INTRODUCTION

This is the fourth and final article in a series on how one might offer several different moral arguments that all (1) focus on our knowledge of moral truth and (2) reinforce each other so that their joint force is even stronger than that of each individual argument.[1] This series of articles has focused specifically on three well-known arguments for God’s existence that have to do with our moral knowledge: the similar arguments of Mark Linville and Alvin Plantinga, an argument by Scott Smith, and an argument put forth by Angus Ritchie. In this article, the series concludes by offering a strategy for how these arguments can be used in conjunction so that they reinforce each other.

A CUMULATIVE CASE MORAL KNOWLEDGE ARGUMENT 

Although there are differences between each of the moral knowledge arguments described in this article, there are also many similarities; moreover, the differences are not such that the Christian moral apologist must choose one to the exclusion of the others. Instead, these versions are complementary and can be used in conjunction to make an even stronger moral argument for God’s existence—one that is sturdier than any one of these versions by itself. In this final section, we will consider how these different arguments may be used in combination and explore a strategy for making this work. To accomplish this, it is helpful to begin by comparing and contrasting these views before integrating them into a cumulative argument.

It is clear that Plantinga and Smith make the most ambitious claim, as they argue that there is reason to doubt that we could have any knowledge at all in a naturalistic world. Nevertheless, Plantinga and Smith make their cases entirely differently, since Plantinga argues that the evolutionary aspect of naturalism provides a defeater for moral knowledge and Smith attempts to show that the physicalist aspect of naturalism removes any basis for the personhood and mental states that would be needed for knowledge. Linville’s point is somewhat less bold, arguing that on naturalism we have a defeater for moral knowledge but not other types of knowledge. In doing so, his approach is more similar to Plantinga’s than Smith’s in terms of arguing that the evolutionary aspect of naturalism undermines knowledge; however, unlike both Plantinga and Smith, Linville does not deny that all knowledge is undermined on naturalism. Ritchie’s claim is the least ambitious, since he grants that moral knowledge is possible even on naturalism. Ritchie takes moral knowledge as a fact that requires explanation and makes the more modest claim that naturalism cannot adequately explain moral knowledge. In terms of how he goes about this, we have seen that his approach is more similar to that of Plantinga and Linville than to that of Smith, as Ritchie’s focus is on the evolutionary aspect of naturalism.

With these differences and similarities in mind, let us now consider how to integrate them into a cumulative moral argument. It is reasonable to begin with the most ambitious claims first. The arguments of both Plantinga and Smith lend themselves well to being the first line of argumentation for two reasons. First, if successful, they raise the sharpest challenge against naturalism by providing a defeater for all knowledge on naturalism. The naturalist who sees the force of either—or both—of these arguments thus has reason to doubt whether naturalism can be rationally affirmed; moreover, if the naturalist finds either of these arguments to be sound and also finds it plausible that we do have knowledge in the actual world, then he has good reason to doubt the truth of naturalism. Second, there is the advantage that these two arguments have an entirely different focus. If the moral apologist’s secular interlocuter is unconvinced by one of them, that does not weaken the force of the other; thus, using these two arguments as the first line of argumentation has an initial cumulative benefit.

Linville’s argument can be used next in order to make a more modest case in the event that the unbeliever sees merit in Plantinga’s argument but finds it implausible that all knowledge would be undermined on evolutionary naturalism. As we have seen, Plantinga rightly contends that it is irrelevant to his argument whether we seem to have knowledge in the actual world, and he argues persuasively that there is no reason to think that adaptive behavior requires the truthfulness of any of our beliefs; however, some may find it more plausible that holding true beliefs would provide a survival advantage. It is at this juncture that Linville’s more modest approach can be used, as he differentiates between moral and nonmoral beliefs by allowing that holding true nonmoral beliefs will often provide a survival advantage whereas the truthfulness of moral beliefs does not seem to provide a survival advantage. Linville makes a good case for why it is not arbitrary to exclude moral knowledge on naturalism even if one allows other knowledge.

If the skeptic of theism is still unconvinced and simply finds it unreasonable to doubt that we would have moral knowledge on naturalism, one can utilize Ritchie’s even more modest argument. Even if the defender of our cumulative moral knowledge argument does not agree with Ritchie that moral knowledge is justified on naturalism, one can grant this for the sake of argument and make the case along with Ritchie that naturalism cannot adequately explain how we have this knowledge. The moral apologist can defend Ritchie’s claim that any viable worldview must be able to explain moral knowledge and point out that Ritchie himself even goes so far as to accept that our nonmoral beliefs can be not only justified but explained by natural selection on naturalism. Ritchie’s dilemma can then be posed to the skeptic to show the difficulty of explaining moral knowledge on naturalism. If the skeptic recognizes that there is no evolutionary advantage to some of our moral beliefs, then she should be able to see that naturalism cannot explain those moral beliefs; however, if she claims that all of our moral beliefs do carry an evolutionary advantage then she faces the problem that some actions that are favorable for the survival and effective replication of the human species seem to be immoral (e.g., killing the weak, disabled, and diseased; mandating rape if the only women left on earth refused to procreate).

In this way, all of these arguments are complementary and fit nicely into a cumulative case strategy. For both apologetic and moral reasons, one does not want a naturalist to give up on thinking that there is objective moral truth or to hold onto her naturalism while abandoning her belief that she has genuine moral knowledge. It is therefore critical when making this case to encourage her to give up naturalism and not give up on moral knowledge. The intent of this cumulative moral argument is to show that naturalism cannot adequately justify—or at least explain—moral knowledge. This can in turn be used as part of an even broader cumulative moral argument that seeks to show that naturalism cannot justify other aspects of morality, such as the existence of objective moral values and duties, moral transformation, and moral rationality.

1. These articles are adapted for a popular audience from an article that I published in the Journal of the International Society of Christian Apologetics (Volume 12, 2019, pp. 49-64) that is titled “Toward a Cumulative Epistemic Moral Argument for God’s Existence.”