Ethical naturalism is the doctrine that moral properties, such as moral goodness, justice, rightness, and wrongness, are among the ‘natural’ properties that things can have. It is the doctrine that moral properties are ‘natural’ and that morality is in this sense an aspect of ‘nature.’ Accordingly, it is a view about the semantics and metaphysics of moral discourse. For example, a utilitarian naturalist might propose that wrongness is the property an action could have of being such as to undermine overall happiness, where happiness is taken to be a psychological property. Unfortunately, it is unclear what the naturalist means by a ‘natural’ property. For my purposes in this chapter, I shall assume that natural properties are such that our knowledge of them is fundamentally empirical, grounded in observation. More precisely, a property is ‘natural’ just in case any synthetic proposition about its instantiation can be known only a posteriori, or with the aid of experience. Ethical naturalism is, in short, the doctrine that there are moral properties and that they are natural properties. It implies that moral knowledge is fundamentally empirical. It is committed to a broadly ‘empiricist’ moral epistemology.

This chapter springs from the fact that certain unsurprising commonsense first-personal observations about our moral thinking can appear to undermine ethical naturalism by undermining the plausibility of the idea that our moral knowledge is empirical.

Let “moral naturalism” be the view that (1) the central semantic function of moral terms such as “right” and “wrong” is to ascribe moral properties, properties such as rightness and wrongness, and that (2) each of these properties is a ‘natural’ property. Let “synthetic moral naturalism” be the view, in addition, that (3) each moral property could, at least in principle, be ascribed by a predicate couched in nonmoral naturalistic vocabulary, where (4) the corresponding identity claim is synthetic. That is, where “M” is a moral predicate that ascribes a moral property Mness, and “N” is a distinct (perhaps very complex) predicate couched in nonmoral vocabulary that ascribes a natural property Nness, if a version of synthetic naturalism implies that Mness is identical to Nness, then it also implies that this is not a conceptual truth.

A fully developed version of synthetic moral naturalism would include a semantics for moral predicates. It would explain what it is that determines which specific natural property is ascribed by a given moral predicate. To avoid being objectionably ad hoc, moreover, synthetic moral naturalism must take on board a general semantic theory of the relation between predicates and properties and apply this theory to the case of moral predicates. Taken together with relevant nonsemantic facts, the resulting moral semantics should determine (nontrivially) which natural property it is that a given moral predicate is used to ascribe in standard literal assertoric uses.

Terence Horgan and Mark Timmons have argued, however, that this strategy cannot work.

Is moral naturalism ruled out by the fact that morality is normative? I want to consider this question in a systematic way, explaining the central thesis of moral naturalism as I understand it, and then clarifying the idea of normativity. The chief point that I want to make is that the issue raised by this question is much more complex than it might seem to be, mainly because of complexity in what philosophers have in mind when they speak of morality as “normative.” There are several dimensions to the complexity, but the dimension I will stress is that philosophers disagree about what we might call the ‘stringency’ of moral normativity. I will distinguish three ‘grades of normativity.’ I will investigate, for each of these grades, the plausibility of the idea that morality has that grade of normativity, and the viability of naturalistic accounts of it. My conclusion will be tentative, but it will be optimistic for moral naturalism.

Moral naturalism holds that in thinking of things as morally right or wrong, good or bad, we ascribe moral properties to these things – properties such as moral rightness and wrongness, goodness and evil. It holds that there are such properties, and it adds that these properties are ordinary garden-variety natural properties – properties that have the same basic metaphysical and epistemological status as the properties a tree can have of being deciduous, and the property a piece of paper can have of being an Australian twenty-dollar bill.

Our thoughts about our moral thinking are Janus-faced. On the one hand, we intuitively and pre-theoretically think as moral ‘realists’ – we take our moral convictions to be beliefs in just the way that our convictions about the weather are beliefs, and of course we take our convictions to be true. Indeed, we take some of them to be self-evidently true. On the other hand, we find ourselves facing intuitively significant challenges that can make moral realism seem problematic or even completely implausible. Ordinary reflection tells us that our moral convictions are different in nature from most other beliefs, such as our beliefs about the weather. Moral judgments are directly relevant to decisions and choices in a way that differs from the way that beliefs about the weather might be relevant to decisions and choices. Intuitively, moreover, a moral judgment speaks to what ‘ought to be the case’ rather than to what ‘is the case.’ We can introduce a term to talk about this. We can say that, unlike judgments about the likelihood of rainfall or the like, moral judgments are ‘normative.’ Unfortunately, however, it can easily seem dubious that there could be something in the world as it is that makes true a judgment about what ought to be the case. There is an obvious tension between these two sides to our thinking.