The “is-ought” fallacy dates at least back to David Hume. The idea is that one may not describe how something is and then conclude this is what one ought to do in terms of morality. In the recent Craig-Harris debate, during the Q&A portion, a young man asked Craig how he was not committing the is-ought fallacy by saying “because God exists, we ought to do this or that.”
The question does need to be addressed: does the Christian or one who believes in objective moral values commit the is-ought fallacy? I think the answer is “no.” First, consider the nature of the fallacy. It is primarily a problem for naturalism, not a theistic view. The idea is that one cannot take a statement of fact itself to be a prescription for moral activity. One cannot say, “it is true I don’t like to be struck; therefore, you ought not to strike me.” This was precisely Hume’s point.
Second, the objection assumes that one derives the “ought” of moral obligation from the “is” of God’s mere existence. This is simply not what the theist does when defending objective moral values. Craig says,
Suppose that God never created a concrete world at all. . .so that there were no created moral agents. In that case, God would not issue any commands, and so there would be no moral obligations or prohibitions of any sort. I suspect the questioner was confusing moral values with moral duties (these are not the same: it would be good for you to become a doctor, but you’re not morally obligated to become a doctor). The former are grounded in God Himself, the latter in God’s commands.
So it is not the case that God’s commands are rooted in his mere existence. Under the comments section on the Craig-Harris debate here at Possible Worlds, I alluded to this. We’re not saying, “God’s nature is good, therefore one ought to behave a certain way,” but rather “God’s nature is good, therefore what he commands we ought to obey.” This distinction between objective moral values and moral duties is critical.
Finally, the objection fails to take into account definitional scenarios. For instance, it is helpful to recognize it is not the “is-ought” fallacy to say, “Because X is a moral obligation, one ought to do it.” The reason is because this is simply what it means to have an obligation! If one wants to be aligned with moral obligations in her life, then she ought to perform these obligations. God’s nature grounds objective moral values, and his commands will always be in alignment with his nature. By this definition, then, she has an obligation to perform whatever commands are binding upon her in order to be in a right standing with morality! In summary, it seems God’s grounding of objective morality is not an instance of the “is-ought” fallacy after all.
 William Lane Craig, “Question
208,” <http://www.reasonablefaith.org/site/PageServer?pagename=q_and_a>, accessed April 11, 2011.
 It’s also worth discussing it seems that atheists may want to do away with the fallacy altogether. For instance, Harris alluded to “health” to be a counterexample of how we may say, “Bill has such-and-such disease, and we can say he ought to do such-and-such to get rid of the disease.” The problem is the “ought” is in accordance with a proposed standard independent of the fact of Bill’s having the disease.
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