For illustrative purposes, let X be the set of all instances where Bob has tortured and eaten babies alive for fun. (Also assume that X is non-empty, to avoid trivial responses.)

Suppose that physicalism and the causal closure of the physical are both true. Then all events in X are physical events, and they cannot have causes outside the physical domain, given that the physical is all that exists.

In this context, the question arises: Is it possible for events in X to be objectively wrong, according to moral realism?

In other words, can moral realism, physicalism, and physical causal closure be all true at the same time?

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    – Geoffrey Thomas
    Mar 4 at 14:19
  • 1
    It is a thin needle to thread, but Jackson’s analytic functionalism apparently does. The trick is that moral facts are physical facts under a different description. It is similar to physicalist accounts of abstractions but less plausible since moral facts are supposed to have some 'real' force or function, even if not exactly causal.
    – Conifold
    Mar 5 at 1:35

2 Answers 2


Physicalism and physical causal closure seem to go hand-in-hand. And both the Wikipedia entry on moral realism that you link to, and the SEP entry on the same topic, have moral naturalism as a specification of the realist thesis:

Putting aside the arguments that appeal to moral disagreement, a significant motivation for anti-realism about morality is found in worries about the metaphysics of moral realism and especially worries about whether moral realism might be reconciled with (what has come to be called) naturalism. It is hard, to say the least, to define naturalism in a clear way. ... Impressed by the plausibility of naturalism, though, many moral realists have tried, in one way or another, to show that the moral facts they are committed to are either themselves natural facts or are at least appropriately compatible with such facts (Boyd 1988, Brink 1989, Railton 1986). If they are right, then naturalism poses no special threat to moral realism.

(The SEP article on moral naturalism is worth reading here then, too.) Now, one can style a moral property as irreducible but physical, since one can believe in irreducible physical properties. That these properties have the second-order property of being physical properties is compatible with their having sui generis first-order determinations, like how a particular shade of red has a sui generis aspect but still reliably falls under the umbrella term "color." But so as the SEP article on normativity in metaethics goes (in a section on Mackie's "argument from queerness"):

Note that on this test, K things don’t have to be queer in virtue of their K-ness in order to be queer. It could be that they entail the existence of some distinct kind of thing, and that kind would constitute a fundamental addition to our ontology. Perhaps, that is, K things actually don’t fall in the fundamental layer of reality, but they entail the existence of L things, and those are both fundamental and not present in otherwise identical worlds without K things.

The problem with this understanding of the queerness objection is that it overgeneralizes. Every fundamental kind of thing turns out to be queer! Supposing that the property of physicalness is fundamental, then, comparing a world W1 without physical things to a world W2 that has physical things, W2 will have more kinds of thing in its basic layer than W1. So, physical things are queer! Combine this with the commitment in premise (2) that there are no queer things, and you get the result that there are no physical objects. (Even on the weaker versions of those premises, we get the result that we shouldn’t believe in physical objects, or that there is strong prima facie reason not to believe in them.) The argument can be run on any possible fundamental kind. (See Morton & Sampson 2014 on this point.)

  • Can you please clarify whether you are using physicalism and naturalism interchangeably? Does naturalism commit you to believing in more things than physicalism does (i.e. anything in addition to quarks, gluons, electrons, space, time, quantum fields, charge, mass, energy, gravity, etc.)?
    – Mark
    Mar 4 at 17:34
  • @Mark I meant that the phrase "moral naturalism" refers to a family of theories whose metaphysics-of-ethics are physicalistic (as far as I know). I should add, pace causal closure, that the very concept of the physical is not closed, so one wonders how we would ever perceive universal physical closure? Mar 4 at 17:54

The question of whether these three perspectives can be held consistently is still up for dispute. Philosophers may discern a fundamental incompatibility that necessitates rejecting or changing at least one of these principles, or they may argue that a kind of compatibilism can be established to reconcile them.

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