I am grateful to Mark Robson for his comments on what I say about evil as privatio boni (absence/privation of good) in my book The Reality of God and the Problem of Evil. Indeed, I agree with most of his observations. This might seem puzzling to readers of Robson's article since it is offered as a criticism of me (and of Aquinas, on whom I draw in what I say about evil in my volume). It might, however, seem less puzzling if I now briefly try to explain why Robson's cogent points do not really engage with what I was arguing in my book (or with what Aquinas is arguing in his treatment of evil as privatio boni).

Robson maintains that if evil or badness is an absence, it cannot be causal. This is obviously true in one serious sense. What is not there is not a causal agent capable of getting something done. So, for example, the absence of dinosaurs in contemporary London cannot be a cause of that city's buses working smoothly. And so on. As Robson says, it would be silly to reify absences or lacks and to think of them as bringing about a change in the world. So it would be silly to reify evil or badness, considered as privatio boni, and to regard it as wandering around waiting to pounce on or modify anything.

Again, Robson is surely right to say that depression is a real thing since it amounts to feelings and reactions that some people have (whatever their causes might be). People who are clinically depressed are, I presume, actually undergoing a range of (undesirable) psychological and behavioral processes (undesirable for them). I would say the same of someone suffering from toothache or someone boiling with unjustified rage. Depression is certainly real, like pain in all its forms and like emotions of various kinds. It is, I would add, as real as any badness we can think of.

These points, though, do not seem to me to undermine what Aquinas says about evil as privation or what I was trying to express in The Reality of Evil and the Problem of God when drawing on him. Indeed, they are compatible with all of this. As Robson notes, in my book I allude to statements such as ‘He got sick because he did not take precautions’. But I certainly do not take statements like these to imply that not doing something should be understood as causally on a level with statements like ‘John kicked the ball down the street’. I take them to draw attention to how it is that an explanation of something happening can sometimes refer (in part anyway) to a lack of attention. Negligence can enter into an account of how certain events come about, not because there is something to be named ‘negligence’ that has a life of its own and is able to wreak havoc, but because not paying attention to something can sometimes (and sometimes culpably) leave the way open to something able to wreak havoc. In this sense, a lack can be referred to as significant when it comes to the coming about of evil or badness and is not a mere illusion. But I do not claim that lacks are causal agents on a par with, say, people kicking balls.

Here I am thinking in terms of what Aquinas calls agent causality. We have this, he thinks, when (abstracting from the notion of God as an agent cause) we have a substance, or an artifact, producing a change in the world. Someone who kicks a ball in the air is, for Aquinas, an agent cause, and so is a lorry that squashes a rabbit on a road. The difference between a substance (an ens per se) and an artifact (an ens per accidens) is important for Aquinas, but it need not detain us for now. The main point to grasp is that he takes agent causation to occur as changes come about by virtue of things that we might loosely refer to as existing in the world as individual spatio-temporal individuals. And, I need now to add, he does not take evil or badness to be any such thing. Nor do I (and nor, I presume, does Robson). Aquinas's point (to which I am indebted) is that evil or badness is no genuine spatio-temporal individual. Or as Aquinas would say, evil or badness lack esse.

Esse, of course, is a word frequently used by Aquinas. Basically, it means ‘existence’ in the sense that we have in mind when we say, for example, that the current U.S. president exists while Julius Caesar does not. Esse, for Aquinas, signifies what we might call ‘actuality’. So, when he says that evil does not exist (that it lacks esse), Aquinas means that it is not a substance, or even a real accident had by a substance (as is a sun tan or a pale face). He means that it is what can be predicated of a substance (as in ‘X is bad or in a bad way’) without itself having esse in its own right or being something that has esse insofar as it is an accidental form of a substance (insofar as it exists in a substance as genuinely amounting to a reality in something that is not essential to the thing in question). Unless I am misreading him, Robson seems to miss this point.

Robson wonders to what extent my (and Aquinas's) account of evil contrasts with the view that evil is an illusion. I am assuming that he does so because he takes my view of evil to, as he puts it, deprive it of ‘causal muscle’. And he is right to argue that my view of evil does just this in the sense I explain above. But this lack of clout when it comes to evil is part and parcel of my (and Aquinas's) view of evil as privatio boni and hardly an objection to it. It is, I think, important to recognize that evil or badness is not something with esse and, therefore, not an agent cause. But that does not render it illusory. I take an illusion or a hallucination to be something that does not exist period. So I take it that when Macbeth thought that he saw Banquo's ghost there was nothing actual/real/having esse that was there for him to see. But ‘is’ can be sensibly used even when it comes to what is not actual/real/or having esse. Thus, ‘Sickness exists’ is a true proposition even though ‘sickness’ is not the name of an individual substance (as ‘Smokey’ is the name of my individual cat). Aquinas makes this point by distinguishing between esse as actuality and esse ut verum (existence in the sense of ‘is true’). We can, he argues (rightly to my mind) happily agree that though it is simply false to say that wizards exist it is true to say that evil or badness exists, or even that the equator exists. Why so? Not because evil or badness or the equator is an individual substance to be picked out as existing in the spatio-temporal world (as my cat is) but because various true statements with ‘is’ or ‘exists’ in them can be formed — as in ‘There is a number between 4 and 6/The number 5 exists’, and ‘Evil is real/Some things are in a bad way’. The problem with the ‘Evil is an illusion’ view is that it does not seem to recognize this obvious fact. But, note, it is not a fact that compels anyone to think that evil or badness is an actually existing substance or an actually existing element or property of any such substance.

What are we complaining about when we take something to be bad or in a bad way? You might say that we are not complaining but merely describing. But judgments to the effect that something is bad or in a bad way are parasitic on judgments concerning goodness and they note that something is not what we want it to be. To say that something is bad or in a bad way assumes that we have a sense of what it would be like for it not to be so (just as to say that someone is ill assumes that we have a sense of what it would be like for someone to be well). If ‘good’ is a logically attributive adjective (as Robson does not seem to deny), it sets a standard for things as we describe them as being bad since its use depends on our understanding of a noun. We do not understand what is being said when told that something is a bad X unless we have a sense of what it would be to be a good X. If rotten apples were the norm, we would not understand what a bad apple is. So we are indeed complaining when calling something bad (which is one reason for denying the distinction between facts and values that some philosophers have championed since the time of David Hume). And in doing so we are, I think, always noting that something is not as good as it could or should be. Robson's example of depression is not a counter instance to this thesis. I presume that someone depressed is in a bad way since they lack what we look for in healthy, thriving human beings. When trying to cure depression we are aiming to restore people to a good human state that they do not currently enjoy, to give them what they lack as human beings (even if the lack here amounts to them being in the grip of what is perfectly real and subject to causal explanation). Again, to call something bad or to say that it is in a bad way is to lament that it lacks some good.

This is the core of the claim that evil or badness is to be thought of as a privation of being. And that is why Aquinas denies that evil is something created by God. On his view, God accounts for what actually exists (that which has esse). But, so he thinks, evil does not have esse. It is not an actually existing substance or an actually existing accident of a substance. It is real in that things with esse (always, thinks Aquinas, good to some extent) lack what perfects them as what they are essentially — whatever the spatio-temporal causes of this lack might be, and however present their effects). And I do not see how Robson has proved otherwise. He asks why evil should be taken seriously on my account of it. But the answer should be obvious to any victim of it. Evil or badness is to be taken seriously because various things suffer as lacking a good that they desire given what they are by nature. Hence the famous problem of evil. Hence, too, our efforts to cure and to make people and other things well.