Aquinas on Intelligent Extra-Terrestrial Life

Author: Marie I. George (St. John's University, Jamaica, New York)
Published in: The Thomist, 65, 2, April 2001, 239-258.
Publication date: April 2001

Aquinas took an interest in the question of whether there were intelligent material beings other than humans in the universe, both as a philosopher and as a theologian. As a philosopher he sought to understand the order of the universe and this entails ascertaining what beings are in the universe. As a theologian he sought knowledge of created beings insofar as it leads to a greater understanding, admiration, and love of the creator, and also insofar as it frees one from superstitious beliefs which pose an obstacle to faith in God.* (1) Although Aquinas was unable to approach the question of the existence of intelligent extra-terrestrial life from the scientific perspective of our day, he does raise some generally overlooked philosophical questions regarding the status of such beings. His theological reflections are helpful for addressing the frequently voiced claim that the discovery of intelligent extraterrestrial life would spell the end of Christianity. Aquinas's position is that it is possible that ETs * (2) of a certain sort exist, but improbable that they do. I will begin by considering Aquinas's philosophical positions on the possibility of ET life, and then will take up his theological views thereon, closing with his arguments regarding the probability of ET life.

When people nowadays think of extra-terrestrial life they generally imagine another earth-like planet inhabited by beings with odd-looking faces who are brainier than ourselves. Aquinas never considers ETs as inhabitants of another earth-like planet. This is because he thinks that there cannot be a more than one world like ours. * (3) Aquinas's views on the material composition of the universe further restricts what he considers possible as far as ET life is concerned. He thinks that celestial bodies are made of a single incorruptible element, whereas the earth is the only place where corruptible elements are found. Thus, his only candidate for ETs is the celestial bodies themselves which some philosophers thought might be animate. He does, however, consider the possibility of the existence of beings, other than humans, composed of the earthly elements. Though he would necessarily see such beings as dwelling on the earth, his reasoning concerning the possibility of their existence is not vitiated by his ignorance that the heavenly bodies are made of corruptible elements and could perhaps provide a dwelling for those beings. I will begin by considering his argument against the existence of beings other than humans made from terrestrial elements.

The arguments against an intellectual substance being united to a single element are straightforward:

[I]f some intellectual substance is united to one of the simple bodies [corruptible elements] as form, either this being will have intellect alone or it will have other powers such as those which pertain to the sensitive or nutritive part as is the case in man. If, however, it has intellect alone, it would be vain for it to be united to the body. For every form of a body has some proper operation through the body. The intellect however does not have some operation belonging to the body except according as it moves the body.... The motions of the elements are from natural movers, namely, from the things which generate them, and they do not move themselves; whence there is no need as far as their motion is concerned for them to be animate. If on the other hand the intellectual substance which is supposed to be united to an element or part of it has other parts of the soul [in addition to intellect], it will be necessary to find in a simple body a diversity of organs; which is incompatible with the body's simplicity. Therefore, an intellectual substance cannot be united as form to some element or part thereof.* (4)

The argument against the possibility that there exists another body composed of many elements is less convincing:

For if it [an intellectual substance] were united to another body, either it would be united to a mixed body or to a simple body. It cannot however be united to a mixed body, because it is necessary that that body be of the most balanced make-up according to its genus among the other mixed bodies, since we see that mixed bodies have nobler forms to the extent that they arrive at a more temperate mixture; and thus that body which has the most noble form, as an intellectual substance, would have to be of the most temperate mixture, if it is a mixed body; whence we even see that softness of flesh and goodness of touch which demonstrate balance of constitution are signs of a good intellect. The most balanced constitution is the constitution of the human body. It is necessary, therefore, that if an intellectual substance is united to some mixed body, it be of the same nature as the human body. The form of this being would be of the same nature as the human soul, if it be an intellectual substances. There would not therefore be a difference according to species between this animal and man. * (5)

It is easy to remain wondering why beings with a rational soul who had better or additional external or internal senses could not exist? And if they did, would they belong to the same species as we do? And what if these beings were reproductively isolated from us (which they most likely would be) or if there was another population of beings like us with the exception of being unable to interbreed with us? Would they belong to the same species?

Aquinas is actually addressing the question of whether there could be a rational being with better or additional senses when he speaks of the need for human beings to possess a body which has the most balanced mixture of elements. To understand Aquinas's position here one must realize that although he thought that there was no best possible world, he did hold that the things in this world were as good as they possibly could be given the overall constitution of the world. * (6)The human body was thus as well disposed as it could be for the rational soul and its operations given the constraints of the elements available in the world. Thus, in response to the objection that "since man is the most noble animal, the human body ought to be disposed in the optimal manner as to those things which are proper to animals, namely, sense and motion," * (7) Aquinas says that:

[T]ouch, which is the foundation of the other senses, is more perfect in man than in any other animal; and it is for this reason that it was necessary that man have the most balanced complexion among all the animals. Also man surpasses all the other animals as to the interior sense powers.... From which it happens by necessity that as to some of the exterior senses man falls short of some other animals. For example, man has the worst sense of smell among all the animals. For it was necessary that man have the biggest brain in proportion to his body among all the animals so that the operations of the interior sense powers in him which are necessary for the operation of the intellect would be more readily exercised.... The magnitude of the brain on account of its moistness is an impediment to smell which requires dryness. * (8)

The rest of Aquinas's response is to the effect that any inferiority in the human body is an unavoidable side-effect of something needed to insure the good functioning of the internal senses and the "perfect equality of complexion" required for a good sense of touch. * (9) One gathers from this that Aquinas thought that there was one perfect balance of elements and arrangement of organs in a human body (albeit varying within a certain range), * (10) and that the presence of any other organs or even better organs would be incompatible with the balance which is needed for two or three things which humans need in order to be human, namely, a good sense of touch, a good memory and imagination, * (11) and perhaps upright posture as well. * (12)

The evidence for the evolution of life forms provides reason to believe that there could be more than one body plan for a rational being. The evolutionary process itself is a kind of material constraint that Aquinas was not aware of. The "making over," so to speak, involved in producing a new life form from its predecessor could account for certain imperfections in the human body, just as the need for good imagination has as a necessary undesirable side effect that areas of the brain which are devoted to other functions in animals have to be reduced in humans. Now, would an intelligent being with a variation of the human body plan be a human being? Aquinas is not really able to consider this as a possibility. However, his minimal criteria for what constitutes a human being gives us some idea of what he might have said had he been aware of the evidence for evolution:

But finger, foot, and hand, and other parts of this sort are outside an understanding of 'man', whence the essential notion of man does not depend on them and man can be understood without them. For whether it has feet or not, so long as there is affirmed a conjunct of rational soul and body mixed from the elements with the proper blending that such a form requires, it will be a man. * (13)

This text maintains that there are some components of humans which are not essential to our nature and thus which do not enter into the human definition, whereas there are others which cannot be dispensed with. A human being could exist without hands or feet, but cannot exist without a mixed body that has the proper mixture of elements which a rational soul as form requires, namely, such as is requisite for an excellent sense of touch. As mentioned above Aquinas also regards a large brain as a necessary component of human beings. He also names bones, which are needed for upright posture. * (14)

We are still left wondering whether Aquinas would rank a being that fit the above criteria and that was endowed with an extra sense or bigger brain than ours in the same species as us or only in the same genus. It cannot have escaped the attentive reader that I am not systematically addressing the question of how a species is determined according to Aquinas. This is a difficult task which demands separate treatment, and so I will continue to skirt the issue dialectically as I am examining the question of the possibility of another species of human-type life.

Up to now I have considered whether it is possible for another human-type body to exist. However, what determines the species of a thing is its form, which in the case of living things is the soul, and so it remains to be considered if there is something about the human soul which prevents it from existing as more than one species. On this point it is helpful to see why separated substances and the human soul are not put in the same species:

Further, any thing whatsoever has its proper being according to the ratio of its species; for of those things of which there is a diverse ratio of being, there are diverse species. The being, however, of the human soul and of the separated substance are not of one ratio ; for a body is not able to share in the being of a separated substances as it can share in the being of a human soul according to which it is united to the body as form to matter. The human soul therefore differs in species from separated substances. ... Moreover, the species of a thing is perceived from the proper operation of it; for operation demonstrates power (virtutem), which indicates the essence. The proper operation, however, of separated substances and the intellectual soul is to understand; however, the mode of understanding of separated substances and of the soul is totally other; for the soul understands by taking from the phantasms, which is not the case of separated substances, which do not have bodily organs in which the phantasms must necessarily be. Therefore, the human soul and separated substances are not of one species. * (15)

The hypothetical ETs and humans would have a rational soul which is the form of the body and which had the same mode of understanding, i.e., abstracting concepts from sense experience. What precludes there being more than one human form given that there exist more than one angelic form? * (16) According to Aquinas:

that there is only one species of rational animal, while there exist many species of irrational animal, arises from the fact that the rational animal is constituted from this that corporeal nature reaches the highest thing it can attain to, [namely], the nature of spiritual substance which [in turn] attains its lowest [grade]. There is only one highest grade, as well as lowest grade, of one nature.... * (17)

One still wonders why one group of human-type beings could not be more capable of understanding than another group, due to better senses. And in fact Aquinas even seems to think that there is room for doubt because after he says that "There is only one highest grade, as well as lowest grade, of one nature" he ends by saying "although it could be said that there were many species of rational animal, if one would posit that the celestial bodies were animate." * (18)

This last statement, despite being couched in hypothetical terms, still strikes me as somewhat odd since Aquinas holds that celestial bodies are simple, and thus would not have sense organs as animals must have. * (19) One cannot get around this by saying that movement is also an animal characteristic because while it is true that only animals move locally, not all animals do so, whereas all sense; sensing is thus the definitory characteristic of animals. A further problem is that not only would such a being not be an animal, it would not be rational either, at least not in the strict sense of rational, since reason is distinguished from pure intelligence by its dependency on sense experience.

Ultimately, I am not sure whether Aquinas really thought several species of human beings could not exist. When he apparently excludes this possibility in SCG II 90 his concern was to refute the erroneous views that demons were physical animals made of bronze and that the elements were animate, and that angels and demons by nature had bodies. If he was presented with an intelligent individual that had a sixth sense and an exceptional memory would he say that it was the same species or that it and we were only generically the same? And how does he situate inability to interbreed * (20) in regard to determining species? I do not see any room to say that he would deny that the beings in question are human, at least in the generic sense, given that they have the requisite sensitive body and an intelligence which starts off tabula rasa and arrives at ideas starting from sense experience.

Aquinas's other candidate for an ET is an animated celestial body. Aquinas's views on this subject might seem of merely historical interest given that we know that the stars and planets are not composed of a fifth incorruptible element. However, a more universal interest lies in seeing the manner in which Aquinas reasons when faced with a possible addition to the chain of being.

Aquinas thinks that the most plausible view concerning the movement of the heavenly bodies is that it is caused by lower ranking separated substances which are not united to those bodies as their forms. * (21) However, he does seriously entertain the possibility that these separated substances are united to the heavenly body as their forms. * (22) In the Disputed Question on Spiritual Creatures (unicus, 6) after giving one argument pro and another a con, he asserts that: "The opinion on each side has the ratio of probability. Though in that disputed question he finds the argument for a negative conclusion more compelling, in the Disputed Question on the Soul, after extensive discussion, he says in the response to an objection:

If, however, it [the separated substance] only possesses intellect [and not sense] it is nevertheless united to the body as form not for the sake of intellectual operation, but for the sake of the execution of its active power .... * (23)

It is somewhat surprising that Aquinas entertains as possible that an intellectual substance be united to a celestial body, since this does not seem consistent with his position that the difference between the human form or soul and angelic forms is that the latter does not have the ability to share its being with a body. * (24) Perhaps Aquinas thinks that human souls may not be the only separated substances that can be united to a body after all, for in another place he says:

This thing able to be perfected that is the human body corresponds to the soul, [whereas] to an angel either no body corresponds or that of another species such as a bronze body as Augustine ... seems to say. * (25)

On the supposition that the heavenly bodies are animate, Aquinas never envisages them as an intermediary between humans and angels. In that odd passage mentioned above, * (26) Aquinas seemed open to ranking them with humans, but more often he maintains that: "if the celestial bodies are animate, their souls belong to the society of the angels." * (27)The reasons for classifying them in this manner is that they lack a sensitive body and they have an angelic mode of understanding.

What can we conclude from Aquinas's hesitancy and at least apparent inconsistency on the issues concerning animated celestial bodies? The fact that his preferred authorities (Aristotle and St. Augustine) do not adopt the position that he thinks is most plausible alerts him to the difficulty of determining the answer. He recognizes that the arguments given on both sides contain probability. Unable to come up with evidence sufficient to allow him to decide the question with certitude, he remains open to the possibility that the opposite position to his own might be true.

Aquinas also takes care to avoid speaking in such a manner as would prejudice divine wisdom. As philosopher he maintains that the works of nature manifest intelligence while themselves lacking it, and as a Christian believing in Divine Providence he was even more firmly convinced that natural things are not junk. * (28) Thus, although the existence of ensouled heavenly bodies appear to him as unlikely because they do not fill an obvious gap in the hierarchy of beings in the universe, he still sees a need to come up with some goodness that would be proper to them, in case they do exist. We see this in the passage quoted above * (29) where he says that such a being would be "united to the body as form not for the sake of intellectual operation, but for the sake of the execution of its active power, according to which it is able to attain to likeness to God as to causality, by [causing] the motion of the heavens".

I will now turn to Aquinas's theological views on the existence of ET life. Aquinas does not see their existence as posing difficulties to the faith. While his treatment of ETs is unfortunately limited by what he considers as possible ETs (namely, animated heavenly bodies), Aquinas does make some generally applicable points.

Aquinas indicates that the fact that Scripture does not mention some being is not a reason to conclude that it does not exist. When addressing the question of whether incorporeal substances are united to celestial bodies as forms, he notes that:

Augustine leaves [this] in doubt and so does Origen. Which nevertheless seems to be rejected by many moderns for the reason that since the number of the blessed according to divine Scripture is made up from human beings and angels alone, those spiritual substances cannot be counted among human souls nor among angels, who are incorporeal. * (30)

Aquinas himself is not persuaded by the moderns' line of reasoning, and he concludes by noting that Augustine also leaves in doubt whether beings of this sort might not also be numbered among the blessed. * (31)

In another passage Aquinas even goes so far as to say that it is of no matter to the faith to make a pronouncement on the existence of animated celestial bodies:

According to what Aristotle says, it is the case that it is necessary to posit an intellect united to the body as form. According to the opinion of Aristotle it is therefore the case that the heavens are composed from an intellectual soul and a body.... ... It will be necessary to say then that the intellect according to its substance is united to the heavenly body as form. ... What is being said, however, about the animation of the heavens we do not say as asserting it according to the teaching of the faith, to which it is of no concern at all (nihil pertinet) whether this matter is said to be thus or otherwise; whence Augustine (Enchird. c. 58): 'Nor am I certain whether the sun and moon and other stars belong to the same society, namely, that of the angels; the brightness of the bodies notwithstanding, they do not appear to have sense or intelligence.' * (32)

I think there are a number of reasons why Aquinas does not think the ET existence is of concern for the faith. * (33) A general reason is that the faith and Sacred Scripture are ordered to our salvation, and not to instructing us in cosmology or science in general:

What the philosopher considers about creatures is other than what the theologian does. [F]or the philosopher [and scientist] consider those things which belong to them according to their proper nature, such as that fire is born upward; the faith, however, considers only those things about creatures which belongs to them according as they are related to God, such as that they are created by God, that they are subject to God, and things of this sort. Whence it is not to be imputed to some imperfection of the doctrine of the faith, if it sets aside many properties of things, such as the structure of the heavens, the quality of motion.... * (34)

It is for the reason that Sacred Scripture is ordered to human salvation that Scripture sometimes omits mention of certain things, as happens, for example, in Genesis in regard to the creation of the angels:

Augustine says..that 'the angels were not left out in the first creation of things, but are signified by the name 'heaven', or even 'lights'. Therefore, however, either they were left out, or they were signified by the names of corporeal things because Moses habitually spoke to uneducated people who were not yet able to grasp [things of an] incorporeal nature; and thus if it had been expressed to them that there were things above all corporeal nature, it would have been for them an occasion of idolatry, to which they were prone, and from which Moses chiefly intended to call them back. * (35)

Knowledge of extraterrestrials is of no more concern for the faith than knowledge about animated celestial bodies is. It belongs to science, and not to faith to catalogue the different beings in the material universe. Moreover, just as untimely knowledge concerning angels was apt to pose an obstacle to the faith of early uneducated believers, this could also be the case with ETs, on the supposition that they exist.

Aquinas does think that it pertains to the theologian to defend the teachings of the faith. * (36) The specific manner in which he envisages ETs does not suggest any conflict with scripture since that sort of ET would be ranked with the angels. * (37)The existence of ETs as we envisage them, however, does raise questions, for example, if ETs are generically human, how is one to understand I Corinthians 15: 22-23: "Death came through one man and in the same way the resurrection of the dead has come through one man. Just as all men die in Adam, so all men will be brought to life in Christ...."? * (38)Thus, if Aquinas had recognized the possibility of ETs as we envisage them, he would have seen it as the theologian's duty to treat these questions. He does nevertheless make at least two applicable observations. First, following Scripture, he insists that all humans have Christ as their head:

Moreover, the one who is not harmed by sin, is not in need of redemption. If therefore there would be someone who was not born in original sin, aside from Christ, there would be someone who was not in need of the redemption accomplished by Christ; and thus Christ would not be the head of all men, which is not fitting according to the faith. Therefore, neither ought one to hold that someone can be born without original sin. * (39)

The question remains, as to what, if any, problem this position would pose were generically human ETs to be discovered.

The argument that Aquinas gives for the possibility of other Incarnations is another contribution that he makes to the theological ET discussion:

On Whether One Divine Person is Able to Assume Two Human Natures: That which is able [to do something] in one case and not in another has its power limited to one. The power of a divine person is, however, infinite, and it ought not be said that a divine person had assumed one human nature is such a manner that another could not be assumed to its personhood, for that is impossible, because an uncreated thing cannot be comprehended by a created thing. It is manifest therefore that whether we consider the divine person according to power, which is the principle of the union, or according to its personhood which is the term of the union, it must be said that the divine person besides a human nature which it has assumed, is able to assume another numerically different human nature. * (40)

Thus far I have considered Aquinas's arguments regarding ET life chiefly with an eye to its possibility. Now I would like briefly to examine a few arguments Aquinas gives in other contexts to the extent they have applicability to the question of the probability of ET life.

An argument for the improbability of ET life can be derived from considering the order of the universe:

The order of the universe includes in itself both the conservation of the diverse things instituted by God, and the motion of them; because according to these a twofold order is found in things, namely, according as one thing is better than another, and according as one thing is moved by another. * (41)

If there were no interaction between us humans in our world and the ETs in their world the universe would lack one kind of order, though it still would have the order of hierarchy. This sort of universe is less probable than one in which the parts had both kinds of order. However, this argument establishes its conclusion in terms of fittingness rather than necessity. * (42) Of course, one response to this argument is that ETs and humans will eventually meet through space travel.

Another argument for the improbability of ET life is based on a notion that Aquinas uses to argue that this world is the only one, namely, that uniqueness is a desirable characteristic:

The third objection is as follows: It is better to multiply the best things than those which are less good; but the world is the best thing; therefore it would be better for there to be many worlds, than it is for there to be many animals and many plants. And to this it ought to be said that this itself belongs to the goodness of the world: that it be one; because one has the notion of good: for we see that something is cut off from its proper goodness through division. * (43)

One could similarly argue that one human species is enough, since it would reflect God's goodness by being unique.

Another argument which Aquinas uses to refute the position that there are many worlds that also serves as an argument against the probability of ETs turns on a characteristic of intelligent agents. In responding to the objection that "it would be better that there exist many worlds rather than one, because many good things are better than fewer," * (44) Aquinas points out that:

no agent intends a material plurality as an end; because a material multitude does not have a fixed term, but of itself tends to infinity; infinity, however, is contrary to the notion of 'end'. When it is said, however, that many worlds are better than one, this is said according to the material multitude. This sort of better, however, does not belong to the intention of God as agent, because for the same reason it could be said that if he made two, it would be better that there were three; and thus ad infinitum. * (45)

God could have populated the universe with any number of reproductively isolated groups of rational animals, but there is no motivation to make more than one human race, since simply having more than one for the sake of having more than one is not something an intelligent being would aim at. The weakness of this argument is that it does not exclude God's creating other groups of human beings who would differ by more than the insignificant differences characteristic of the different races. And indeed thomistic positions on the diversity of being in the universe seem to lean in the direction of an affirmative answer to the question of whether ET life is probable. Aquinas maintains that:

... God produces things for the sake of communicating his goodness to creatures, and through them to represent his goodness. And because it cannot be adequately represented through one creature, he produces many and diverse creatures, so that what is lacking to one for the purpose of representing divine goodness, is filled up by others; for the goodness which exists simply and uniformly in God, in creatures is multiple and divided. Whence the whole universe more perfectly shares in and represents divine goodness than any other creature whatsoever. * (46)

Of these diverse beings which God creates, Aquinas holds that it "certainly agrees with the affluence of divine goodness that those things which are more noble are more abundantly produced in being." * (47) A question remains as to whether the greater abundance of more noble things refers to the number of individuals within a species or to the number of species or types. Aquinas addresses this question when discussing the number of angels there are:

[T]he Platonists were saying that to the extent that something is closer to the first principle to that extent it is smaller in multitude; just as to the extent that a number is nearer the unit to that extent it is less in multitude. And this opinion stands up well as to the number of orders: three assist, while six minister.--But Dionysius held...that the multitude of angels transcends lower bodies in greatness by something immense, so that the higher incorporeal natures transcends all corporeal natures because what is better is more intended by God and more multiplied. And according to this, since those assisting are superior to those ministering, there are more assistants than ministers. * (48)

According to these principles, there should be fewer sorts of humans than animals, and more individual humans than animals, but the latter is not the case. Nor if one regards humans as the lowest of separated substances do the principles apply: there should be more sorts of the lower form, i.e., humans, and fewer individual humans than angels, but the former is not the case. Ultimately, Aquinas seems to acknowledge that the numbers of created things are not dictated by necessary rules:

It does not therefore appear to be universally true that the more imperfect difference of a genus is multiplied in more species. For body is divided into animate and inanimate: nevertheless there appear to be more species of animate bodies than inanimate, especially if the heavenly bodies are animate, and all the stars differ from one another in species. But even in plants and animals there is the greatest diversity of species. ...so it is even in the whole universe of things that to the extent that something is superior among beings, to that extent is has a greater formal multitude, which is judged according to the distinction of species: and in this Dionysius' saying is saved: it has less, however, material multitude which is judged according to the distinction of individuals in the same species, in which the saying of the Platonists is saved. That there is, however, only one species of rational animal when there exist many species of irrational animals comes from the fact that rational animal is constituted from corporeal nature attaining its highest grade and spiritual substances attaining its lowest grade. The supreme grade, however, of a nature, and also the lowest grade, is only one: although it could be said there were many species of rational animal, if one held that the celestial bodies were animate. * (49)

Aquinas notes that there are more species of animals than species of elements. I wonder if he ever also noticed that there are more species of insects than species of cats or of boars. This fact, and the preceding observation of the comparative numbers of animals vs. elements make it plain that there are no absolute rules concerning the numbers of the different sorts of beings. In any case, it is apparent, especially from the last sentence in the passage above, that Aquinas is proceeding inductively in order to establish what, if any principles are operative here, rather than deriving these principles deductively. The principles he arrives at are not then of such a certain character as to exclude the possibility that there exist more than one species of human being. And Aquinas is open to this possibility, as one can see from the last sentence in the quotation. If God indeed does create more of the higher sorts of beings, this could serve as the basis of a very weak argument in favour of the probability of ET life.

I have tried to show that Aquinas, despite lacunae in his scientific knowledge, does make contributions to both philosophical and theological discussions concerning the existence of intelligent extra-terrestrial life.

Aquinas calls to our attention that one sort of ET that could exist is a separated intelligence joined to body as its mover. He himself thinks that there are intelligences of this sort which move the heavenly bodies. As for the other sort of ET which would consist of a separated substance united to a body as its form, Aquinas points that it is extremely unlikely that a pure intelligence be united to a body as its form, since the pure intelligence in no way profits from its union to the body. However, an intellectual substance of the rational sort is suitably united to a body since an intelligence of this sort can only acquire its ideas through sense experience.

Aquinas points out that the sort of body the composite being must have is specified to some extent by the requirements of the intellectual substance that is united to it. The body cannot be a simple body such as air or iron, because sense organs require a balance of elements, and indeed, a most subtle blend of elements; otherwise the being will lack a good sense of touch and well-functioning internal senses that provide reason with the starting points it needs for forming ideas. Aquinas further points out that rational beings need not have fingers, hands, and feet as humans do; he holds that even humans would still be human without them.

Aquinas does not favor the idea that other human-type beings exist because he thinks that the human soul represents the very lowest type of intelligence, whereas the human body represents the very highest material body. However, he does remain open to the possibility.

From a theological standpoint, Aquinas explains that there is no reason for concern here because it is not the task of Scripture to classify the beings in the universe. Since Aquinas does not think that there in fact are other human-type beings, he has little reason to investigate any apparent conflicts between their existence and scriptural statements. His examination of whether many Incarnations are possible is useful for theological discussions of ET existence.

Aquinas explicitly denies that it is probable that other human-type bodies exist, for the reason noted above. There are two other probable arguments that can be drawn from Aquinas, one against and one in favour of the existence of other human-like creatures. On the one hand, the human species would reflect God's goodness in a special way by being unique, while on the other hand, it is befitting to God's goodness that he create more of better creatures. Aquinas leans in the direction of the former view, but realizes that the latter could in fact be the case. And by doing so, he gives us an example of the circumspection that this matter demands.

Notes

1. Cf. Summa Contra Gentiles, ed. C. Pera, O.P. et al. (Turin: Marietti, 1961), II 2&3. Hereafter cited as SCG.
2. Henceforth ET will be used as an adjective in place of "intelligent extra-terrestrial" and will also be used as a noun to name beings of this sort.
3. Cf. In Libros Aristotelis De Caelo et Mundo. Leonine Edition. (Rome: Society for the Propagation of the Faith, 1886), Bk. 1, c. 9, lec. 19. Hereafter cited as De Caelo. Cf. also Summa Theologiae I 47.3.
4. SCG II 90.
5. Ibid.
6. Cf. Summa Theologiae, Ed. Instituti Studiorum Medievalium Ottaviensis, (Ottawa: Commissio Piana, 1953), I 25.6 ad 3: "the universe cannot be better, supposing these things, on account of the most fitting order attributed to them by God, in whom the good of the universe consists. If some one of them would be better, it would corrupt the proportion of order; just as if one chord was more than duly stretched, it would destroy the melody of the cithara. Nevertheless God could make other things, or add other things to the things already made; and thus there would be a better universe. " Hereafter cited as ST.
7. ST I 91 3 obj. 1.
8. ST I 91 3 ad 1.
9. Aquinas insists in many places on the importance of "perfecta complexionis aequalitate" which insures that humans have the best sense of touch. Cf. Quaestio Disputata de Anima in Quaestiones Disputatae. Vol. II. (Turin: Marietti, 1965), art. 8: Whether the Rational Soul Ought to be United to Such a Body of the Sort that the Human Body is. Hereafter cited as DQDA.
10. Aquinas is not ignorant that there is a certain range in human composition. Cf. Scriptum super Sententiis, (Paris: Lethielleux, 1956) Bk. II, dist. 15, q. 2, art. 1: "For when some make-up (complexio) is assigned to some genus, this is not according to some indivisible grade, but with a certain latitude; so what is found beyond certain limits no longer preserves the make-up belonging to that genus. But there is much diversity between those limits ... Nevertheless there is some level of heat or cold that the human make-up cannot withstand". Hereafter cited as Sent.
11. DQDA art. 8.
12. Cf. ST I 91.3 ad 3.
13. In Librum Boethii de Trinitate. Ed. Decker. (Leiden: E.J. Brill, 1959), 5.3.
14. Cf. ST III 5.2 "Carnes et ossa in hominis definitione poni oportet."
15. SCG II 94. Cf. II Sent. dist. 3, q. 1, art. 6: Utrum Angelus et Anima Differant Specie.
16. Cf. ST I 50.4 obj. 2: "More and less does not cause a diversification of species. But angels do not appear to differ from one another except according to more and less, namely, according as one is more simple than another, and is of more perspicacious intellect. Therefore, angels do not differ in species. Ad 2: More and less, according as it is caused from the intensification and remission of one form does not cause a diversification of species. But more and less according as they are caused from forms of diverse grades do thus cause a diversification of species; for example, if we say that fire is more perfect than air. And in this manner angels are diversified according to more and less."
17. Quaestio Disputata de Spiritualibus Creaturis in Quaestiones Disputatae. Vol. II. P. Bazzi et al. (Turin: Marietti, 1965), unicus 8 ad 10. Hereafter cited as De Sp. Cr. Cf. De Ente et Essentia, c. 5: "if the nature of the possible intellect would be unknown, we could not discover the multitude in separated substances. There is, therefore, a distinction of each separated substance from the others according to the grade of potency and act; so that the superior intelligence, the nearer it is to being first, has more act and less potency, and so with the others; and this [series] comes to an end in the human soul which holds the last grade in intellectual substances. Whence its intellectual potential stands to intelligible forms as prime matter (which holds the last grade in sensible being) to sensible forms...and therefore the Philosopher compares it to a blank slate on which nothing is depicted".
18. De Sp. Cr. 8 ad 10.
19. Cf. DQDA unicus 8 ad 12: "the body of man could not be a simple body, neither a heavenly body on account of the passibility of organic senses, and above all the sense of touch; nor a simple elementary body: because in an element there are contraries in act. It is necessary, however, that the human body be brought to a middle [state]". Cf. SCG II 70.
20. To my knowledge Aquinas says very little about reproduction in relation to species. He determines that men and women do not belong to different species on the grounds that male semen could not produce both sexes if they were not of the same species. It does not however follow from this that if a human cannot reproduce with an alien that the two are not of the same species. Aquinas was aware that the mule is an offspring of a horse and an ass. However, this does not help much in determining whether there could be another species of humans, because Aquinas regards horse and ass as two species to start with, and he does not provide us with the criteria whereby he determined this.
21. Cf. De. Sp. Cr. 1.6: "It ought to be said further that it is contrary to the incorruptibility of the heavenly bodies that they be animate as are lower bodies which are rendered vegetative and sensitive through the soul. It is therefore to be denied that celestial bodies are animate in the same manner in which these lower bodies are animated. It is nevertheless not to be denied that heavenly bodies are animate, if what is understood by animation is nothing other than the union of mover to mobile."
22. Cf. Quaestiones Disputatae de Veritate, in Quaestiones Disputatae. Vol. I. Raymundi M. Spiazzi, O.P. (Turin: Marietti, 1964), 5.9 ad 14: "Or it can be said that the soul is the perfection of the human body both as form and as mover; the heavenly body which is perfect does not require another spiritual substance which will perfect it as form, but which will perfect it as motor only..... Although even certain held that the movers of the orbs were joined to them as their forms; which is left in doubt by Augustine.... Damascene, however, says the contrary." Hereafter cited as DV.
23. DQDA unicus 8 ad 3.
24. Cf. SCG II 94 cited above. Cf. also DQDA 3: "belonging to the notion of the human soul is that it is unitable to a human body, since it does not have in itself the complete species; but what makes up the whole of the species is in the composite itself. Whence that the soul be unitable to this body or that body multiplies it according to number, not, however, according to species...."
25. II Sent. dist. 3, q. 1, art. 6 c (Utrum angelus et anima differant specie). The passage reads: "Praeterea, eidem formae vel perfectioni respondet idem perfectibile. Sed anima et angelus sunt quaedam formae, prout communiter omnes substantias a subject separatas formas dicimus, quorum formae Materials sunt imagines, ut Boethius dicit.... Cum ergo animae respondeat hoc perfectibile quod est corpus humanum, angelo vero vel nullius vel alterius speciei, ut corpus aereum, secundum quod Augustinus...videtur dicere, vel etiam corpus caeleste sec. opinionem Avicennae....et quorumdam philosophorum; videtur quod anima et angelus non sunt unius speciei."
26. De Sp. Cr. unicus 8 ad 10
27. DQDA 8 re 5. The objection states "that if they are animate, in eternal beatitude there would be not only angels and men, but also a certain middle nature." Cf. De Sp. Cr. 1.6 "Whether a Spiritual Substance is United to a Heavenly Body. Sed contra 3: The heavenly society of the blessed consist of angels and souls. But the soul of the heavens, if the heavens are animate, would be contained under neither group. Therefore, there would be some rational creatures which could not be sharers in beatitude; which seems to be unfitting. Ad sed contra 3: If the heavenly bodies are animate, the spirits governing them are to be counted in the society of the angels".
28. Aquinas does exclude some combinations as absolutely impossible, e.g., an animate body made only of air.
29. DQDA unicus 8 ad 3.
30. Quaestiones Disputatae de Potentia in Quaestiones Disputatae. Vol. II. Ed. P. Bazzi et al. (Turin: Marietti, 1965), 6.6. Hereafter cited as De Pot.
31. It is somewhat ironic that Aquinas uses the absence of any statement to the contrary to conclude that there is no more than one world: The only theological statement he makes concerning the question regards a passage from John: 'The world was made through him'. Aquinas's comment: "[John] names the world in a singular way, as if there were only one world that existed." ST I 47.3 sc.
32. SCG II 70
33. Of course Aquinas's faith eliminates any worry on his part that evidence could irresolvably conflict with scripture: "[W]hat is divinely said through apostles and prophets is never contrary to those things which natural reason dictates. "DV 14.10 ad 7.
34. SCGII 4.
35. ST I 61.1 obj. 1 and ad 1. Cf. De Substantiis Separatis, c. 17.
36. Cf. SCG II 3.
37. Cf. DQDA unicus 8 ad 5 I cited above with De Sp. Cr. 1.6.
38. To my mind the most problematic text is Colossians 1: 15-20: "As he is the Beginning, he was first to be born from the dead, so that he should be first in every way; because God wanted all perfection to be found in him and all things to be reconciled through him and for him, everything in heaven and everything on earth when he made peace by his death on the cross". This could be explained by saying that fallen ETs were redeemed through Christ's death on earth. Another alternative is that they never fell.
39. II Sent. dist. 31, q. 1, art. 2. Cf. SCG IV 83: "Furthermore, if after the resurrection there will be generation of human beings, either those who are generated will be later corrupted, or they will be incorruptible and immortal. If, however, they will be incorruptible and immortal, a number of unsuitable consequences follow. First, it will be necessary to hold that those men are born without original sin, since the necessity of dying is a punishment consequent upon original sin; which is contrary to what the Apostle says in Romans, 5:12, that 'through one man sin was transmitted to all men, and through sin, death.' Whence it would follow that not all were in need of the redemption which is from Christ, if some were born without original sin and without the necessity of dying; and thus Christ will not be the head of all men; which is contrary to the view of the Apostle who says in I Corinthians 15:22 that 'as all died in Adam, so too all are made alive in Christ.' "
40. ST III 3.7 Cf. also ST III 3.7 ad 2: "The assumed nature, however, as to something stands in the manner of a vestment, granted that there is not a likeness on all points, as was said above. And therefore if a divine person would assume two human natures, on account of the unity of the supposit, one would speak of 'one man having two human natures'".
41. ST I 103 4 ad 1.
42. A somewhat similar question is whether the angelic realm and human realm were created at the same time. Aquinas does not want to say that a negative answer to this question is to be reputed as erroneous because certain saints were of that opinion. He himself, however, does not think that it is correct for the reason that "if, however, the angels would have been created separately, they would seem to be totally alien from the order of corporeal creatures, as if constituting of themselves another universe." De Pot. 3.18.
43. De Caelo, Bk. 1, c. 9, lec. 19. This argument is already found in Plato's Timaeus, and perhaps Aquinas took it from there: "To the end that this world may be like the complete and living Creature in respect of its uniqueness, for that reason its maker did not make two worlds nor yet an indefinite number, but the Heaven has come to be and is and shall be hereafter one and unique." Timaeus 31a, b in The Collected Dialogues of Plato, eds. Hamilton and Cairns, (New York: Pantheon Books, 1961). A number of authors maintain that the discovery of ET life would prove Christianity is false since it would imply that there were many Adams and many Incarnations. Cf. Thomas Paine quoted by Michael J. Crowe in The ET Life discussion 1750-1900 The Idea of a Plurality of Worlds from Kant to Lowell, 163: "From whence, then, could arise the solitary and strange conceit that the Almighty, who had millions of worlds equally dependent on his protection, would quit the care of all the rest and come to die in our world, because, they say, one man and one woman had eaten an apple! And, on the other hand, are we to suppose that every world in the boundless creation had an Eve, and apple, and serpent, and a redeemer? In this case, the person who is irreverently called the Son of God, and sometimes God himself, would have nothing else to do than to travel from world to world, in an endless succession of deaths, with scarcely a momentary interval of life". Of course this in not the only possible scenario, even if ET life existed in great numbers. However, it does support Aquinas's claim that uniqueness is a kind of perfection.
44. ST I 47.3 obj. 2.
45. ST. I 47.3 ad 2. Cf. De Caelo, Bk. 1, c. 9, lec. 19: "...if God would make other worlds, either he would make them like this world or unlike. If they were completely alike, they would exist in vain: which does not belong to his wisdom".
46. ST I 47.1.
47. De Pot. 6.6. Cf. SCG II 92 which also says that there should be more of better things, giving as reason that the lower are for the sake of the higher.
48. ST I 112.4 ad 2.
49. De Sp. Cr. 8 ad 10.