Some conservative Christian apologists argue that whenever atheists engage in moral reasoning, they are stealing from the Christian worldview. As I understand their argument, the point is supposed to be that only Christianity can metaphysically ground objective morals and values and that this would be obvious if only we were not so blinded by sin (herein: the obviousness thesis). In their unrighteousness, atheists deny Christianity while simultaneously presupposing Christianity in their moral discourse, or so it is claimed. Call this the Presuppositionalist Moral Argument (PMA). In what follows, I show that PMA is implausible because, due to the controversy over Hume’s Is/Ought dichotomy, the obviousness thesis is probably false.
Hume’s is/ought dichotomy is the view that no statement about what ought to be can be derived from a statement about what is the case. While Hume used his dichotomy to argue that moral statements should be understood non-cognitively, many contemporary metaethicists utilize the dichotomy to argue that moral facts constitute an independent domain of facts which cannot be reduced to non-moral facts. Other metaethicists argue that moral facts can be reduced to non-moral facts. At present, there does not seem to be a consensus between the two views and it is difficult to determine which side is correct. Given the debate, I think it reasonable to assign a 50/50 chance as to whether the is/ought dichotomy is true:
P(Hume’s dichotomy is true|debate) ~ P(Hume’s dichotomy is false|debate) ~ 0.5
But now the kicker: Hume’s dichotomy is incompatible with theistic metaethics (as I will show), which entails that morality can be explained by Christianity only if Hume’s dichotomy is false. There are multiple metaethical theories compatible with the conclusion that Hume’s dichotomy is false; thus, the probability of Christian metaethics is less than 0.5:
P(Christian metaethics|debate) < 0.5
Far from being obviously true, Christian metaethics is less likely to be true than the alternative.
Why should we think that theistic metaethics is incompatible with the Hume’s dichotomy? Here I quote from one of my working papers:
Although many classical theists find arguments for moral realism convincing and, from this, conclude that God exists, we argue in the following that the conjunct of moral realism and the is/ought dichotomy entail that theism is false. […]
Let’s suppose that both moral realism and the is/ought dichotomy were true. There are two candidates for a theistic metaethics: first, that morality is reducible to God’s commands or, second, that morality is reducible to God’s nature (for example, perhaps the Good is identical to God’s essence).
There are well-known reasons why it is implausible for moral truths to be reducible to God’s commands. First, this would render moral truths arbitrary due to the fact that God could not possess moral reasons for Her commands (Euthyphro’s Dilemma). Furthermore, given the is/ought dichotomy, it is difficult to see how God could possess non-moral reasons for Her commands, for this would entail that moral facts could be reduced to various non-moral facts (perhaps concerning God’s nature or desires). But it seems equally strange to suppose that God possesses no reasons at all for her commands. Second, reducing moral truth to God’s commands involves reducing moral facts to facts about commands, which violates the is/ought dichotomy.
The second option, that morality is reducible to God’s nature, seems to be the best candidate for a theistic metaethics. However, the second option entails that moral facts are reducible to non-moral facts about God (Greg Bahnsen suggests, for example, that ethical facts are reducible to facts about God’s character); thus, the second option is not a viable candidate for theistic metaethics. Moral facts might be reducible to facts about God’s essence or nature, but unless we identify facts about God’s essence or nature as moral facts, this reduction violates the is/ought dichotomy.
Perhaps the theist will argue that facts about God’s essence or nature are identical to moral facts. For example, some theists have argued that, given Divine Simplicity, God is identical to the property of Goodness, itself, in virtue of which all other moral facts can be explained. However, we find this to be mysterious, if not plainly contradictory. How is it that a specific person can be identified with an abstract property or with a universal?
The remaining option is that the ethical facts, somehow, constitute an independent domain of facts not created by God. Yet, given God’s aseity, everything else that exists is somehow caused or explained by God. […] Given moral realism and the is/ought dichotomy, the moral facts are not caused or explained by God. This is a contradiction. Thus, if moral realism and the is/ought dichotomy are true, theism is not, or so it seems. Perhaps the theist has a way of resolving this difficulty, but whatever that way might be is not obvious.
There is a third difficulty with the notion that ethical absolutes entail theism: theistic metaethics appears to render the order of explanation of moral truths incorrectly. Bahnsen has argued that God’s commands are both good and non-arbitrary because they issue forth from God’s good character, which is the ultimate measure against which all else must be compared. Put aside the difficulties with the is/ought dichotomy. Most theists conceive of God as good because She is virtuous, fair, loving, and so on; Bahnsen’s view entails that virtuousness, fairness, and lovingness are good because God possesses these traits. That’s a reversal of what seems to be the correct order of explanation.
Wes Morrison puts the point this way: “Is God good because he is loving, generous, just, faithful, kind, and so forth? Or are these attributes good-making because God has them?” The first possibility is more plausible than the second. Theists say that God is good in virtue of Her properties. If Bahnsen chooses the second, it is difficult to make sense of the notion that God is good (in virtue of what would God be good?).
Thus, Christians should not say that atheists steal from their worldview; the obviousness thesis fails.
I do not think that this is surprising. Ordinarily, when we engage in philosophical debate, we do not accuse our interlocutors of theft when we think that they made unwarranted assumptions. Instead, we say that our interlocutors have not presented a convincing case (or something similar) for their conclusions. The accusation of theft sounds, to my ears, like little more than bullying for ideological purposes.