Understanding Evil: Aquinas’s Dual Conception of Moral and Physical Evil

How can a world created by an all-powerful, all-good God contain so much suffering? This question – known as the problem of evil – has haunted philosophers and theologians for centuries. Thomas Aquinas, the 13th-century Dominican friar and one of the most influential thinkers in Western philosophy, offered a remarkably systematic answer. Rather than treating evil as a mysterious dark force, Aquinas argued that evil is not a “thing” at all. It is a privation – an absence of the good that should be present. He then drew a careful distinction between two types of evil: moral evil, rooted in human free will, and physical evil, arising from the natural limitations of the material world. Together, these ideas form one of the most enduring frameworks for understanding why evil exists.

What does Aquinas mean by “evil”? 🔗

To grasp Aquinas’s position, we first need to understand what he means by “good.” Following Aristotle and Augustine, Aquinas holds that goodness and being are fundamentally the same thing. Everything that exists, insofar as it exists, possesses some degree of goodness. A healthy dog, a blooming tree, a functioning human eye – all of these are good because they are actual, existing things fulfilling their natures.

Evil, then, is not a separate substance or entity lurking in opposition to good. Aquinas explicitly rejects any dualistic framework that treats good and evil as two equally real, competing forces. Instead, evil is the lack of a good that ought to be present in something. In the Summa Theologiae (I, Q. 48), Aquinas argues that evil falls between pure being and pure non-being – it is neither a positive reality nor a simple negation, but a privation.

Privation vs. simple absence 🔗

This is a critical distinction. Not every absence of good counts as evil. A rock cannot see, but we do not call a rock “blind.” Blindness is only an evil when it occurs in a creature that is supposed to see – like a cat or a human being. Aquinas’s classic example is exactly this: the privation of sight in a being naturally equipped for vision is evil, but the absence of sight in a stone is merely a negation, not a deficiency.

The technical term Aquinas uses is the absence of a “due good” – a good that belongs to a thing’s nature or proper functioning. A human being who cannot reason, an animal that cannot walk when it should be able to, a plant that withers instead of growing – these all represent privations. Evil, in this sense, always depends on a good subject in which it resides. Without something good that exists, there is nothing to be deprived of goodness.

The convertibility of being and goodness 🔗

Aquinas’s view of evil rests on a deeper metaphysical claim: that being and goodness are convertible. In the Summa Theologiae (I, Q. 5, Art. 1), he writes that goodness and being are really the same and differ only conceptually. Everything that exists has some degree of perfection, and that perfection is what makes it good. The more fully something actualises its nature – the more it lives up to what it is supposed to be – the more good it possesses.

This principle has a powerful implication. If goodness is identical with being, then evil – as the opposite of goodness – must be a kind of non-being. It cannot exist independently. It can only exist as a deficiency within something that is itself good. As Augustine argued before Aquinas, if something were stripped of every last shred of goodness, it would cease to exist altogether. Evil is parasitic: it requires goodness as its host.

This is why Aquinas insists that God did not create evil. God, as the source of all being, creates only good things. Evil enters the picture not as something God made, but as a falling-short within created things – a gap between what something is and what it should be.

The two types of evil: moral and physical 🔗

With this foundation in place, Aquinas draws a crucial distinction between two categories of evil, each with a different origin and a different relationship to human responsibility.

Moral evil: the corruption of free will 🔗

Moral evil – what Aquinas calls peccatum (sin) – is the evil that arises from the choices of rational, free agents. When a human being lies, steals, or commits violence, the wrongness of that action is a privation of the rational order that the action should have possessed. In other words, moral evil is a deficiency in the will: the person freely chose a lesser or inappropriate good when reason demanded they choose otherwise.

For Aquinas, no one chooses evil as evil. Following Aristotle, he maintains that every act of the will is directed toward something perceived as good. A thief steals because they desire the good of wealth; a liar deceives because they desire the good of avoiding consequences. The evil lies not in the desired object itself, but in the disordered choice – the failure to align one’s action with what reason and the natural law require.

This is where free will becomes essential. Aquinas believes human beings are endowed with the capacity to make genuinely free choices. This freedom is a great good – it makes love, virtue, and moral responsibility possible. But it also opens the door to moral failure. God causes free agents to act freely, but the deficiency in a free act belongs entirely to the creature, not to God. As Aquinas explains in the Summa Theologiae (I, Q. 49), good is the cause of evil only accidentally, not directly. The human agent, not God, is responsible for the privation of rationality in a sinful action.

Could God have created free beings who never sin? Aquinas considers this and concludes that a will entirely incapable of moral failure would have to be perfectly actualised – identical, in effect, to the Divine Will itself. Since created beings are by nature a mixture of actuality and potentiality, the possibility of moral evil is built into the very structure of created free will. God could eliminate moral evil only by eliminating free will itself, which would mean sacrificing the immense goods – love, virtue, courage, compassion – that only free agents can achieve.

Physical (natural) evil: the limits of material existence 🔗

Physical evil, also called natural evil, encompasses suffering that is not caused by human choice: diseases, earthquakes, birth defects, predation, decay, and death. These are evils that creatures undergo passively rather than commit deliberately.

In Aquinas’s framework, physical evil too is a privation – a lack of some perfection that belongs to a thing’s nature. A disease is a privation of health. A birth defect is a privation of the bodily integrity a creature should possess. The death of a living thing is the privation of its life.

But what causes these privations? Aquinas’s answer is rooted in his understanding of material nature. The physical world is made up of corruptible, changeable substances. Material beings sustain themselves by consuming and transforming other material beings. A lion lives by killing a gazelle. Fire sustains itself by consuming wood. This is not a flaw in creation – it is how material nature works. The good and perfection of one thing often requires the privation of good in another.

As Aquinas writes in the Summa Theologiae (I, Q. 22, Art. 2), God’s providence does not eliminate natural evil because doing so would mean eliminating the natural order of material things altogether. A world of indestructible physical beings would be logically incoherent given the nature of matter. Material things, by their very constitution, are subject to change, corruption, and eventually destruction.

Why does God permit evil? 🔗

If God is all-good and all-powerful, why does He allow any evil at all? Aquinas addresses this directly, and his answer has two parts.

First, preventing all evil would require the elimination of goods that are even greater than the evils prevented. Without corruptible material beings, there would be no physical world at all. Without free agents capable of sin, there would be no possibility of genuine virtue, love, or moral growth. Aquinas quotes Augustine’s principle that God is so powerful He can bring good even out of evil. There would be no patience without suffering, no courage without danger, no forgiveness without wrongdoing.

Second, Aquinas holds that God’s omnipotence does not extend to logical impossibilities. God cannot create a square circle. Similarly, God cannot create material beings that are by nature immune to corruption, or free agents that are by nature incapable of sin, without fundamentally changing what those beings are. Evil, then, is not a sign of God’s weakness or indifference – it is a consequence of the kind of world that allows for the greatest overall good.

Evil is not a force – it is a deficiency 🔗

One of the most important takeaways from Aquinas’s account is the rejection of evil as an independent power or substance. In many popular narratives, evil is portrayed as a force equal to or rivalling good – a cosmic antagonist. Aquinas dismantles this view entirely. Evil has no formal cause (it is not a form that shapes anything). It has no final cause (it is not directed toward any end). It only has a material cause – the good subject in which the privation resides – and an accidental efficient cause, meaning some good agent whose action incidentally produces a deficiency.

This might sound abstract, but it has a deeply practical implication. If evil is always a deficiency in something good, then evil can never fully win. Complete evil would mean complete non-existence. As long as anything exists, there is some goodness in it. The worst atrocity still involves agents who exist, who possess intelligence and will – goods in themselves, even when tragically misdirected.

Aquinas’s distinction and human responsibility 🔗

Aquinas’s separation of moral and physical evil also clarifies the boundaries of human responsibility. We are accountable for moral evil – for the choices we make that fall short of what reason and the moral law require. But physical evil is not our fault in the same way. An earthquake, a genetic disease, a pandemic – these are not the products of anyone’s sin (a point Aquinas makes carefully, distancing himself from simplistic readings of suffering as divine punishment).

This distinction allows for a more compassionate understanding of suffering. It acknowledges that some pain is the result of human cruelty and can be addressed through moral reform. Other suffering is woven into the fabric of material existence and calls not for blame but for solidarity, care, and the pursuit of knowledge to alleviate it where possible.

Criticisms and ongoing debates 🔗

Aquinas’s framework is not without its critics. Some philosophers, both contemporary and historical, have argued that calling evil a “privation” is merely a semantic move – replacing “the problem of evil” with “the problem of the absence of good” without actually reducing the weight of suffering. The 18th-century theologian Francesco Antonio Zaccaria, for instance, contended that the privation theory fails to adequately address the felt reality of human pain.

The British philosopher John Hick offered an alternative approach, arguing that evil exists because the world is designed for the moral and spiritual development of human beings – a view he traced back to the early Church Father Irenaeus rather than Augustine. More recently, philosopher J.L. Mackie posed a formidable challenge: if God is omnipotent, why couldn’t He create free beings who always freely choose the good? Aquinas’s metaphysics offers a response – that such a will would need to be purely actual and thus identical to God’s own will – but the debate continues.

Despite these challenges, the privation theory remains a foundational position in the philosophy of religion, particularly within the Thomistic and neo-Thomistic traditions. Its strength lies in its internal coherence: it explains how evil can exist in a world created by a good God without requiring evil to be a positive thing that God created.

Why Aquinas’s view still matters 🔗

Aquinas’s dual conception of evil provides more than a theological defence. It offers a way of thinking about suffering that is both intellectually rigorous and deeply humane. By insisting that evil is not a thing but a deficiency, he reminds us that the world is fundamentally oriented toward goodness and being. By distinguishing moral from physical evil, he clarifies where human responsibility lies and where it does not. And by grounding moral evil in the misuse of free will, he preserves the dignity of human agency while acknowledging its tragic potential for failure.

Whether one accepts his theological premises or not, Aquinas’s analysis forces us to think more carefully about what we mean when we call something “evil” – and that clarity, in itself, is a philosophical achievement of lasting value.

What do you think? Does defining evil as a privation of good reduce the felt weight of suffering, or does it actually sharpen our understanding of what goes wrong in the world? And can free will truly be worth the moral evil it makes possible?

References

1. https://iep.utm.edu/thomasaquinas-moral-philosophy/
2. https://www.newadvent.org/summa/1048.htm
3. https://aquinasonline.com/nature-of-evil/
4. https://en.wikipedia.org/wiki/Augustinian_theodicy
5. https://onlinelibrary.wiley.com/doi/10.1111/nbfr.12579
6. https://www.newadvent.org/summa/1049.htm
7. https://aquinasonline.com/problem-of-evil/
8. https://en.wikipedia.org/wiki/Absence_of_good
9. https://en.wikipedia.org/wiki/Problem_of_evil
10. https://plato.stanford.edu/entries/aquinas-moral-political/