of humans to bring the summum bonum about, because we cannot ensure that virtue always leads to happiness, so there must be a higher power who has the power to create an afterlife where virtue can be rewarded by happiness. Philosopher G. H. R. Parkinson notes a common objection to Kant's argument: that what ought to be done does not necessarily entail that it is possible. He also argues that alternative conceptions of morality exist which do not rely on the assumptions that Kant makes – he cites utilitarianism as an example which does not require the summum bonum. Nicholas Everitt argues that much moral guidance is unattainable, such as the Biblical command to be Christ-like. He proposes that Kant's first two premises only entail that we must try to achieve the perfect good, not that it is actually attainable. Argument from objective moral truths Both theists and non-theists have accepted that the existence of objective moral truths might entail the existence of God. Atheist philosopher J. L. Mackie accepted that, if objective moral truths existed, they would warrant a supernatural explanation. Scottish philosopher W. R. Sorley presented the following argument: If morality is objective and absolute, God must exist. Morality is objective and absolute. Therefore, God must exist. Many critics have challenged the second premise of this argument, by offering a biological and sociological account of the development of human morality which suggests that it is neither objective nor absolute. This account, supported by biologist E. O. Wilson and philosopher Michael Ruse, proposes that the human experience of morality is a by-product of natural selection, a theory philosopher Mark D. Linville calls evolutionary naturalism. According to the theory, the human experience of moral obligations was the result of evolutionary pressures, which attached a sense of morality to human psychology because it was useful for moral development; this entails that moral values do not exist independently of the human mind. Morality might be better understood as an evolutionary imperative in order to propagate genes and ultimately reproduce. No human society today advocates immorality, such as theft or murder, because it would undoubtedly lead to the end of that particular society and any chance for future survival of offspring. Scottish empiricist David Hume made a similar argument, that belief in objective moral truths is unwarranted and to discuss them is meaningless. Because evolutionary naturalism proposes an empirical account of morality, it