SUPERVENIENCE AND MORAL REALISM ALISON HILLS University of Oxford 1 INTRODUCTION According to objective moral realism, there exist moral properties that are not dependent on our beliefs or our attitudes about them. In Mackie’s fa- mous first chapter of his book Ethics, he gives three arguments against this theory. His first argument is that the nature and quantity of moral dis- agreement gives us reason to deny that there are any objective moral prop- erties. His second argument is that the very nature of putative objective moral properties or moral facts – as essentially motivational (or perhaps, essentially normative) – is reason to doubt that any such thing could exist. Moral realists have not been too impressed by these arguments, however. It is perfectly clear that moral properties can coexist with widespread and persistent disagreement about their nature. In response to the queerness ar- gument, moral realists have either opted for naturalism, trying to show that moral properties are not really peculiar at all – in fact they simply are natu- ral properties – or they have conceded that moral properties are non-natural and that at least to that extent they are queer, but have argued that this is not a sufficient reason to deny that they exist. The third argument, concerning supervenience, has been compara- tively neglected, however, though it was taken up by Simon Blackburn, with responses from a variety of moral realists. In the following sections I will make the supervenience argument against moral realism more precise, showing that it must be directed at non-naturalist forms of moral realism. I will consider one possible response to the argument: I will look at the pos- sibility of denying that supervenience is true. 2 THE SUPERVENIENCE ARGUMENT It is widely, perhaps even universally believed that moral properties neces- sarily depend on other properties. To be more specific (but still quite rough): Supervenience: there can be no difference in moral properties with- out a difference in the natural properties.1 Supervenience in one form or another is accepted by a range of philoso- phers who otherwise have quite different meathetical views, including G.E. Moore, R.M. Hare, Blackburn, Shafer-Landau, and Ridge. Some of these philosophers accept moral realism, others do not. But according to Mackie’s supervenience argument, moral realists cannot comfortably ac- commodate Supervenience. The problem is not that Supervenience is actu- ally inconsistent with moral realism, rather that Supervenience generates an explanatory demand which moral realism cannot meet but certain ver- sions of non-realism can. If the argument succeeds, moral realism is at a considerable disadvantage to a theory like Blackburn’s non-cognitivism. We can distinguish between different versions of Supervenience. I 2 will assume that the necessity at stake is metaphysical: 1 What are the natural properties? It is well-known that “the natural” is difficult to de- fine. Typically philosophers understand the “natural” properties as something like: those properties that are studied by science (or perhaps those that would be studied in a future, complete science). Do mental properties – being happy, being in pain, having a desire or intention, deciding to act – count amongst the natural? One might say no, they do not, and deny Supervenience on that basis. Do supernatural properties – being prescribed or proscribed by God – count as natural properties? Again, one might say no, and deny Supervenience for that reason. But I will assume for the purposes of this paper that the “natural” can be defined sufficiently clearly and sufficiently broadly that supervenience is a plausible claim. 2 Alternatively, Supervenience could be primarily a semantic thesis about the meaning of moral terms (or the requirements on a competent user of moral terms), with refer- ence to analytic necessity. In Blackburn’s presentation of the supervenience argument, he seems to be discussing analytic necessity, and requiring a moral realist to explain why it is part of our concept of morality that there can be no moral changes without accompanying changes in natural properties. However, it seems to me more difficult for the moral realist to explain the metaphysical connection between the moral and the natural (at least, this is a difficulty for a non-naturalist moral realist), so I will focus on the metaphysical supervenience claim. 164 Metaphysical Supervenience: it is metaphysically impossible for there to be a difference in moral properties without a difference in natural properties. But we can distinguish a stronger and a weaker claim, depending on whether we are talking about differences within one world, or between worlds: Strong Metaphysical Supervenience: it is metaphysically impossible for there to be a difference in moral properties (between any two possible worlds) without a difference in natural properties. Weak Metaphysical Supervenience: it is metaphysically impossible for there to be a difference in moral properties (within one possible world) without a difference in natural properties. That there is a dependence between moral and natural properties is not itself inconsistent with moral realism. But a necessary connection be- tween two kinds of property is the sort of thing that calls for explanation and it is cost to a theory if it is committed to such a connection without be- ing able to explain it. The problem for moral realists is that they have not offered an adequate explanation – in fact in many cases they have offered no explanation at all – of why this dependence should hold. There seems to be a particular difficulty in explaining why weak supervenience is true, if strong supervenience is not. Suppose that in World 1, natural property N1 underlies moral property M, so that there is never a change in property M without a change in N1 in that world. And suppose that in world 2, natural property N2 underlies moral property M, so that in world 1, there is never a change in property M without a change in N2. According to weak super- venience, there can be no “mixed worlds” in which a change in M is some- times accompanied by a change in N1 and sometimes by N2. But why not?3 Moral realists have some explaining to do. 3 Blackburn thinks that his non-cognitivism can explain this better, because on that view the function of moral discourse is to praise, recommend or condemn actions, states of affairs etc in terms of their natural properties, in order to regulate our behav- iour. So it is clear why weak supervenience is true, given non-cognitivism. But since there is no single best way of carrying out this function, strong supervenience is not true. There is nothing mysterious to explain. 165 3 NATURALIST AND NON-NATURALIST MORAL REALISM According to some moral realists, moral properties simply are natural properties. So it is obviously true that necessarily, there can be no change in moral properties without a change in natural properties – a change in moral properties is a change in natural properties. In which case there is no need to offer a special explanation for the supervenience of the moral on the natural.4 But non-naturalist moral realism is in a more difficult position. Ac- cording to non-naturalists, moral properties are not natural properties: the two are distinct. So why is there a necessary connection (either strong or weak) between the two? Non-naturalists need not offer a naturalist expla- nation for this connection (they need not commit themselves to what has been called “superdupervenience”).5 But it remains true that they think that there is a metaphysically necessary connection between two distinct kinds of properties, natural and moral, and it is not unreasonable to expect this sort of connection to be explicable. The non-naturalist might simply bite the bullet and refuse to give any explanation at all. It is simply a brute fact, she might say, that there is this necessary connection between moral and natural properties. This rejection of the demand for explanation is not very appealing, but the non-naturalist might bolster her position by claiming that others are in an equally difficult place. For example, she might cite other topics where realists have ac- cepted supervenience theses, including the thesis that mental properties de- 4 Naturalist moral realists can claim that there are analytic connections between moral and natural predicates, but more typically they deny this, claiming that moral terms cannot be analysed in natural terms, but nevertheless they refer to natural properties (on an analogy with the identity between water and H2O – water cannot be analysed as H2O, but nevertheless water necessarily refers to H2O). Analytic naturalism is not very plausible, since it has not proved possible to provide naturalistic analyses of moral terms; the non-analytic version is more appealing. However, there are serious ques- tions over whether the semantics of moral terms really are similar to those of natural kind terms and it is not obvious that either type of naturalism is sustainable. 5 Superdupervenience is “ontological supervenience that is robustly explainable in a materialistically explainable way.” (Horgan 1993 p.566). This is more than a moral realist needs to accept, unless they are committed to naturalism. But of course, if they accept non-natural moral properties and they accept a necessary connection that is not naturalistically explicable, they violate naturalism twice over. But if naturalism is false, as the non-naturalist believes, two departures from naturalism are no worse than one. So it is not a compelling objection to a non-naturalist moral realist that she has no naturalistically respectable explanation for supervenience. 166 pend on physical properties, that chemical properties depend on atomic properties, that colour properties depend on physical properties. It is rare for anyone to offer explanations for the proposed necessary connections between these sets of properties.