One common objection to moral arguments for the existence of God is not that God’s existence would fail to secure objective moral values and obligations if God existed, but that God’s existence is irrelevant because there just are no objective moral values and obligations to be secured in the first place. Morality is the product of evolution, and that is all (or so they say). Take that, you sentimentalist, you.
First, let’s define some terms.
Objective means mind-independent and hence not contingent upon our thinking about it. For example, the squirrel Robert outside my window is an objective reality (though his name is my invention): he would exist even if I thought about or looked at something else. Further, Robert is the original cause of my thinking about him and not the other way around.
Conversely, if we say something is subjective, we mean it is rooted in the person (subject) only, like a personal preference. For example, I, Pat Flynn, prefer unsweetened iced tea on hot summer days to diet Tang, though I would not say anybody who does not share this preference is wrong in the sense that somebody who denies the squirrel outside my window is wrong.
Finally, moral value refers to something’s metaphysical goodness or worth (like when we say every human life is infinitely valuable), whereas moral obligation relates to prohibitions or prescriptions regarding behaviors pertaining to rational agents that can set themselves on courses of action that promote, or frustrate, their attainment of the objectively good life.
Strategically speaking, I believe that the first thing a person should point out before attempting to rebut an evolutionary debunking objection (as they are often called) is the costs the critic assumes in promoting it, of which there are two. The first is the acceptance of not just moral relativism, but nihilism. To those denying objective morality, we ask: is the skeptic willing to embrace the repugnant conclusion that there is nothing really and truly wrong with acts of rape or racism or pollution for fun and profit and that all condemnations of such behavior reduce to nothing but differences of personal preference?
If you make this argument, you’re equally implying that moral progress is impossible. Moral change? Sure, that could happen—but without an objective standard, there is no sense to be made of progress in the moral life, either individually or collectively.
Perhaps the critic in evading theism is willing to acknowledge these costs and assume them, to reduce all morality down to the realm of opinion, with no possible way to adjudicate by means of an external measuring stick between the moral positions of one person (or society) and another. Perhaps he is willing to accept that, but perhaps not, and so it is worth mentioning.
The second cost is the guarantee of an eventual if not immediate performance contradiction—that is, of the skeptic saying one thing but acting contrariwise. To offer just one example of this, the other week I received correspondence from somebody who in their opening paragraphs explained to me that all moral sentiments and evaluations are nothing but fantastic projections fashioned by evolutionary pressures for the purpose of getting us to make love and avoid bears, and then, in their closing paragraphs, relayed how thankful they are that we are no longer collectively burdened by Catholic morality since our ethical systems these days are far better and more moral than whatever the cobwebbed Magisterium has to offer. So, in one breath, all talk of anything being objectively better (or worse) is meaningless babble, corresponding to nothing beyond our personal or collective fantasies, and in another breath, something is objectively better, morally speaking. Which is it?
Granted, a contradiction in performance is not necessarily a contradiction in logic. Fortunately, there is more to say about these evolutionary arguments against our sense of objective morality, including their being question-begging. For one thing, we have presumably evolved to discover rather than invent many aspects of everyday experience—for example, mathematical truths and Robert the squirrel and the fact that there are no unmarried bachelors on Mars (or anywhere, because contradictions cannot exist). We have powers—sense windows, if you will—that bring us into contact with a world beyond our head. Most of us take these powers to be generally reliable even if they are not infallible. That is, we tend to sense and infer things that are really there, even though we admit that illusions and deception and mistakes intermittently creep in. From our power of sight to our power of reasoning, we believe that these powers put us into contact—again, for the most part, anyway, if not always—with states of affairs not dependent upon our conceptions of them.
Why, then, we should wonder that just because some power of ours has an evolutionary history, we should declare it delusory—that is, a power that projects instead of discovers, invents instead of detects, despite it presenting precisely the opposite impression upon us? If theism is true, it seems reasonable, if not expected, that we could have evolved a power to detect a moral realm just as we evolved powers to detect other aspects of reality, like sight in relation to squirrels. And just as being comes in many ways, so does knowledge; thus, we should not expect to know all things in just the same way. Furthermore, to suggest morality is different simply because it is not something we can examine under a microscope would be equally question-begging, and in favor of not just naturalism, but scientism, which, so far as crummy, self-defeating epistemologies go, is king of the scrap heap.
In short, just because something helps us to survive does not preclude that same something telling us about reality as it is. Our sense of sight helps us to survive, and it also tells us about something objectively real: that there is a squirrel out front. It is frequently useful and true (corresponds to what is). Why assume that our moral sense is not similarly evolved—namely, that it comes with a host of survival and reproductive benefits, but also helps us to discern what the good life truly is, given the sorts of creatures we are? In other words, things can have an evolutionary history and still be windows into the world, and it is not enough to tell any just-so evolutionary origin story of something (senses of sight, reason, morality, etc.) to bear doubt upon the reliability of it. We would need further considerations to justify the latter inference, and apart from assuming, rather than proving, naturalism in addition to evolutionary theory, the evolutionary account is for the most part irrelevant and cannot decide the issue either way.
What is more, as soon as someone starts admitting that evolutionary forces could have given us unreliable but useful sense powers or cognitive states, he is going to have a difficult time preventing that line of argument from dealing skeptical destruction to all human powers of sense and probably reason. For few arguments could be given from an evolutionary origins standpoint that cast doubt on the reliability of our moral power that would not also cast doubt on the reliability of everything else—and if our beliefs-forming mechanisms are generally unreliable (that is, something aimed at not truth, but survival), and especially if false beliefs can just as well help a person survive as true beliefs, then we should be doubtful of the truth of any of our beliefs, including that belief. Self-defeat is knocking at the skeptic’s door. Neither will it do for the skeptic to say we can at least test our other senses or powers (say, by touching and feeling things), because all such tests already assume the reliability of the senses in question, in which case the skeptic will be arguing in a circle. What’s more, our sense powers must be generally reliable to make a case for evolutionary theory in the first place—so, yeah.
Our powers of sense are something of a package deal: begin to cast doubt on the (general) reliability of any of them, and probably you ruin most if not all of them, in which case, say goodbye to any legitimate basis for science, reasoning, truth, and especially arguments against the existence of God.
But we are not done yet. There is another self-defeat issue for the skeptic, since to say that nothing has any objective value applies to statements and opinions as well. But if a statement or opinion—including the statement that nothing has any objective value—has no value, then why adopt it? Naturally, the reason anybody would adopt that statement or opinion is because 1) he believes that it is true and 2) he believes that it is objectively better (for whatever reason) to believe true and not false things. Further, if there are no objective moral obligations, then why should anybody accept such a statement even if it is true, particularly if they prefer otherwise? Again, what response could be forthcoming except that we ought to believe true and not false things, regardless of how we feel about them?
Clearly, there is no way to sustain such an assertion without it becoming pointless unless the skeptic retracts his claim of there being no objective moral values or obligations to begin with. One way or another, the skeptic puts himself into a series of intellectual knots that cannot be undone from within his skeptical position. You’ll have to forsake skepticism to remove yourself from this mess. And that is precisely what you should (moral language!) do.
Once we see that these evolutionary debunking arguments fail, what other reasons do we have for denying that morality is objective apart from assuming a worldview (say, physicalism) that strictly precludes it? The answer is hard to find. Arguments from moral disagreement are weak: we disagree about many things we believe can be adjudicated by some objective standard, even if those adjudications are difficult. Plus one does not need to have perfect sight of everything (whether every situation is right or wrong) to have clear sight of some things (that at least some situations are wrong). Combine that with our everyday, obvious grasp that at least some things are wrong (say, torture and murder of innocents) or even good (say, acts of self-sacrificial love), and we have all the support we need to launch into our arguments for God to ultimately explain how such things are the case.