Two chapters this week on the (non-controversial) claim that evil must have a cause. The nature of this cause is, however, of some interest.
1 From what has been said above it can be shown that, though evil has no direct cause of itself, still there must be an accidental cause for every evil.
2 Whatever exists in another thing as in its subject must have some cause, for it is caused either by the principles of the subject or by some extrinsic cause. Now, evil is in the good as in a subject, as has been indicated, and so it is necessary for evil to have a cause.
3 Again, that which is in potency to either of two contraries is not advanced to actuality under one of them unless through some cause, for no potency makes itself be in act. Now, evil is a privation of something that is natural to a man, and which he ought to have. This is why anything whatever is called evil. So, evil is present in a subject that is in potency to evil and to its contrary. Therefore, it is necessary for evil to have some cause.
Notes Reminder: this is (one of the) definitions of cause: the move of a potential to an actuality by something actual.
4 Besides, whatever is present in something and is not due to it from its nature comes to it from some other cause, for all things present in existing beings as natural components remain there unless something else prevents them. Thus, a stone is not moved upward unless by something else that impels it, nor is water heated unless by some heating agent. Now, evil is always present as something foreign to the nature of that in which it is, since it is a privation of what a thing has from its natural origin, and ought to have. Therefore evil must always have some cause, either directly of itself, or accidentally.
5 Moreover, every evil is the consequence of a good, as corruption is the result of an act of generation. But every good has a cause, other than the first good in which there is no evil, as has been shown in Book One . Therefore, every evil has a cause, in regard to which it is an accidental result.
1 It is plain, from the same consideration, that evil, though not a direct cause of anything by itself, is, however, an accidental cause.
2 For, if a thing is the direct cause of something, then that which is an accidental concomitant of this direct cause is the accidental cause of the resultant. Take, for instance, the fact that a builder happens to be white, then whiteness is the accidental cause of the house. Now, every evil is present in something good. And every good thing is the cause of something in some way, for matter is in one way the cause of form; in another way the converse is so. The same is true of the agent and the end. Hence, the result is not a process to infinity in causes if each thing is the cause of another thing, for there is a circle involved in causes and effects, depending on the different types of cause. So, evil is an accidental cause.
Notes We can learn a lot from accidental causes in application to physical and statistical models, too! Note very carefully the word cause is not here restricted to physical efficient cause, as is (unfortunately, all too) common in the sciences. See paragraph  below.
3 Again, evil is a privation, as we have seen before. Now, privation is an accidental principle in beings subject to motion, just as matter and form are essential principles. Therefore, evil is the accidental cause of something else.
4 Besides, from a defect in a cause there follows a defect in the effect. Now a defect in a cause is an evil. Yet, it cannot be a direct cause in itself, for a thing is not a cause by the fact that it is defective but rather by the fact that it is a being. Indeed, if it were entirely defective, it would not cause anything. So, evil is the cause of something, not as a direct cause by itself, but accidentally.
5 Moreover, evil is found to be an accidental cause in a discursive examination of all types of cause. This is so, in the kind of cause which is efficient, since a defect in the effect and in action results from a deficiency of power in the acting cause. Then, in the type of cause that is material, a defect in the effect is caused by the unsuitable character of the matter. Again, in the kind of cause which is formal, there is the fact that a privation of another form is always the adjunct of the presence of a given form. And, in the type of cause that is final, evil is connected with an improper end, inasmuch as the proper end is hindered by it.
6 Therefore, it is clear that evil is an accidental cause and cannot be a direct cause by itself.