Full Idea
Moore's account leaves it entirely unexplained and inexplicable why something's being good should ever furnish us with a reason for action.
Gist of Idea
Moore cannot show why something being good gives us a reason for action
Source
comment on G.E. Moore (Principia Ethica [1903]) by Alasdair MacIntyre - A Short History of Ethics Ch.18
Book Reference
MacIntyre,Alasdair: 'A Short History of Ethics' [Routledge 1967], p.252
A Reaction
The same objection can be raised to Plato's Form of the Good, but Plato's answer seems to be that the Good is partly a rational entity, and partly that the Good just has a natural magnetism that makes it quasi-religious.