear from the context”. Even though he thinks this is not Aristotle’s opinion, he explains why this would be the case, i.e., that the human does the thinking, not the soul. Even if the soul is considered to be an accident or a part of a larger being (therefore, not capable of existence per se) it can still be the case for a part of a whole to operate by itself, he argues. Hence, in the same way that it is possible to say ‘the eye sees’ (even though is the human seeing) it is possible to say ‘the soul thinks’. So, even when Aquinas recognizes that the accurate expression is ‘the human thinks through his soul’, it is not wrong to say that ‘the soul thinks’, offering a way out to the problem raised in [T2].

Let us see what he says about [T1]. In reply 3 he seeks to show that the soul does not need the body to perform its own activity, i.e., that it is subsistent. This is key for Aquinas; if he can prove this point and the soul indeed has an operation per se, immortality is the next logical step. He states in this reply: “The body is necessary for the action of the intellect, not as its origin of action, but on the part of the object; for the phantasm is to the intellect what color is to the sight”. He compares the activity of the soul with the activity of the eye; color is necessary for the eye only as material for its operation, but the ability to operate resides in the eye, not in the color. The color is merely a tool, something that is necessary to carry out the activity, but the power is in the eye itself. As stated before, it is the same for the soul. The intellect works with noêmata, which are formed from images which, in turn, require sensible perception. However, phantasms belong to the person executing the activity, not to the perceived object. If this is the case, the noêmata are present in the intellect after perception, not during it. Therefore, perception, as well as phantasms, are required solely as information to think and produce mental notions (verbum mentis/noêmata); and when these materials already exist, the intellect can operate using them with independence of the source of these materials. They already exist and do not require the body to be present in the intellect. Therefore, the need for the body disappears once the soul has all it needs to carry out its operation. Aquinas continues in reply 3: “Neither does such a dependence on the body prove the intellect to be non-subsistent; otherwise, it would follow that an animal is non-subsistent, since it requires external objects of the senses in order to perform its act of perception”. He starts by stating what can be concluded by the previous sentence, that is, that the dependence on the body is not strict and related to existence, but only to acquire information, which is one of the main pillars of his soft hylomorphism. The he goes on and offers a new comparison: the subsistence of the soul and the subsistence of the animal. If the soul of the animal, incapable of intellect, has a strict necessity of an object to have the power of perception, the animal would not be subsistent, that is, it would exist only as an accident or as a property of something subsistent in itself. Given that this is not the case and that the animal is subsistent per se,¹⁰ the intellectual soul must be able to think with independence of the body, because to exist is previous to perceive. This way, Aquinas offers a solution to the problem Aristotle states in [T1], and he does so by strictly making inferences from the philosopher’s ideas, not appealing to Neoplatonism or Revelation. Let us see if he offers a solution to [T3] and [T5].

In the same question (I q.75), article 6, he asks whether the soul is corruptible or not, and he presents three objections for which it can be said it is indeed corruptible. The first and second objections deal with biblical passages which seem to point in the other direction. Given our hypothesis and its philosophical irrelevance, we will not go over these. The third one is a problem he has already dealt with: thinking is impossible without phantasms, and phantasms require perception, therefore, thinking requires the body. The reply he gives is very short and adds nothing significant to what he already said in article 2. Hence, we will focus in the respondeo, where he approaches the corruptibility problem in greater depth. He starts by stating his own opinion on the subject: “We must assert that the intellectual principle which we call the human soul is incorruptible”. To prove his point, he mentions the distinction between substantial and accidental corruption. For the soul to be corruptible (not immortal) would mean to be substantially corruptible, not accidentally, because accidental corruption is impossible for a substance. Likewise, non-substantial items, i.e., accidents and material forms, are corruptible only accidentally: “things which do not subsist […] acquire existence or lost it through the generation or corruption of composite things”. As seen in article 2, the human soul, being a form, is subsistent. Therefore, the human soul cannot be corruptible but by substantial corruption. In other words, when it comes to the body, its existence depends on other, while the soul, being a form, exists by itself:

For it is clear that what belongs to a thing by virtue of itself is inseparable from it; but existence belongs to a form, which is an act, by virtue of itself. Wherefore matter acquires actual existence as it acquires the form; while it is corrupted so far as the form is separated from it. But it is impossible for a form to be separated from itself; and therefore it is impossible for a subsistent form to cease to exist.

Thereafter, Aquinas approaches the idea that the soul is composed of both matter and form and state that the outcome of accepting this premise will still be an incorruptible soul.¹¹ Since this issue does not tackle [T3] or [T5] we will focus only on the final idea of that paragraph:

Moreover, we may take a sign of this from the fact that everything naturally aspires to existence after its own manner. Now, in things that have knowledge, desire ensues upon knowledge. The senses indeed do not know existence, except under the conditions of "here" and "now," whereas the intellect apprehends existence absolutely, and for all time; so that everything that has an intellect naturally desires always to exist. But a natural desire cannot be in vain. Therefore, every intellectual substance is incorruptible.

Here Aquinas constructs an interesting syllogism. He starts with an Aristotelian observation: “desire ensues upon knowledge”. It is impossible to desire in the absence of knowledge, and since everybody seeks knowledge (Metaph. 1.1, 980a 22) everybody’s desire is guided by the knowledge we have over the object of that desire. And he goes on: “the intellect apprehends existence absolutely, and for all time, so that everything that has an intellect naturally desires always to exist”. This idea is not hard to grasp: there is no indication in Aristotle’s words that the intellect has some sort of limit. In fact, our common experience is that, in normal conditions, the intellect tries to understand as much as possible with no regards for limitations but for natural human limitations. If that were not the case, human knowledge and progress would not exist at all. Perhaps the main philosophical attitude is precisely this, i.e., the constant search for knowledge and understanding of the world, humanity and God. Aquinas closes with another Aristotelian idea: “natural desire cannot be in vain”. Although he might have some Christian teleological basis for this, the idea is clearly a derivation from the famous “Nature does nothing in vain”.¹² If human intellect desires to understand as much as possible for as long as possible, this desire must have a correlation with reality.¹³ Therefore “everything that has an intellect desires always to exist” is not a hypothesis as much as an observation of the natural world. If these premises are correct, then it follows that “every intellectual substance is incorruptible”.

If we are to state a common idea between [T3] and [T5], it must be the body-soul relationship. In these passages Aristotle argues for a strong hylomorphism. However, as seen above, there is room for a soft hylomorphism in the De Anima as well. It is clear that Aquinas is choosing the latter for his argumentative structure in the Summa Theologiae. Strong hylomorphism considers that human existence occurs only when body and soul are together as one, and that those principles are incapable to exist separated from each other. That seems to be the case in [T1], [T2], [T3] and, [T5]. On the other hand, soft hylomorphism understands that, while being true that human existence takes place when body and soul are together, one of these principles, i.e., the soul, can exist when the body has been corrupted.¹⁴ All Aristotelian evidence in the De Anima aims to the intellect as responsible for this capacity of self-existence. See, for example [T4], [T6], [T7] and, [T8].¹⁵Hence, Aquinas constructs in the Summa Theologiae his entire theory of immortality based on the intellect, which can be summarized as follows:

As usual, if we are to accept the premises, we must accept the conclusion. The logical consequence of the soul’s subsistence is its immortality, as Aristotle himself recognizes in [T1]. Up to this point, there is no indication that Aquinas is betraying the peripatetic anthropology or appealing to Revelation to support his theory. However, there is another source where we can witness how the saint deals with these problems: its commentary to the De Anima. In the next part we will go over that work and see if he has anything to add to the reflections presented so far.

3. Aquinas’ Commentary

In Book 1, Lesson 2, Aquinas goes over [T1] and offers a summary of the problem. One of the main challenges when investigating the soul is to determine whether it requires the body for its specific operation, i.e., thinking. It seems that it is in fact the case since thinking requires phantasms and phantasms depend on the senses (§18). Likewise, if we pay attention to other activities, all of them seem to require both body and soul: to get angry or experience any sensation. However, the evidence seems to indicate that thinking could belong only to the soul (§17). In §19, even though he recognizes Aristotle offers a solution to this problem in Book 3 of the De Anima, he wants to address briefly the difficulties involved in [T1]. The soul depends on the body to think in two different senses: the first as an object and the second as an instrument. The first involves the necessity of an object of knowledge: although the intellect is immaterial, many times it requires external bodies to acquire knowledge. For example, if one does not know what a poodle is, the only way to fully solve the mystery is when actual sensory perception of the material body of a poodle occurs. Thus, it seems that the soul requires external bodies to carry out its own operation. Following the example, although the essence of a poodle may be known by the soul, that activity “cannot occur except through the instrumentality” (§19) of a bodily organ; in this case the eyes, hands, nose, and so on. This constitutes the second sense in which the soul depends on the body: there is no knowledge without sensation. To perceive the external body (first sense) the soul requires a bodily organ (second sense); only then knowledge is possible. However, intellectual knowledge comes from understanding, in which the senses are not needed. Consequently, Aquinas arrives to the same conclusion he did in the Summa, i.e., the body is needed to collect data from the outside world, but knowledge comes from the soul. From this, he concludes that it is possible to infer the soul’s subsistence:

Two things follow from this. (1) Understanding is an act proper to the soul alone, needing the body, as was said above, only to provide its object; whereas seeing and various other functions involve the compound of soul and body together. (2) Whatever operates of itself independently, has also an independent being and subsistence of its own; which is not the case where the operation is not independent. Intellect then is a self-subsistent actuality, whereas the other faculties are actualities existing in matter. And the difficulty in dealing with this type of question arises simply from the fact that all functions of the soul seem at first sight to be also functions of the body (§20).

Taking this as base, he then tackles the second part of [T1]: if such an independent activity can be found, then the soul is separable from the body, because “whatever can operate on its own, can exist on its own” (§21). To illustrate the point, Aquinas uses the same example Aristotle does right after [T1]. Just as it happens with the soul, a straight line can touch a bronze sphere as long as it has a material dimension. Many things can be predicated of the line qua line, but to predicate that it can touch a bronze sphere at a certain point, it means that these predications are accurate only when in matter. Likewise, in the case of the soul, if there is the case that it has no proper activity, many things can be predicated of it, but these characteristics apply only when in a body.

Let us move on to [T2]. As stated before, he had already discussed and discarded this difficulty in the Summa (I, q.75, a.2, ad. 3) arguing that it is not Aristotle’s opinion but of those “who said that to understand is to be moved” and he arrives to the same conclusion here in the commentary: while it is better to say that it is the entire human who operates, it is not wrong to say that it is the soul that does it (§152). Regarding [T3] the saint takes a methodological approach. When Aristotle says “it is clear that the soul is inseparable from its body”, Aquinas points out that this statement is a conclusion that must be true if, and only if, the ideas mentioned previously by the philosopher are true. The first of those ideas is that the soul, as a whole, is the actuality of the body, and second, that it is the actuality of the whole body. Since he has already proven that the soul is substantial and can operate without the body, these premises are wrong, which means the conclusion is wrong (§242). Does it mean that Aristotle says something false or contradictory in [T3]? Aquinas replies in §244 the following: the philosopher ends this passage pointing out that these ideas are just a “sketch or outline” (413a 10), which means that he is only raising questions and anticipating answers, much like ‘if A and B are true, C must follow’. He is not actually supporting A and B, he is just saying that if A and B happen to be true, C must be true as well. Furthermore, Aquinas says that when Aristotle approaches in more depth the nature of the soul and that of the intellect, the issue becomes clearer (§244).¹⁶

In [T4] we can find one of the first instances where Aristotle refers to the intellect as separable and it is so far removed from the other types of soul as what “is eternal from what is perishable”. Aquinas provides some context to explain how Aristotle arrives at this conclusion. [T4] is located in Book 2, Chapter 1, where the traditional division of the types of souls is first outlined in the De Anima. In the preceding lines, the Greek philosopher is setting out the different powers of the souls and discussing if they can be located in different bodily parts of animals and vegetables. Now, when it comes to the intellect, things seem to be different. For the intellect appears to be of a unique kind, much different from the other powers described previously. Then, in §268 Aquinas states:

Then, at ‘But as regards intellect’, he points to one part of the soul over which doubts may arise. About the intellect, or whatever we call the percipient or speculative faculty, we are still, he says, uncertain. No proof has yet been given of its location in any special or particular organ of the body. Yet even at first sight it would seem to be of a different nature from the other parts of the soul, and to exist ‘in a different way; and that it alone is separable from the rest of the soul (and may even exist apart from any organ of the body) as what is immortal from what is mortal. That the other parts of the soul are not locally separated is now clear.

He starts by clarifying that when the philosopher is talking about the “percipient or speculative faculty”¹⁷ he is referring to the intellect. Next, he provides the basis for the separability, i.e., that Aristotle gives no indication that the intellect is located in one specific organ. On the contrary, this power is special and different from the others, as different as a mortal item from an immortal one. It is evident that this particular characteristic is not different from the others in the same way as nutrition is different from sensation. Since the language used by Aristotle, it is evident that he is vouching for a significant ontological difference: aidios (eternal) and phthartos (corruptible). This is supported by his next statement: only the intellect is choristos (separable), which Aquinas explores in §269. Vegetative and sensitive powers can be “mentally” separable. But the separation of the intellect is not just theoretical; it is not a mere rational distinction, it is loco (locally), i.e., it constitutes a real separation.

Let us move to [T5]. This passage has three linked statements in the De Anima: a) the soul cannot be without a body, b) it is not a body, but rather something relative to a body, and c) it is in a body, but a body of a definite kind (an organic, natural body). While b) and c) pose no problem for Aquinas’ interpretation, a) does imply that the soft hylomorphism is not possible. For some reason, Aquinas focuses on b) and c), and does not pay too much attention to a), which is a clear argument against his overall interpretation of the Aristotelian psychology. It could be argued that, since he discusses this particular problem elsewhere, i.e., the Summa Theologiae, he does not feel the need to do it here as well.

[T6] and [T7] take on the differences between perception, imagination, and understanding. In §629-633, Aquinas deals with these passages and expresses some key ideas. First, he picks up Aristotle’s idea that perceiving and understanding are not the same. The main difference is that, when it comes to rational knowledge or understanding, there is room for correct and incorrect knowledge (§630-631). On the other hand, when it comes to perception, the senses are infallible in perceiving their proper objects (§630).¹⁸ Also, the object of knowledge are immaterial substances and, ultimately, true understanding, while the senses are concerned with the material world (§630-631). Next, Aquinas elaborates on the differences expressed in [T7]: imagination is different from the two powers already treated, especially from discursive thinking because it is voluntary, it is not concerned with the notions of correct and incorrect and, therefore, it is not an opinion (§632). Serious opinions, according to Aristotle, must have some sort of foundation, some kind of evidence that allows us to state this or that. Imagination, on the other hand, does not have this limit. We can imagine all we want, not being concerned with the correlation our imagined object might or might not have with reality (§632-633). Why are these passages relevant? Mainly because they allow Aquinas to establish a difference considerable enough between the intellect and the other powers to make immortality more plausible and truer to Aristotle’s psychology.

Finally, [T8] elaborates further on what has been said on [T6] and [T7] and provides a new feature for imagination: it can take place in the absence of perception. The example used for this are dreams: even when we sleep, and we are not having any sensation of the world (potentially or actually) we are capable of producing imaginary objects of all sorts of shapes and forms (§641). This way, it seems that imagination works in a similar fashion as the intellect: it may require sensation to acquire data from the outside world, but once this has been provided, its proper operation can be performed in the absence of that stream of information. However, there is no indication in Aristotle or Aquinas’ work that imagination is an activity only proper to the soul. That characteristic belongs only to the intellect. Therefore, immortality can be predicated only as a characteristic that is proper to the soul, given the intellectual activity it carries out, not because of the possession of the ability to produce mental images (imagination).

4. Conclusion

In this article we have proposed and tried to prove some key ideas when it comes to the possibility for the human soul to be immortal from the Aristotelian approach. First, that within Aristotle’s ideas, this problem does not have an apparent solution, constituting an aporia. Second, that Thomas Aquinas produced an effective theory to sort this aporia and offer a solution to the problem. Third, that the saint managed to do so by achieving two objectives: a) keeping his reflections withing the anthropological and metaphysical limits of Aristotle’s psychology, and b) not needing to appeal to Revelation, proposing his theory on the basis of natural philosophy rather than revelated theology. According to the evidence exposed here, we consider that the medieval philosopher accomplishes this, which corroborates our initial hypothesis.¹⁹

As part of Aquinas’ elaboration, we also have tried to show that, in Aristotle’s De Anima, there is room for both a strong and a soft approach to anthropological hylomorphism. While the first considers that body and soul can exist only when together, soft hylomorphism accepts the premise that one of them, i.e., the soul, can exist after the body has been corrupted. As it has been shown, the softer approach is far more coherent with the saint’s proposal which, in turn, does not imply an offense to the philosophy of the Stagirite. It is true, nonetheless, that Aquinas does make use of a stronger version of hylomorphism when he approaches the problem of knowledge in the Summa contra Gentiles.²⁰ A future study could compare both accounts and establish its compatibility or incompatibility.

Finally, a theory on immortality as proposed by Aquinas may imply some trouble when it comes to explain the connection between individual immortality and Christian resurrection.²¹ The term used for immortality is athanasia, while resurrection is referred as anastasis nekron. While the second is the one used in the New Testament, the first is not an object of concern. From this, it is valid to ask whether Aquinas’ theory is conclusive regarding Christian faith and if his theory on the subject is coherent with the corpus of believes he professes.²² These and other problems that arise from Aquinas’ consideration on immortality are still open for discussion and could enrich the approach we have presented here.