What would Aquinas have said if he had our science

Moral theology is often separate from science but ignoring science in this modern age would leave it looking reactionary and irrelevant. In light of some recent understanding from the world of science, I set out some questions that I feel should be incorporated into moral theology, not as a challenge but to complete it.

I have always believed that while religion & philosophy and science do not necessarily conflict, neither is inherently superior. Rather, they inform each other, with the language & conclusions of religion & philosophy being refined to incorporate scientific understanding to remain relevant and science owing its body of science ethics to religious & philosophical traditions. It is, therefore, not the intention of this article to be an article on ensoulment or to challenge existing theology or even to participate in the war between religion and science but I have always wondered how the great philosophical and theological works of the past would have been written in the light of the latest scientific understanding.

Thomas Aquinas

Thomas Aquinas wrote his Summa Theologiae in a time when science and philosophy were unified subjects in a way it is not today. In classical Greece, there was no categorisation of knowledge into science, maths, humanities, philosophy, or any other fields we have today. Philosophy means love of knowledge and knowledge was integral. Socrates, Pythagoras and the like taught the single subject of philosophy but what is now classified as maths, political science, sociology, psychology and the various sciences. Indeed, science was known as natural philosophy as late as the early nineteenth century.

The Summa was written in a scientifically rational manner but based on the scientific understanding of his day. Today, we tend to read the Summa divorced from the scientific knowledge of today. The question is whether Aquinas would have written it differently in light of scientific knowledge of today, other than the obvious difference of language and style?

I would not presume to challenge Aquinas. Rather, I would ask whether some of his conclusions or formulations in the Summa might require revision or rewording with the benefit of modern scientific knowledge to remain relevant to a more educated audience. The questions are largely to do with the mechanics of metaphysical-physical interactions, an approach which Aquinas seems to have addressed constantly in his great work. The aim is not to replace Thomism with biology, but to explore whether Thomistic categories themselves would have been articulated with greater nuance if applied to contemporary science.

What have souls?

The kingdom of life

As we understand more about the biology of life, we find the simplistic division of all life into the animal and plant kingdoms no longer works. We now know that life on Earth is more complex than that. Modern taxonomy added the fungi, protista and monera kingdoms. So, is the classical Western philosophy classification into vegetative souls, animal souls and human souls (or nutritive, sensitive and intellectual souls respectively in the terminology of Aristotle) that Aquinas used still good for purpose? Are the three new life kingdoms intended to fit into the original vegetative and animal souls classification or do they get their own classifications inline with developing biological knowledge? What metaphysical criteria beyond biological taxonomy determine whether an organism has a vegetative or sensitive soul?

Do single cells have souls?

If biological classification itself no longer aligns neatly with traditional metaphysical categories, the difficulty becomes more acute when we consider life at its smallest scales. Would single-cell protozoa, bacteria and fungi have souls as well, as they are obviously alive? If they don't, at which point do we cut off which species have souls and which do not, and what are the criteria? As species exist within a spectrum of increasing biological and behavioural complexity, does the Thomistic classification imply a gradation in the powers of vegetative or sensitive souls, or are such souls strictly binary - one species has a complete animal/vegetative soul, while the next one down the spectrum has none at all? Was Aquinas' rejection of the mixing of souls based on his understanding of the knowledge of the day or on metaphysical reasoning independent of scientific understanding?

Would souls extend to below the cellular level - what about viruses, which are even debated whether they are life or not? Do souls need a minimum threshold to exist, and if so, what is the threshold based on - cellular/electrical/chemical activity, level of consciousness? Effectively, how does Thomistic philosophy apply to organisms and attributes not known to Aquinas? Or is it that the questions on which classical philosophy and theology are silent because they are irrelevant? Meaning that any advancement in human knowledge is irrelevant to one's philosophical and theological understanding?

What have spirits?

In Western philosophy, souls are the material nature that distinguishes a living organism from a non-living one. Only humans, though, have spirits capable of moral will. Under Thomistic definitions, the human spirit-soul is a single entity and is the only one that survives death in an afterlife (Summa Theologiae I, q. 75, a. 2–4). I need to stress that the following inquiry does not assume that animal cognition constitutes rational intellect in the Thomistic sense. Rather, it asks whether the growing empirical overlap in cognitive capacities would have led Aquinas to articulate sharper gradations in sensitive powers, even while maintaining the uniqueness of rational will.

Koko with her keeper, Francine Patterson

Modern scientific research challenges the notion that some traits are exclusively human. Great apes can be taught to use language, communicating in American Sign Language. Whether they understood language the same way as human is debatable but they certainly made themselves understood by humans, including use of abstract terms. One primatologist, raised by two deaf parents, met Koko, the first ape who signed, and reflected that she had just spoken to another species in her mother tongue. Tool-making is widespread in many species; some of these are carefully planned and kept for future use, indicating foresight and intention.

Behaviours interpretable as abstract thinking, including artistic expressions in works of art, creating new words, etc., have been observed and documented in non-human animals, although these remain contested and distinguished from full human conceptual abstraction. Science only examines rigorous research, which excludes as many anthropomorphic assumptions as possible from these seemingly human traits, while actions undertaken due to instincts required by their genes are also excluded as far as is known.

If Aquinas had the benefit of these observations, would he still have held animal souls and human spirits to be binary classifications? Or would it be a spectrum, with the possibility that an animal/individual be at an intermediate stage with part animal soul and part human spirit, or a full animal soul with a bit of human spirit overlaid on it, growing with its increasing humanness? We know that we share, depending on the methodology used, 96-99% of our DNA with chimpanzees and bonobos. Is the defining criterion for whether an individual has a human spirit (or the proportion of human spirit if non-binary) grounded in a quantitative genetic similarity threshold or perhaps, resulting in qualitative human-like cognitive capabilities?

Chimp preparing a twig to use to fish for ants

While Thomistic philosophy does not equate intelligence with moral agency (Summa Theologiae I, q. 79, a. 1-4), the increasing cognitive overlap nevertheless poses uncomfortable pastoral and educational questions. Primatologists have noted that a chimpanzee can attain the intelligence of a six-year-old human. Today, we know that chimpanzee and human brain structures differ, leading to human babies surpassing chimpanzee babies at different ages, depending on the tasks involved. In some tasks, human brains will never exceed chimpanzee brains and vice versa. But let's take age six for this inquiry: if a six-year-old child is deemed capable of basic moral instruction, it is at least worth asking what kind of moral consideration, if any, is owed to non-human animals that demonstrate comparable levels of cognitive functioning, without assuming equivalence of moral status.

Salvation

These observations raise not merely biological or psychological questions, but theological ones. If certain non-human animals display traits traditionally associated with moral agency, it becomes necessary to ask whether this has implications for salvation itself. Fundamentally, would a non-human animal qualify for salvation if it could demonstrate a certain level of abstract thinking for certain virtues, virtues that mainstream religionists agree enable adherents of any religion to attain salvation?

Thomistic philosophy precludes animals from having rational, immortal souls because they lack free will and the capacity for sin (Summa Theologiae, I, q. 83, a. 1). Modern primatological research, however, has uncovered many observations of what seems to be a sense of reciprocal justice among individual primates, as well as displays of cruelty. Granted that a sense of reciprocal justice does not equate to moral agency, but it is a stepping stone to it. Would Aquinas have reworded or even reconsidered his conclusions if he had access to this understanding?

Ultimately, traditional Christian soteriology posits that salvation frees us from original sin, which is the root of all suffering: pain, death, loss. Observations of species as varied as whales and primates reported mothers carrying the body of a dead offspring around for days and refusing food. So have elephant herds standing guard at the body of a dead relative and even occasionally placing stones on the body been observed. While we need to take care not to anthropomorphise another species, it is equally hard not to feel the familiar anguish of loss at such sights.

Thomistic philosophy sharply distinguishes intelligence from rational will (Summa Theologiae, I, q. 82, a. 2), and nothing here presumes their equivalence. Rather, the question centres only on those individual non-human animals that have the cognitive intelligence to learn the consequences of wrongdoing and the benefits of salvation, as well as a rudimentary will to choose to do right and wrong. If such animal individuals can feel the effects of original sin, even if they did not participate in the Genesis story of original sin, would they also benefit from the balm of salvation?

There was a view among Jews at the time of Jesus that the human world was binary: Jews destined for salvation and Gentiles who were not. Jesus showed a different criterion for salvation that was not dependent on binary definitions (Mt 3:9). Are we on the cusp of a similar shift in our understanding of salvation away from discrete definitions of species towards a continuous spectrum of mental capabilities?

Evolution

At this point, a tension becomes unavoidable. Thomistic metaphysics treats the soul as a categorical substantial form, while evolutionary biology presents a continuous, incremental account of biological and cognitive development. The central tension explored here is not between theology and science, but between Thomism’s categorical metaphysics and modern biology’s continuous models. Aquinas worked within a framework of discrete natural kinds; contemporary evolutionary theory describes gradual transitions, overlapping traits, and populations rather than fixed essences. The questions raised in this section arise precisely from that shift in scientific perspective since his time.

When the classical classification of souls is applied to evolution, it would seem that there were pre-humans with animal souls. At which point in human evolution did pre-humans acquire a human spirit? Was it that the animal souls were reduced in succeeding generations to be gradually replaced by human spirits? If, alternatively, the classification was binary, wouldn't that mean that one generation had a wholly animal soul and their offspring would have a wholly human spirit? Effectively, how does Thomistic philosophy apply to evolution, a body of knowledge totally unknown in Aquinas' day?

Do our closest cousins have animal or human souls? - Wikipedia

We know today that early humans interbred with Neanderthals and the DNA of humans of non-African origins has 1-4% Neanderthal DNA. These evolutionary questions arising from this scientific revelation are posed counterfactually: not to overturn Thomistic anthropology, but to ask how Aquinas might have expressed it if he had known that multiple hominin populations interbred and shared genetic continuity. A large body of philosophical and theological thinking, including mainstream Catholicism, holds that Neanderthals have animal souls. Let's assume they did for this inquiry (if they had human spirits, then we go back to a pair of pre-human species which interbred but with one having an animal spirit and the other having a human soul). Does that mean that that particular breeding Neanderthal individual had a wholly human soul because of their capacity to produce human soul offspring? And would the other Neanderthals that did not have the same capacity have animal souls, meaning that some individuals in the species had human spirits and some had animal souls? Or did that individual have a part-human spirit of sufficient threshold to beget a human spirit offspring, raising the question as to what that threshold was? This, of course, assumes that this apparent violation of the Thomistic strict binary distinction between animal and human souls is nuanced in light of modern understanding of evolution.

Multi-level souls?

When the first single cells clumped into multiple cells, did they retain their own souls or did they merge souls?

Aquinas’s account of ensoulment presupposes organisms as unified biological wholes, an assumption entirely reasonable given the scientific knowledge of his time. Modern cell biology, however, reveals that multicellular organisms are composed of semi-autonomous living units with their own metabolism, replication, and survival behaviours. This does not refute Thomism, but it introduces ontological complexity that Aquinas did not have the conceptual tools to address explicitly, simply because the science did not exist in his day.

While Aquinas did opine that there can only be one soul for each organism, his adoption of Aristotelian hylomorphism deals more with whether an organism has multiple animal, vegetative and human souls in a single organism, not to the concept of cells. So we can still explore whether Aristotelian and Thomistic philosophies provide for the idea of multi-level souls. The question, therefore, is whether modern cell biology introduces ontological features that his framework did not have the science to explicitly address.

If single cells have animal/vegetative souls, would multicelled organisms have two levels of souls: one at the cellular level and one at the organism level? If there are, what is the relationship between the two, and does the afterlife also exist for the human subsidiary soul? If so, does the afterlife of one affect the other?

Single cells have been shown to survive and even reproduce by mitosis in an appropriate nutrient-rich and oxygen-rich medium at the right temperature. If single cells have souls but cells within an organism do not, do the latter acquire souls when they separate from the organism: fall off, are removed, etc.? Or do they never have souls, no matter how long they survive outside the body: meaning not everything alive has a soul. If individual cells with souls survive the death of the organism, for a certain length of time anyway, do each soul at both levels commence its post-death transition at different times?

What then determines whether a particular organism has a separate soul, anyway? Is the criterion tied to a biological definition? For instance, an organism could have a separate soul if it has (i) DNA seeking to replicate itself (thus, ensouling viruses), or (ii) demonstrates metabolism and reproduction. Or is the criterion one that is physical but non-biological? Or is the criterion purely metaphysical or even a philosophical tool utilised to explore certain philosophical concepts and questions?

Fetus-in-situ

Twins

Fetus-in-situ has always intrigued me and raised distinct metaphysical questions concerning identity, individuality, and the temporal conditions of ensoulment. Basically, these are twins in the womb, one of which did not develop as a fully viable baby. The non-viable twin often dies and would normally be reabsorbed back into the mother's body. In rare cases (about 2 in a million live births), however, it is not reabsorbed, and the surviving twin grows to envelop it, resulting in stories of patients having a tumour removed, which distressingly turned out to be a sibling. The non-viable twin may be dead and was removed as a calcified mass, like bone, or still alive and was removed as a mass of cells, like a cyst or tumour, sometimes with recognisable organs & limbs.

If a twin eventually becomes non-viable while still a fetus with an undeveloped nervous system (ie, incapable of any sensation), did it have a human soul at conception, which then passed to the afterlife like any other human soul, albeit without any human experience? If the non-viable twin was eventually removed as a tumour, it is alive insofar as the cells comprising it are alive as it feeds and the cells reproduce, but it has no consciousness of a human and is not alive as a human. So, does it have a vegetative or animal soul or is it a collection of souls at a cellular level?

If we accept that the soul existed at conception, did the non-viable twin have a soul at conception and make its post-death transition on becoming unviable? Did it have a soul as long as a possibility of viability remains, only to lose it the moment the last possibility of viability was extinguished, meaning it could be alive but had no soul because it had no future viability? Or did it never have a soul because it was doomed even at conception due to existing congenital traits or the future non-temporal knowledge of future unviability prevents its ensoulment in the first place, even if the unviability results from an accident or condition which did not exist at conception?

As a separate point on ensoulment of identical twins, how many souls were there before the fetus split up? Were there two souls at conception because there would be two fetuses eventually, sort of pre-ordained, even though it has not happened yet? Or was there only one soul, which split up into two souls when the fetuses split up? Or was there only one soul which continued after the split, with another soul starting its existence from the split - meaning, there are an elder and a younger sibling souls metaphysically when both fetuses have identical physical age?

I know this sounds convoluted but I hope I have adequately represented Aquinas's line of reasoning had he the benefit of knowledge about twinning and fetus-in-situ that only became available after his time. I have left the questions unanswered, not being equipped with Aquinas's mind. Still, it seems obvious that the Summa would have been a much thicker tome if Aquinas had access to the scientific knowledge of today, in this field and in others.

Conclusion

Should philosophy return to its roots of being integrated into other bodies of knowledge, informing and being informed by developments in research anywhere? Should our understanding of theology, the science of God, be contextual and conditional on our comprehension of science and theology being aligned with each other? If so, would Thomistic or any other forms of philosophy and theology benefit from being revisited from time to time in dialogue with the wisdom of the contemporary community? Not to be undermined, but to relate to modern minds educated with textbooks of today. Or should philosophy and theology, which study an atemporal God, also be atemporal and be totally insulated from our developing understanding of other bodies of knowledge?

This inquiry does not presume that Thomistic philosophy is mistaken, nor that empirical science determines theological truth. Aquinas has reasoned coherently within the limits of the empirical evidence available to him at his time: an understanding of biology unaware of cells, evolution, human-like traits in non-human animals and a far more complex taxonomy of life forms. Rather, it deliberately raises, rather than resolves, the question of how Aquinas' careful wording, or even conclusion, would have unfolded with modern understanding of biology being continuous gradations, as in our day, rather than discrete categorisations, as in his? The questions raised here arise precisely from this shift in scientific understanding, not from seeking to abandon his philosophical reasoning.

To refuse such questions out of fear of unsettling established categories risks rendering moral theology frozen in archaic understanding and conceptually brittle, not philosophically living and learning. Aquinas himself engaged deeply with the science of his time; fidelity to his method may therefore require ongoing dialogue with our time. A theology confident in the robustness of its reasoning need not fear a deeper understanding of how the physical and living world actually works. It would speak carefully to contemporaries in the language of contemporary understanding and, by contrast, honours the Thomistic commitment to reason, coherence, and truth, wherever that truth may be found.