Full Idea
Mackie's 'error theory' of ethics says that if a fact is something that corresponds to a true proposition, there are actually no moral facts, hence no knowledge of what moral statements are about.
Gist of Idea
The 'error theory' of morals says there is no moral knowledge, because there are no moral facts
Source
report of J.L. Mackie (Ethics: Inventing Right and Wrong [1977]) by Pascal Engel - Truth §4.2
Book Reference
Engel,Pascal: 'Truth' [Acumen 2002], p.107
A Reaction
Personally I am inclined to think that there are moral facts (about what nature shows us constitutes a good human being), based on virtue theory. Mackie is a good warning, though, against making excessive claims. You end up like a bad scientist.