¹ This is the contrapositive of OIC.

² OIC is one of many ‘ought-implies-can’ principles, each of which is a variation on a logical theme, taking some sense of ‘ought’ and some sense of ‘can’ and saying for those senses that one ought to φ only if one can φ. The ‘ought’ in OIC is that of moral obligation. I have deliberately left the sense of ‘can’ unspecified. It won't matter for my purposes, since everything I want to say in this article will apply to OIC for any interpretation of ‘can’.

³ Though there is some controversy about whether Kant should be interpreted as subscribing to it. See Stern, Robert, ‘Does “Ought” Imply “Can”? And Did Kant Think It Does?’, Utilitas 16 (2004), pp. 42–61CrossRefGoogle Scholar.

⁴ See, in particular, the essays collected in Gowans, Christopher, Moral Dilemmas (New York, 1987)Google Scholar and Mason, H. E., Moral Dilemmas and Moral Theory (Oxford, 1996)Google Scholar.

⁵ See Mason, Elinor, ‘Consequentialism and the “Ought Implies Can” Principle’, American Philosophical Quarterly 40 (2003), pp. 319–331Google Scholar, and Andric, Vuko, ‘Objective Consequentialism and the Rationales of “Ought” Implies “Can”’, Ratio 29 (2015), pp. 1–16Google Scholar.

⁶ See Graham, Peter, ‘“Ought” and Ability’, Philosophical Review 120 (2011), pp. 337–82CrossRefGoogle Scholar; Mason, ‘Consequentialism’; Andric, ‘Objective Consequentialism’.

⁷ See Haji, Ishitiyaque, Moral Appraisability: Puzzles, Proposals, and Perplexities (New York, 1998)Google Scholar; Haji, Ishitiyaque, ‘Moral Anchors and Control’, Canadian Journal of Philosophy 29 (1999), pp. 175–203CrossRefGoogle Scholar; Haji, Ishitiyaque, Deontic Morality and Control (New York, 2002)CrossRefGoogle Scholar.

⁸ See van Inwagen, Peter, An Essay on Free Will (Oxford, 1983)Google Scholar; Widerker, David, ‘Frankfurt on “Ought Implies Can” and Alternative Possibilities’, Analysis 51 (1991), pp. 222–4CrossRefGoogle Scholar; Copp, David, ‘Defending the Principle of Alternate Possibilities: Blameworthiness and Moral Responsibility’, Nous 31 (1997), pp. 441–56CrossRefGoogle Scholar; Copp, David, ‘“Ought” Implies “Can”, Blameworthiness, and the Principle of Alternate Possibilities’, Moral Responsibility and Alternative Possibilities: Essays on the Importance of Alternative Possibilities, ed. McKenna, Michael and Widerker, David (Burlington, VT, 2003), pp. 265–99Google Scholar.

⁹ For a good overview of the broader debate, see Vranas, Peter, ‘I Ought, Therefore I Can’, Philosophical Studies 136.3 (2007), pp. 351–84CrossRefGoogle Scholar.

¹⁰ My view is that OIC is false, and so every argument employing it as a premise is unsound. But I won't argue for that claim here. My ambition is only to show that the guidance argument in particular is a failure. This leaves open the possibility that the principle may be established on other grounds (though as I said, I'm sceptical).

¹¹ See amongst others: Hare, R. M., Reason and Freedom (Oxford, 1963)Google Scholar; Driver, Julia, ‘Promises, Obligations, and Abilities’, Philosophical Studies 44 (1983), pp. 221–3CrossRefGoogle Scholar; Smith, Holly, ‘Moral Realism, Moral Conflict, and Compound Acts’, Journal of Philosophy 83.6 (1986), pp. 341–5CrossRefGoogle Scholar; Williams, Bernard, ‘Internal Reasons and the Obscurity of Blame’, Making Sense of Humanity (Cambridge, 1998)Google Scholar; Griffin, James, ‘The Human Good and the Ambitions of Consequentialism’, The Good Life and the Human Good, ed. Paul, E. F., Miller, F. D. and Paul, Jeffrey (Cambridge, 1992)Google Scholar, pp. 118–32; Copp ‘“Ought” Implies “Can”’; Andric, ‘Objective Consequentialism’.

¹² Usually the argument is not laid out precisely, step by step, but instead put forward in a more casual and offhand way. This is unfortunate, as it can make it difficult to see exactly how it is supposed to go. In ‘Guidance, Obligations, and Ability: A Close Look at the Action Guidance Argument for Ought-Implies-Can’, Utilitas (forthcoming), I argue that if it is to have any plausibility at all the argument must be understood as taking the form of (1)–(3).

¹³ Copp, ‘“Ought” Implies “Can”’, p. 273.

¹⁴ Provided you know that you cannot φ, at least.

¹⁵ Sartre, Jean-Paul, Existentialism and Humanism (London, 1948)Google Scholar. Sartre's response wasn't ‘both’, by the way. It was that no moral theory can help the student; he must simply choose. Perhaps he wouldn't have accepted the guidance argument, then.

¹⁶ A referee has suggested that we should also reject premise (2) of the argument. If you are unable to φ, but don't know it, couldn't an obligation to φ guide you in trying to φ? Perhaps. But those who endorse the guidance argument may maintain that deliberating about whether or not to try to φ is not the same thing as deliberating about whether or not to φ. I might know that I can't pick up a large boulder, but nevertheless decide to try to, because by trying and failing I will thereby demonstrate to my friend that it can't be done (see Hornsby, Jennifer, ‘Reasons for Trying’, Journal of Philosophical Research 20 (1995), pp. 525–39)CrossRefGoogle Scholar. I've certainly deliberated about whether or not to try to pick up the boulder in this case, but have I deliberated about whether or not to pick up the boulder simpliciter? One reason to think not is that it would be irrational for me to deliberate about whether or not to pick it up, since I know that I can't, but it isn't irrational for me to deliberate about whether or not to try.

¹⁷ Some readers might think that one cannot be obligated to φ but not in a position to know about it. If you are one of them, I ask you to hold that thought. I will address it in the next section.

¹⁸ Smith, Holly, ‘Using Moral Principles to Guide Decisions’, Philosophical Issues 22 (2012), pp. 369–86CrossRefGoogle Scholar. I don't mean to suggest that conforming to Levitical law is actually morally obligatory (nor, I assume, does Smith). It merely provides a useful example for the point I want to make.

¹⁹ We are interested here (and later) in the obligation to give the patient an injection of morphine on this occasion, rather than the more general conditional obligation to give the patient an injection of morphine if he's in extreme pain. The conditional obligation should be read as having narrow scope (that is, as having the logical form A → OB, rather than O(A → B), where ‘A’ = the patient is in extreme pain, ‘B’ = the patient is given an injection of morphine and ‘O’ = it is obligated that), otherwise we cannot detach the obligation to give the patient an injection of morphine on this occasion from the conditional obligation. Thanks to a referee for pointing this out.

²⁰ I'm not saying anything new here. The observation that you can only be guided by an obligation to φ if you know about it has been made many times before. See amongst others: Hudson, James, ‘Subjectivization in Ethics’, American Philosophical Quarterly 26.3 (1989), pp. 221–9Google Scholar; Gibbard, Allen, Wise Choices, Apt Feelings (Oxford, 1990)Google Scholar; Jackson, Frank, ‘Decision-Theoretic Consequentialism and the Nearest and Dearest Objection’, Ethics 101(1991), pp. 461–82CrossRefGoogle Scholar.

²¹ Doubtless there are other conditions which must also be satisfied, but they won't matter for my argument.

²² The class of trivial conditions is comprised mainly of those that obtain in all or no possible worlds. Clearly there are non-trivial moral obligations in this sense. Asha may be obligated to inject her patient with morphine in this world, because the patient is in extreme pain, but there is a possible world in which Asha has no such obligation, because the patient isn't in pain at all. There may be trivial conditions that don't obtain in all or no possible worlds – the condition that one exists (under a first-personal mode of presentation), for instance – but as Williamson points out, if such conditions do exist they are curiosities at best. They won't affect my argument.

²³ Srinivasan, Amia, ‘Normativity without Cartesian Privilege’, Philosophical Issues 25.1 (2015), pp. 273–99CrossRefGoogle Scholar, also draws consequences for ethical theory from Williamson's anti-luminosity argument. Her goal is to show that if no non-trivial condition is luminous, then we must accept a tragic view of the normative realm, according to which it is ineliminably suffused with luck.

²⁴ Adapted from Williamson, Timothy, Knowledge and its Limits (Oxford, 2000)Google Scholar.

²⁵ My presentation of the argument here draws on Berker, Selim, ‘Luminosity Regained’, Philosophers’ Imprint 8.2 (2008), pp. 1–22Google Scholar, and Srinivasan, Amia, ‘Are We Luminous?’, Philosophy and Phenomenological Research 90.2 (2013), pp. 294–319CrossRefGoogle Scholar.

²⁶ To see why, notice that if C obtains in t⁰⁺¹ then by (LUM) K(C) obtains at t⁰⁺¹. And if K(C) obtains at t⁰⁺¹ then by (MAR) C obtains at t⁰⁺². And if C obtains in t⁰⁺², then by (LUM) K(C) obtains at t⁰⁺². And if K(C) obtains at t⁰⁺² then by (MAR) C obtains at t⁰⁺³ . . . and so on all the way to tⁿ.

²⁷ If you don't find this argument convincing, please hold that thought. I will address it in the next section.

²⁸ Again, the conditional obligation should be read as having narrow scope here, and our interest is in the detached obligation to give the patient an injection of morphine on a particular occasion (see n. 17).

²⁹ For instance: Hudson, ‘Subjectivization in Ethics’; Gibbard, Wise Choices, Apt Feelings; Jackson, ‘Decision-Theoretic Consequentialism’.

³⁰ It occurs to me that this argument might seem to rely on a sleight of hand. I've argued that there is one way in which you can fail to be guided by a would-be obligation – by not knowing about it – and I have used this point to argue that for a different way in which you can fail to be guided – by not being able to fulfil the would-be obligation – the fact of this failure does not undermine the possibility that the obligation is nevertheless a genuine one. Is this a legitimate form of argument? Indeed it is. If you are in a situation in which you already can't be guided by an obligation, because you don't know about it, then there is simply no motivation for insisting that you must be able to fulfil the obligation because otherwise you can't be guided by it. It's too late. You can't be guided anyway. That ship has already sailed.

³¹ See amongst others: Neta, Ram and Rohrbaugh, Guy, ‘Luminosity and the Safety of Knowledge’, Pacific Philosophical Quarterly 85 (2004), pp. 296–406CrossRefGoogle Scholar, and Conee, Earl, ‘The Comforts of Home’, Philosophy and Phenomenological Research 70 (2005), pp. 444–51CrossRefGoogle Scholar.

³² Weatherson, Brian, ‘Luminous Margins’, Australasian Journal of Philosophy 82 (2004), pp. 373–83CrossRefGoogle Scholar; Vogel, Jonathan, ‘Luminosity and Indiscriminability’, Philosophical Perspectives 24.1 (2010), pp. 547–72CrossRefGoogle Scholar.

³³ Wong, Wai-hung, ‘What Williamson's Anti-Luminosity Argument Really Is’, Pacific Philosophical Quarterly 89 (2008), pp. 536–43CrossRefGoogle Scholar; Berker, ‘Luminosity Regained’. A tolerance principle is a principle of the form ‘If C obtains in case n, then C obtains in case n ^±1’. It is by relying on the apparent plausibility of some tolerance principles that soritical arguments create a paradox.

³⁴ Srinivasan, ‘Are We Luminous?’.

³⁵ How pervasive is the problem? If, as a referee has suggested, we say that all obligations except the ones that depend on vague predicates are action guiding, will that help? It won't, for two reasons. First, as Williamson himself points out, the anti-luminosity argument doesn't only apply to conditions articulated with vague predicates, it applies equally to conditions articulated with sharp predicates (Williamson, Knowledge and its Limits). Second, even if that wasn't true, the most that those who put forward the guidance argument would be able to conclude, on this construal of it, is that OIC holds in cases where the obligation doesn't depend on vague predicates. But if so, it isn't fit to play the roles in moral theory described in section I, which depend on its holding in every case.

³⁶ This isn't just true of obligations. In general, in order for my φ-ing to have been guided by some fact P, P must be amongst my motivating reasons for φ-ing. For instance, for my decision to take an umbrella to have been guided by the fact that it's raining, the fact that it's raining must be amongst my motivating reasons for deciding to take an umbrella.

³⁷ It is important to pay attention to the fact that we're talking about motivating reasons. When explaining a person's φ-ing, we can cite reasons that are not amongst their motivating reasons. For instance, the fact that Bill has a brain tumour may be a reason why he just lashed out (the tumour has changed his personality and made him prone to violence). It is an explanatory reason, but it need not have been amongst his motivating reasons, nor need it have guided his decision to φ – Bill may be totally unaware of the tumour. A referee has suggested that in cases in which a person is guided to φ by an obligation to φ, we can give a full explanation of their action whilst appealing only to their psychological states (beliefs, desires, etc.). I disagree. In appealing only to their psychological states, the explanation will miss out the fact that they φ’ed not (merely) because they believed that they were obligated to, but because they were in fact obligated to.

³⁸ This claim is defended by: Unger, Peter, Ignorance (Oxford, 1975)Google Scholar; Hyman, John, ‘How Knowledge Works’, Philosophical Quarterly 49.197 (1999), pp. 433–51CrossRefGoogle Scholar; Hyman, John, ‘Knowledge and Evidence’, Mind 115.460 (2006), pp. 891–916CrossRefGoogle Scholar; Hyman, John, Action, Knowledge, and Will (Oxford, 2015)CrossRefGoogle Scholar; Williamson, Knowledge and its Limits; Hornsby, Jennifer, ‘Knowledge in Action’, Action in Context, ed. Leist, A. (Berlin, 2007), pp. 285–302;Google Scholar Littlejohn, Clayton, ‘The Russellian Retreat’, Proceedings of the Aristotelian Society 113.3 (2013), pp. 293–320CrossRefGoogle Scholar. I argued against it in Hughes, Nick, ‘Is Knowledge the Ability to φ for the Reason that P?’, Episteme 4.11 (2014), pp. 457–62CrossRefGoogle Scholar (see also Locke, Dustin, ‘Knowledge, Explanation, and Motivating Reasons’, American Philosophical Quarterly 52.3 (2015), pp. 215–32)Google Scholar. I have since changed my mind.

³⁹ Hyman, ‘How Knowledge Works’.

⁴⁰ As a referee has pointed out, this stands in need of further clarification. Asha's belief isn't her motivating reason if by this we mean that she treats the fact that she believes that Nadal has lost to be a reason to write him a letter. After all, it is not as though she'll think to herself: ‘I believe that Nadal has lost the match, and I want to write him a letter if he has lost, so I'll write him a letter’. Rather, she'll just think ‘Nadal has lost the match, and I want to write him a letter if he's lost, so I'll write him a letter’. So, what is her motivating reason then? One popular answer is that it is the content of her belief, which she takes to be a fact, rather than the fact that she believes it (see, amongst others, Dancy, Jonathan, Practical Reality (Oxford, 2000)Google Scholar; Hornsby, Jennifer, ‘A Disjunctive Conception of Acting for Reasons’, Disjunctivism: Perception, Action, Knowledge, ed. Haddock, Adrian and MacPherson, Fiona (Oxford, 2008), pp. 244–62;CrossRefGoogle Scholar Schroeder, Mark, ‘Having Reasons’, Philosophical Studies 139 (2008), pp. 57–71)CrossRefGoogle Scholar. Another, more radical, answer is that she had no motivating reason at all, only the illusion of one (Alvarez, Maria, Kinds of Reasons: An Essay on the Philosophy of Action (Oxford, 2010)CrossRefGoogle Scholar. I don't find either of these responses fully satisfying, but going into the issue further here would take us too far afield. Hereafter I will continue to speak of her belief being her motivation, but only for the sake of convenience; nothing much should be read into it.

⁴¹ Once again, the conditional obligation for Asha to pay up if she's lost a bet should be read as having narrow scope, and our interest is in the detached obligation (see n. 17).

⁴² A referee has suggested that you might be indirectly guided by an obligation to φ even if you don't know that you're obligated to φ. You might worry that, for all you know, you are subject to the obligation, and think to yourself ‘better safe than sorry’. Has the obligation thereby (indirectly) guided your decision to φ? My answer won't come as a surprise: no, it is at most your belief or knowledge that you might be subject to the obligation that has guided you here, not the obligation itself. Compare: for all I know China will top the medal table at the next Olympics. This fact about my epistemic position may guide my behaviour by, say, leading me not to accept certain bets against it happening (better safe than sorry). But it surely cannot be the case that the fact that China will top the medal table has guided my decision. For one thing, on some theories of facts, there is at present simply no such fact. But even if there is, it hasn't guided me, even indirectly. If it had, then supposing that China does indeed top the medal table, in 2020 I will be able to claim legitimately that the fact that they did (or would) was among my reasons for not betting against them in 2017. But that would be a ludicrous claim.

⁴³ For example: Davidson, Donald, ‘Actions, Reasons, and Causes’, Journal of Philosophy 60.23 (1963), pp. 685–700CrossRefGoogle Scholar, and Smith, Michael, The Moral Problem (Oxford, 1994)Google Scholar. Thanks to an anonymous referee for pointing this out, and for encouraging me to think about how psychologism and the guidance argument interact.

⁴⁴ Dancy, in Practical Reality, is one of many to press this objection against psychologism.

⁴⁵ Might those who go in for the guidance argument somehow revise it in order to avoid this problem whilst nevertheless maintaining that you don't need to know to be guided? I find it hard to see how they could. But at any rate, even if it can be done, it is incumbent on them to show how.

⁴⁶ It is difficult to briefly summarize Williamson's argument for these claims, and I won't attempt to do so here. The reader should consult Williamson, Timothy, ‘Very Improbable Knowing’, Erkenntnis 79.5 (2014), pp. 971–99CrossRefGoogle Scholar.

⁴⁷ John Hawthorne and Amia Srinivasan, ‘Disagreement without Transparency: Some Thoughts, Bleak’, The Epistemology of Disagreement, ed. Christensen, David and Lackey, Jennifer (Oxford, 2013), pp. 9–30Google Scholar.

⁴⁸ I should note that this second reply doesn't apply to the claim that one can be guided by an obligation if one believes that one is subject to it, since the Williamsonian argument cannot be extended to show that one can be subject to an obligation even though one is not in a position to believe that one is subject to it. I am content to rely on my first reply to that claim here.

⁴⁹ It is also questionable whether moral theories which accept OIC actually do better when it comes to giving guidance than those which reject it. See Talbot, Brian, ‘The Best Argument for “Ought Implies Can” Is a Better Argument against “Ought Implies Can”’, Ergo 3.14 (2016), pp. 377–402Google Scholar, for an argument to this effect.

⁵⁰ See, amongst others, Williams, Bernard, ‘Ethical Consistency’, Proceedings of the Aristotelian Society, Supplementary Volumes 39 (1965), pp. 103–38CrossRefGoogle Scholar; van Frassen, Bas, ‘Values and the Heart's Command’, Journal of Philosophy 70.1 (1973), pp. 5–19CrossRefGoogle Scholar; Marcus, Ruth Barcan, ‘Moral Dilemmas and Consistency’, Journal of Philosophy 77.3 (1980), pp. 121–36CrossRefGoogle Scholar; Nussbaum, Martha, The Fragility of Goodness (Cambridge, 1986)Google Scholar.

⁵¹ I should note though that there is a way of reconciling OIC with the possibility of tragic dilemmas: by rejecting the agglomeration principle, according to which if one ought to φ and one ought to ψ, then one ought to perform the complex action φ & ψ (see Williams, ‘Ethical Consistency’, and Hughes, ‘Dilemmic Epistemology’, Synthese (forthcoming). But taking this approach comes with costs of its own. OIC is at least in tension with the idea that there can be tragic dilemmas, then.

⁵² See, amongst others, Kekes, John, ‘Ought Implies Can and Two Kinds of Morality’, The Philosophical Quarterly 34.137 (1984), pp. 459–67CrossRefGoogle Scholar.

⁵³ Here I have given two examples of problems for OIC, but more could be given.

⁵⁴ Thanks to John Hyman, Jennifer Hornsby, an audience at University College Dublin and two anonymous referees for this journal, for helpful discussion and feedback.