The Similarity Hypothesis in Metaethics In the introduction to his new book Being Realistic about Reasons T.M. Scanlon writes: "Contemporary metaethics differs in two important ways from the metaethics of the 1950's and 60's and even the later 1970's when John Mackie wrote Ethics: Inventing Right and Wrong. In that earlier period, discussion in metaethics focused almost entirely on morality…. Today, although morality is still much discussed, a significant part of the debate concerns… reasons for action and even more broadly reasons for belief and other attitudes." (2014, 1). Scanlon is capturing a thought that many contemporary metaethicists would both agree with as a sociological description of their subject and endorse. We should approach metaethics with the big picture in mind: a picture that includes not just morality, but 'reasons for belief and other attitudes'. Doing so is interesting in its own right as well as, potentially, a useful means of shedding light on morality itself. There is a sense in which this represents a new 'metanormative' approach to, or method in, metaethics. But what results has it actually had? One interesting example is the widespread use of what I shall refer to as 'The Similarity Hypothesis'. This is the view that practical normativity-what we ought to doand theoretical normativity-what we ought to believe-are similar in metaphysically important respects. It is a view that is increasingly appealed to in arguments in metaethics and beyond. The aim of this paper is to assess the Similarity Hypothesis. I do so by engaging with one of the most interesting arguments against it. The paper proceeds as follows. I'll begin by saying a bit more about the Similarity Hypothesis. I'll then introduce the challenge to it that I'm interested in. The challenge concerns the contrasting natures of the 'aims' of belief and desire (or intention) respectively. I'll briefly rehearse and set to one side familiar attempt to articulate this argument before engaging at length with Stephen Darwall's articulation. This will be my main focus. I'll claim that despite its promise, Darwall's argument leaves the Similarity Hypothesis untroubled. 1. The Similarity Hypothesis According to The Similarity Hypothesis theoretical and practical normativity are, at the metaphysical level, the same kind of thing. There are probably ways of reading this claim on which it is trivially true and ways of reading it on which it is trivially false. But I intend something like this: The Similarity Hypothesis: With respect to the big picture metaphysical questions about their nature and metaphysical grounds, theoretical normativity and practical normativity look similar: for example, if non-natural realism is true about one, then it's true about the other, if expressivism is true about one, then it's true about the other, if one is grounded in our attitudes, or the activity of reasoning, then so is the other. And so on. This view figures prominently in contemporary metaethics, sometimes as an assumption in arguments, sometimes as de...