Ethical non-naturalists tend to be intuitionists. Non-naturalists hold that moral properties are fundamentally different to non-moral properties, and so cannot be analysed in non-moral terms. To attempt a reductive analysis of morality is, according to non-naturalists, futile; there can be no such analysis.

This raises a problem for moral epistemology: if moral properties are fundamentally different to non-moral properties, then how can we learn moral facts? It is only the physical world that can be experienced through the five senses, so if morality is not a part of the physical world then it is mysterious how we can come to possess moral knowledge.

Intuitionism is a response to this problem; it postulates the existence of a special moral intuition, a sixth sense, by which we can perceive the moral realm. According to intuitionists, we acquire knowledge of morality by intuition; moral facts are evident to us.