The challenge to objective values › How could such values be known?

The epistemological part asks what kind of access we could have to objectively prescriptive properties. If moral knowledge requires a special intuition quite unlike ordinary ways of knowing, Mackie thinks that requirement counts against the view.

He also asks how moral properties would relate to the natural features on which they depend. Suppose two actions differ in cruelty. A moral explanation often appeals to what the agents did, their intentions, and the suffering involved. Why should those features have the additional property of objective wrongness, and how would we know the connection?

This is not answered merely by repeating that cruelty is wrong. The question concerns the link between the descriptive features and the allegedly objective value. An account of moral knowledge must explain both what is known and how the relevant connection is accessible.

A possible response is that moral properties are natural properties, rather than a separate category apprehended by a special faculty. That would alter the metaphysical picture Mackie is attacking. The remaining question is whether such a natural account can explain the authority claimed in moral judgment.

Another response could accept fundamental normative facts while rejecting the demand that all genuine knowledge resemble empirical observation. This creates a challenge to Mackie's standards of explanation. He must explain why the alleged peculiarity is a reason to reject objective value rather than a reason to broaden the account of what exists.

These are possible lines of argument, not reports of a settled resolution. Calling a property strange identifies a burden of explanation. It does not dispense with examining whether the burden can be met.