"The divine command theory (DCT) of ethics holds that an act is either moral or immoral solely because God either commands us to do it or prohibits us from doing it, respectively. On DCT the only thing that makes an act morally wrong is that God prohibits doing it, and all that it means to say that torture is wrong is that God prohibits torture. DCT is wildly implausible for reasons best illustrated by the Euthyphro dilemma, which is based on a discussion of what it means for an act to be holy in Plato's Euthyphro. Substituting "moral wrongness" for "holiness" raises the dilemma: Is torture wrong because God prohibits it, or does God prohibit torture because it is already wrong?
While DCT takes the the first route, Euthyphro takes the last one: If a good God prohibits torture he does so because torture is intrinsicly wrong, not merely because he declares torture to be wrong by fiat. But if torture is intrinsicly wrong, then it is wrong regardless of whether or not God exists. Either certain acts are wrong regardless of anyone's opinions or commands (including God's), or else all that we mean by "torture is wrong" is "God prohibits torture." Rather than grounding the objectivity of ethics, DCT completely undermines it by insisting that God's commands (like those of individuals or societies) do not require justification in terms of any external principles.
DCT is thus a kind of moral relativism: what's right or wrong is what one's God (like one's self or one's society) says is right or wrong--and there are no moral standards apart from this. Yet if God said that 2+2=100, 2+2=100 would nonetheless be false because 2+2=4 is true regardless of what God says. The same point holds for moral propositions like "inflicting unnecessary suffering solely for fun is wrong." If that proposition is true, then it is true regardless of whether God commands or prohibits inflicting such suffering.
If there is no standard of "being morally right" apart from God's commands, then God could literally command us to do anything and it would be right for us to do it by definition. Whatever God commands becomes the standard of moral rightness, and there are no moral values external to God to constrain what he would or would not command. So if God commanded one person to rape another, DCT entails that that rape would be moral because "doing the right thing" is logically equivalent to "doing what God commands." A highly implausible implication is that it is impossible to even imagine God commanding a wrong act. What counts as moral or immoral behavior on DCT is completely subjective--dependent upon God's fiat--and thus arbitrary.
While some retort that goodness flows from God's nature, this merely changes the form of the dilemma: Is compassion good because it is a part of God's nature, or is compassion a part of God's nature because it is already good? The first option produces problems parallel to those for DCT. If malice were a part of God's nature, for instance, it is doubtful that malice would automatically be good. If there are any objective moral standards at all, then a god can be either good or evil, and the assessment of a god's character would depend upon appealing to standards independent of any god's commands, opinions, statements, nature, or character."
See more about the Divine Command Theory @ Infidels.org
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