The Open Question Argument

Many moral religious and moral philosophers have attempted to equate good either with a deity (God is good) a divine command (what is pleasing to God) or with some natural phenomenon (pleasure is good). The Open Question Argument was formulated by G. E. Moore in his 1903 work Principia Ethica in order to refute such identifications. The argument is as follows:

(Premise 1) If X is good, then the question "Is it true that X is good?" would be  meaningless.
(Premise 2) The question "Is it true that X is good?" is not meaningless (i.e. it is an open question).
(Conclusion) X is not (logically) equivalent to good.

This known as the open question argument as it depends on the second premise being an open question; that is a question whose answer has to be investigated rather than reasoned about. A closed question would be something like, Is that widow's husband dead? 

In response to criticism that the original argument assumed its own answer in the second premise G E Moore's original argument has been restated in the following manner.

(Premise 1) If X is good, then X will in itself motivate an individual to pursue it.
(Premise 2) A sane and rational speaker of English can understand that Action X* produces X, yet not pursue X*.
(Conclusion) X is not (analytically equivalent to) good.

The first premise follows Plato and Kant on the equvalence of knowledge of the good and right action. Hume would agree with premise 2 in that both belief and desire are needed to motivate actions.