Socrates on the topic of morality is quoted as saying we are not talking about something little but how we ought to live. As morally conscious individuals, we are concerned with the qualities required for living a moral life, to be good people and with our actions being deemed morally praiseworthy. However, this concern presupposes that there are correct answers to moral questions and that there exists a domain of moral facts about which we can form beliefs and which we may be mistaken about. This distinctive nature of correctness as concerns morality is maintained by view that moral propositions are in fact matter of fact and that our answers concerning morality correspond to the facts about the world. The view that maintains this is called Moral realism, which is the metaphysical (or ontological) view that there exists, in reality, moral facts. The two alternative metaphysical views are moral irrealism and moral nihilism. Irrealism is the view that such moral facts do not exist and are not necessary for moral practice. On the other hand, moral nihilism is the view that there are no moral facts which are necessary for moral practice, and that without these moral facts, morality is illusory.
Traditionally morality was grounded in God. God was the transcendental grounding for objective morality. God provided the transcendental vantage point in order to escape the subjective, partial personal preference based views concerning morality. Now, what becomes of morality if we no longer have God, can we establish a realist outlook about morality? Is it possible to maintain the essential feature of objectivity that ensures morality is valid and binding whether we believe in it or not? Is nihilism avoidable without God? The prominent Humanist philosopher Paul Kurtz writes, “The central question about moral and ethical principles concerns this ontological foundation. If they are neither derived from God nor anchored in some transcendent ground, are they purely ephemeral? 
Many theists and atheists concur that without God moral skepticism becomes an unavoidable reality. Moral skepticism can be introduced by reference to two main arguments for it: that from the existence of plurality, disagreement of morality and that from the queerness of moral properties. 
The objection of plurality and disagreement within the discourse on morality, which is also an objection facing a theistic realist moral viewpoint, argues that due to the extent of disagreement on what is right and good for human beings between cultures and within human cultures, are the demonstration that moral thought is the inevitable outcome and expression of culture and cultural conditioning. Since there exists widespread moral disagreement, which is more radical and more persistent than disagreements about matters of fact, there is a qualitative difference between matters of fact and morality, implying that moral facts do not exist.
Renford Bambrough, in his book Moral Scepticism and Moral Knowledge provides a cogent rebuttal to these common objections. Bambrough begins by discussing the statement Moral disagreement is more widespread, more radical and more persistent than disagreement about matters of fact. He writes that we are asked to compare a matter of fact i.e. a glass of water on the table which has no disagreements, with moral issues pertaining to capital punishment, abortion, birth control and nuclear disarmament which have a great degree of disagreements pertaining to them. However, Bambrough disagrees with this comparison which suggests that discussions about matter of fact are distinct from moral discussions due to their uniformed acceptance. To illustrate this, Bambrough presents a counter example that shows a disagreement about matters of fact and a general agreement about a moral matter. Bambrough gives a counter example where the agreement and disagreement are reversed and matters of fact are in disagreement and where there is an agreement on a moral issue. His counter example is whether to give a child anesthetic to avoid a painful surgery (which we would all agree is right) contrasted with cosmologists and radio astronomers about the interpretations of certain radio astronomical observations. By presenting this counter example Bambrough shows that even matters of fact are subject to disagreements, making the inference from the proposition inaccurate. Another counter example to show disagreements concerning matters of fact and uniformed agreements about moral issues, would be to compare the topic of quantum mechanics with the decision to torture a child for fun. There are ten interpretations for quantum activity and the correct interpretation can not be known as they are all empirically equivalent, whereas in the case of torturing a child for fun, we can all agree that it would be morally wrong even morally abhorrent to do so. Similarly, the duty to give a child anesthetic to avoid painful surgery is equally binding.
His second objection with regards to the proposition is that even if it were true that there were widespread disagreements and conflicting views regarding moral issues, it would not logically follow that there is no such things as moral knowledge. He argues, that with a dispute it could well be that they could just be wrong, and that it could be that one party is right and the other two wrong. That is to say that one cannot infer that moral knowledge does not exist from a dispute, as one would need to rule that a disagreement could not be the result of human error between the moral truth and our perceptions. Just as matters of fact are independently true whether people believe in them or not, the same could be said for moral knowledge too. In short, one could easily account diversity, relativity and wide bases disagreement on the bases of human error intervening in its attempt in discovering moral facts in the world. Therefore, the realist position can rightly refuse to see the plurality of morality as a defeater against objective morality.
What about the queerness of moral properties? In order to provide a non-theistic moral realism, the realist must sufficiently provide an alternative explanation for the existence of moral properties. Take for example the statement The ball is hard. If this statement is true, it reflects the way the world is. That is to say that a specific ball exits and that it possess a real property, namely hardness. Now consider the following value statements: Compassion is a virtue or friendship is good. Now, these statements are true, but not true in the same way as the statement The ball is hard, since Compassion or friendship are ascribing to a non-natural properties, virtue and good, i.e. an attribute that is not a scientific, physical characteristic of physics or chemistry.
The late J.L. Mackie on the queerness of moral properties writes, Moral properties constitute so odd a cluster of qualities and relations that they are most unlikely not have arisen in the ordinary course of events, without an all-powerful god to create them.  Mackie considers that there is a special problem concerning our knowledge of objective values. He argues that if moral proprieties did exist, then it would be the case that they would be very different to what currently exists in the universe. This is due to the fact that the properties that currently exist in the universe, those that we can currently observe, are subject to our five senses, touch, smell, auditory, taste and sight.
Mackie notes that there is a special problem regarding our knowledge of objective values, and this on two fronts. Mackie compares objective values to Platos Forms, The Form of the Good would have to provide the knower with both a direction and an overwhelming motive, explains Mackie. An objective good if apprehended, has to be pursued not because of any other reason or contingent fact, but rather because an objective good would intrinsically have a power to motivate. According to Mackie, what this means is that if moral values did exist, then they would function in the same way. We would know exactly what they were, in order to know what to do and that there would be an overwhelming urge to follow them. Mackie makes these points to show that we dont have such distinctive aspects of morality, of clear knowledge and intrinsic compliance, therefore objective values do not exist.
The summary of Mackies queerness argument is that we would need a strange faculty to discern these moral properties, faculties that are very different to what currently exists in the world, i.e. to discern nonmaterial properties. In addition, he claims that even if we did have these faculties, then we would know exactly what to do and they would thus have an affect on us to behave in this way . For Mackie the lack of all these aspects renders objective morality contentious.
Mackies point about the existence of morality is similar to David Humes, the famous Scottish philosopher of the eighteenth century fact value distinction. For Hume, matters of fact can be observed in our ordinary lives, whereas our passions are not of the outside world but within us. According to Hume, there are two main kinds of psychological state. The first is that we hold beliefs that purport to represent the way the world is. Since our beliefs purport to represent the world, they can be falsified in terms of truth and falsehood, according to whether or not they succeed in representing the world to be the way it really is. The second are states that represent our desires. These differ with beliefs in that they are not a matter of truth and falsehood since they do not purport to represent how the world is but how the world should be. Therefore, desires cannot be falsified, hence they are rationally neutral.
David Hume encapsulates this problem with respect to morality with the following in A treaties of human nature. “Take any action allow’d to be vicious: wilful murder, for instance. Examine it in all lights, and see if you can find that matter of fact, or real existence, which you call vice. In which-ever way you take it, you find only certain passions, motives, volitions and thoughts. There is no other matter of fact in the case. The vice entirely escapes you, as long as you consider the object.” “You can never find it, till you turn your reflexion into your own breast, and find a sentiment of disap-probation, which arises in you, towards this action. Here is a matter of fact; but ’tis the object of feeling, not of reason. It lies in yourself, not in the object.”
Hume is arguing here that in any given situation where one is deducing a moral judgment, there is a subject and an object. For instance, stealing is an evil act. In the given scenario the person stealing is the object and the subject is the moral judgment evil. Here the object is a matter of fact since it is within the outside world defining it as an objective observable fact. In contrast, we have the subjective view â€œis an evil act as it is merely within us and not outside of the view itself. Similarly, the statement â€œI think chocolate ice-cream is the tastiest ice-cream, there is the object which is â€˜chocolate ice-creamâ€™ and the subject is the I, as the ice-cream doesnt taste but tastes, therefore there is nothing objective about the tastiness of the ice-cream. I am simply tracing a feature of my own consciousness rather than of reality itself.
Confronted with these challenges moral philosophers have resorted to Irrealism. Irrealisms claim to morality is based on the view that moral facts are not required in order to make sense of our moral notions and that we can have moral knowledge by means of viewing our moral assertions as simply expressing desires about how people behave. For example when we judge it wrong to murder we are expressing our desire that people should not murder, which is equivalent to saying yuck to murder. Although, this provides a practical solution to a world where moral facts do not exist i.e. that we can understand ones moral position regarding a particular action, however there is still a problem here, no moral advancement is achieved with respect to the particular act it self. (The assessment would be in fact if I hold that desire genuinely or not, which seems to be an absurd question since its my desire.) Since desires are neither true nor false and lack the ability to be assessable according to a true criteria. My entire expressions only amount to reveal something about myself that I have a certain desire, not that the expressions are true propositions in that they represent the world outside of me.
When we engage in moral argument within an irrealist outlook, it is to get our opponent to have the same desires as we have. Not on the bases that the opponent rationally should have these desires, as desires arent supposed to be subject to rational criticism, but rather because these are the desires we want him or to have. This would be like asking someone who was a great admirer of vanilla ice cream to stop eating it because you disliked it, which would be absurd. Therefore, irrealism lacks any moral reflection and more importantly, it is impossible to maintain any genuine moral high ground, which is essential feature of moral discourse. What then becomes our notion of morality? Unavoidably they are relegated to merely expressions of personal and unreasoned prejudice. It would seem that what logical follows from moral irrealism is moral nihilism.
In summary, a non-theistic moral realism is false by virtue of the fact that moral facts cannot be successfully established and that irrealist view cannot maintain a genuine notion of morality. What logically follows is therefore moral nihilism. Given that nihilism is true, it implies that morality does not exist and that our use of the vocabulary and structures of cognitive enquiry are a mere mask. This reality hides the fact that moral judgments are nothing other than the expression of personal preference or socially imposed rules without God as the locus and paradigm of morality.
 Michael Smith, Realism, in Peter Singer, A Companion to ethics, (Basil Blackwell 1991, 1993) p402
 Paul Kurtz, Forbidden Fruit (Buffalo, N.Y.: Prometheus Books, 1988) p. 65.
 Peter Byrne, The philosophical and theological foundations of ethics (St. Martinâ€™s press, INC), p10
 Renford Bambrough, Moral Scepticism and Moral Knowledge (London: Routledge and Kegan Paul, 1979), px
 J. L. Mackie, The Miracle of Theism (Oxford: The Clarendon Press, 1982), p115- p116
 J. L. Mackie, Ethics: Inventing Right and Wrong (Harmondsworth: Penguin, 1977),
 David Hume, A treaties of human nature: III, 1