In this chapter, Baggett and Walls motivate using an abductive moral argument over a deductive moral argument. They first review what they call the Anti-Platonist Moral Argument (APMA), such as the one proposed by William Lane Craig:
P1. If God does not exist, then objective moral values and duties do not exist. P2. Objective moral values and duties do exist. C3. Therefore, God exists
To prevent ruling out theistic ethics, Baggett and Walls take Craig's definition of objective moral (deontic) truths to be facts according to which some actions and motivations are right or wrong independently of whether any human mind believes it to be so.
As previously mentioned in Good God, some theists have reservations about the argument. C. Stephen Layman, a theist himself, rejects APMA because of P1 which he thinks is unappealing to nonbelievers, especially Platonists. If Platonism is true, then P1 is false, because one can appeal to a possible world where God does not exist but objective moral value and duties do (since moral values and duties exist necessarily on their view). One may counter that God’s existence is necessary, so there is no such world and Platonism is false. However to do so would be to appeal to C3 which would be circular. Hence, Layman thinks that P1 lacks non-circular justification, wide enough support, adequate intuitive force, and sufficient obviousness.
John Milliken, another theist, similarly rejects P1. He imagines a world such us ours without God. He thinks intuitively that, in such a world, morality still holds. In more philosophical terms, he takes P1 to be a nontrivially false counterpossible (since he is committed to Divine necessity).
In Good God, Baggett and Walls have also previously offered their criticisms, and in John Hare's (another theist) review of Good God, Hare says that Baggett and Walls have argued convincingly that Craig's view that atheism leads to moral nihilism is unlikely to be persuasive. Note that Baggett and Walls do not think Craig's argument is bad or unsound, rather that it is relatively unpersuasive to many atheists for a few reasons. Hence they propose one should adopt their abductive moral argument instead.
For the classical theist, a world such as ours could not even exist without God, while for the atheist, the world is possible without God. Hence a world such as ours, with at least the appearance of love, relationships, satisfactions of morality, social harmony, clear moral apprehensions, etc., is possible without God for the atheist. Baggett and Walls think that it is better to approach atheists by affirming their common convictions about moral truth and then asking what better explains such facts, rather than encouraging them to assume such a world like the actual world is consistent with atheism, providing them a lot of theoretical resources to use, and inviting them to construct a secular moral theory. It would be strange if atheists could not come up with a substantive moral theory using the rich resources of a world like ours, which is only here (if theists are right) because God created it with such features. This allows atheists to travel some distance down the road in building an ethical theory. Hence Baggett and Walls prefer an abductive moral argument that does not rely on P1.
Craig however thinks that his formulation has the advantage of meeting the atheist in the world as he conceives it to be and asking whether morality would be objective without God. The problem however is that while one allows such a world with such rich resources to be consistent with atheism, one (potentially anyway) dismisses too hastily the atheists' serious efforts to build a secular ethic. This explains why so many secular theories have emerged and sport considerable merit. It is not as if secular theories fail altogether to explain anything morally. They can get somewhere given the resources of the world. Baggett and Walls think however that this world conjoined with God provides a better explanation of the full range of moral facts.
Other theistic ethicists seem to recognize this. Robert Adams, for example, thought social requirement theory had its strengths in explaining the idea that obligations are owed to persons. Human social requirement theory is not without resources to make sense of this, but adding God is crucial to complete the theory. Another example is Linda Zagzebski who grounds morality in motivations and admits that the first half of her theory can be constructed without reference to God at all. She then completes her theory by bringing God in.
Craig however thinks that when it gets down to showing that the best explanation of objective moral values and duties is God, one will slip into arguing that, given atheism, objective moral values and duties would not exist. So the abductive argument still ends up doing the same thing as the deductive argument. Contra Craig, Baggett and Walls think that they are doing something different. The deductive argument says, "Imagine the world is atheistic, now try to make sense of morality, you can't." The abductive argument instead says, "Suspend belief on whether the world is atheistic or theistic, try to make sense of morality. Given its features, you can make some progress. But what better explains the fuller range of moral facts in need of explanation, the world alone or the world and God?”
In short, they list five main problems in total with the deductive argument (TDA). In a footnote, they construct an Acrostic called CARBS:
1. Counterpossibles. Counterpossibles are counterfactuals with an impossible antecedent. For example, "If a necessarily good and loving God commanded murder and torture for fun, it would be right to do so," or "If there were a square circle, mathematicians would be puzzled." For the classical theist, if God exists necessarily, then "God does not exist" would be impossible. Hence P1 is a counterpossible (according to classical theists), and a particularly intractable counterpossible—one in which the being presumed to be the very ground of being doesn’t exist. Presuming to know the features of such an intractably impossible world strains credulity. (Note that “If God doesn’t exist, then God didn’t create the world,” doesn’t seem particularly problematic, but it’s also analytic.)
2. Acknowledging the rich features of a world like this if it could exist without God. The deductive method doesn't allow enough room to acknowledge what would be the simply amazing features of a world like this if it could exist without God, whereas the abductive approach allows the world without God to explain some of morality, while providing the explanation for why it can.
3. Rejecting realism instead of naturalism. By allowing the atheist to think this world with its features is compatible with atheism, it is easy for them to reject moral realism instead of naturalism, contributing to the escalation of nihilism Nietzsche predicted would ensue from the “death of God.” The abductive method instead keeps the moral facts in question front and center as the starting data in need of explanation.
4. Bridge-breaking. The deductive version can sever the bridge with naturalists by focusing on differences rather than similarities. The abductive argument agrees that the world can account for some of the moral data to a certain extent, and then shows how adding God to the picture offers a considerably more robust explanation.
5. Saying uncomfortable things. The deductive version makes us say very uncomfortable, unintuitive, and unnecessary things like "If God does not exist, then rape is not wrong." The abductive version avoids this.