A goal to defend the objectivity of moral values on an atheistic worldview is atheistic moral Platonism.

Ancient Athenian Greek philosopher Plato (428/427-348/347 BCE) argued that there exists a realm of Ideas or Forms. This realm endures objectively and independently of the world of senses. The atheist Platonist finds this notion inviting because he can argue that values such as love and justice exist as ideal abstract objects that need not be grounded in God.

The atheist moral Platonist is therefore warranted in rejecting the need for a God to affirm the existence of objective moral values and duties. As such, moral values and duties are not subjective based on an individual’s own opinion or point of view. They are also not relative to culture or geographic region which may have over time produced disparate conceptions of moral values and duties distinguishing them from other cultures existing elsewhere. Moral relativism has been a challenge that atheist philosophers realize they need to wrestle with.

William Lane Craig, a theistic philosopher and Christian theologian, offers three reasons why he thinks atheistic moral Platonism is an untenable position.

Atheistic moral Platonism is unintelligible

Craig argues that moral Platonism does not make sense because moral values and duties are properties of persons,

“I can understand what it means to say that a person is just or compassionate. But I have no inkling what is meant at all by justice or compassion just existing as abstract objects… Things like justice, compassion, fairness, loyalty, and kindness seem to properties of persons. They are grounded in persons, but the idea that these things can just exist on their own is unintelligible.”

Craig further claims that the atheist moral Platonist’s view affirms the existence of moral vices, such as greed, cruelty, murder, and so on, also as abstract objects.

So, why, asks Craig, should one be obligated to align his life with one set of these abstract objects rather than another? The challenge of moral relativism emerges once again.

Atheistic Moral Platonism cannot affirm moral duties.

Despite providing an explanation for moral principles, atheist moral Platonism fails to provide an explanation for moral obligation. Duties are things one has to do or not be allowed to do.

Craig is unable to see how he has any moral obligation to act with goodness, compassion, or kindness rather than cruelty or selfishness, even if abstract qualities like these exist. Moral imperatives, or a directive to perform this or that action, seem to give rise to moral duties. According to Craig, moral obligations make sense in the context of theism because there is a divine law-giver who gives humans commands like “thou shalt not kill,” “thou shalt love thy neighbor,” and so on.

Atheistic Moral Platonism posits an incredible coincidence

Craig explains that this coincidence is that the exact kind of creature that happens to adhere to the causally unrelated abstract realm of moral standards and obligations should arise in the naturalistic sphere. It is a remarkable coincidence, according to Craig,

“that this independent, physical realm through a blind evolutionary process should evolve just that kind of being that corresponds to the abstractly existing, casually independent moral realm.”

Because theism provides stronger justifications for moral values and duties, Craig concludes that it is more reasonable position than atheistic moral Platonism.