Aquinas and the Cause of Evil

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By Elizabeth Zahorick, University of Notre Dame

In the Middle Ages, university professors commonly taught their students using Peter Lombard’s Sentences, a book of commentary on the Church Fathers and other authoritative texts that attempted to explain apparent discrepancies in Christian teaching [1]. Students would then write commentaries on this book. In 1265, Thomas Aquinas, a Dominican friar and university lecturer, decided to write his own text that could be used by students as a compilation of philosophical and theological knowledge. This work, the Summa Theologiae, addresses a vast range of questions about God, the angels, and humanity [2].

In Article 1 of Question 49 of the Prima Pars of the Summa Theologiae, Aquinas examines the question of the cause of evil [3]. Intuitively, it seems that evil must have some sort of cause. However, it also seems impossible that evil could be caused by good, or that evil could in some way cause itself. Aquinas first proves that evil must have a cause and that its cause must be good. He then demonstrates how exactly good can be the cause of evil, using the four causes as delineated by Aristotle to distinguish between the different ways in which good causes evil. He demonstrates that evil has no form or end and therefore no formal or final cause. He then shows that that good is the subject, or material cause, of evil. Finally, he proves that good is also the per accidens efficient cause of evil, distinguishing three ways in which good can efficiently cause evil accidentally. In this paper, I will focus on Aquinas’ initial proof that evil must be caused by good and reconstruct the premises and arguments used by Aquinas to prove this conclusion. I will then evaluate whether his argument sufficiently answers the posed question.

Aquinas’ argument for good as the cause of evil is as follows:

“I answer that, it must be said that every evil in some way has a cause. For evil is the absence of the good, which is natural and due to a thing. But that anything fail from its natural and due disposition can come only from some cause drawing it out of its proper disposition. For a heavy thing is not moved upwards except by some impelling force; nor does an agent fail in its action except from some impediment. But only good can be a cause; because nothing can be a cause except inasmuch as it is a being, and every being, as such, is good.”

He begins with a definition of what evil is. For Aquinas, evil is not something that exists in itself, but is rather a privation of the goodness that is natural and due to a thing. While it is common to think of evil as something having substance or some force opposed to good, it is impossible for that to be true. In Article 1 of Question 48 of the Prima Pars of the Summa, Aquinas proves that evil does not have actual existence in itself [4]. In this article, he first shows that we can understand something by understanding its opposite, such as when we come to know what darkness is by our understanding of what light is. It follows from this that we can understand evil by understanding its opposite, which is good. Aquinas defines good as that which is desired by the appetites. Since that which is desired by the appetites of any nature is to be, it follows that being is good. Evil, since it is a lack of good, must therefore be a lack of being.

Evil is not merely a lack of being generally, but is most properly understood as a privation, which is a way of non-being that is different from negation. Negation signifies a lack of being generally, whereas privation is non-being where being ought to be present. For example, the fact that a couch cannot see is a negation, because couches cannot naturally see. However, a person being unable to see is a privation because people are naturally able to see. A negation is not considered to be evil, but a privation is, because it signifies a lack of something that a thing naturally possesses.

We can also understand that evil is not a being in its own right by examining the theological implications of an evil that exists substantially. If evil did exist in its own right, it would have to either have been created by God or exist without being created by God. However, all things created by God are good, according to the Book of Genesis, which states, “And God saw all the things that he had made, and they were very good.” [5]. This means that God did not create evil.  Nor can it be the case that evil could exist and not be created by God, for God is the First Principle of being. Aquinas describes God as “the essentially self-subsisting Being” [6] from which all other beings receive existence due to their participation in His essence. This is evident in the beginning of the Gospel of John, which says of the Word of God that “all things were made by Him: and without Him was not anything made that was made” [7]. Therefore, it must be the case that evil does not exist in its own right but is only a privation of the natural and due goodness of all things God has created.

Next, Aquinas presents the premise that anything that fails from its natural and due goodness must have some sort of cause. To illustrate the truth of this premise, he provides the example of a heavy object, which has a natural disposition towards falling to earth. In order for it to be deprived of this disposition, it must be moved upwards by a heavy force. More broadly, any agent only fails in its action if there is some impediment preventing it from performing that action. Aquinas demonstrates earlier in Article 4 of Question 4 of the Prima Pars that all beings are naturally ordered towards God, who is good, as their end [8]. This means that if something fails to achieve the goodness and perfection intended for it by God, it only does so because of some cause.

 If evil is a failure of a thing from its natural and due disposition, and a thing only fails in its natural and due disposition from a cause, then evil must have a cause. If this were not the case, then evil would exist without a cause. However, it has been shown that God is the First Principle that causes all other things, and therefore God is the only Uncaused Cause. Since God is good, and evil is the lack of good, evil cannot be an uncaused cause. Therefore, evil must have a cause.

Next, Aquinas sets out to prove that the only cause for evil is good. His first premise is that nothing can be a cause except insofar as it exists. This claim makes intuitive sense, because in order for something to “be a cause” it seems like it would have to, at first, “be”. We can see that this is true by considering the implications that would arise were it not true. If something could be a cause without existing, that would mean that non-existence could cause something. But non-existence cannot cause anything, because it is nothing and therefore has no principle of existence to drive any sort of motion or change. Therefore, it must be the case that something must exist in order for it to be a cause.

Aquinas’ next premise is that if something exists, it must be good insofar as it exists. This is one of the foundational premises of his philosophical system. Aquinas proves this premise in Article 3 of Question 5 of the Prima Pars of the Summa [5]. In this article, he first states that all being, as being, has actuality. This comes from the definition of actuality, which is the state in which something exists in reality. Next, he claims that all being is in some way perfect. This conclusion follows from the premise that all act implies some sort of perfection. This premise follows from the sense in which we understand “perfection” as a motion from potency towards act. If act implies perfection, and being implies actuality, then it follows that all beings, as beings, possess some form of perfection.

Furthermore, perfection implies some sort of goodness and desirability. Aquinas proves this claim in the first article in the aforementioned question [10]. As was stated earlier, goodness is anything that is desirable. What people desire is perfection, which means that a thing is only desirable insofar as it is perfect. If something is only desirable insofar as it is perfect, it follows that it is only good insofar as it is perfect. As was shown above, a thing is only perfect insofar as it actually exists. Therefore, something is only good insofar as it exists. Having proven that all evil must have some sort of cause and that all causes are good, it follows necessarily that all evil is caused by good.

In this article, Aquinas effectively demonstrates that evil must be caused by good. Although evil does not exist in its own right, it does exist in the sense that it is something about which we can make true statements. Aquinas uses this sense of the definition of existence to show what evil is, and to prove that evil must be caused by something. He then shows that only good is capable of causing anything, which means that evil must be caused by good. This proof is perfectly valid, and its premises have been shown to be true. If we take validity of argument and soundness of premises to be the two criteria by which we judge whether a question has been sufficiently answered, then it is true that Aquinas has sufficiently answered the question of whether evil is caused by good.

In this paper, I have reconstructed the argument by which Aquinas proves that good is the cause of evil. I have then broken down each premise used to draw this conclusion and evaluated its truth, then brought the premises back together to show how they lead to the conclusion. Later in this question, Aquinas explains precisely how good can be the cause of evil, distinguishing between the four types of causes. He proves that evil has no formal or final cause, but that good is both the material and accidental efficient cause of evil. These proofs are beyond the scope of this paper but effectively support Augustine’s claim, cited in this article, that “there can be no cause for evil except good” [11].


[1] “Four Books of Sentences.” Encyclopædia Britannica, Encyclopædia Britannica, Inc.,

[2] “Summa theologiae.” Encyclopædia Britannica, Encyclopædia Britannica, Inc.,

[3] ST I.49.1.

[4] ST I.48.1.

[5] Gen. 1:31.

[6] ST I.44.1.

[7] John 1:3.

[8] ST I.4.4.

[9] ST I.5.4.

[10] ST I.5.1.

[11] Contra Julian. i, 9.

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