The Open Question Argument
According to the philosopher David Hume a moral judgement that an object or action was good or bad referred not to the bare facts of the object or action, but was something else that was produced in the mind. This principle became known as Hume's Law.
Faced with the results of an action there is a wide chasm between the facts of what was done and what a person ought to have done in a particular situation. Some philosophers believed that they could cross this chasm by defining what good meant based on the properties or essence of good actions. They made many attempts to define what was considered good or bad and many of their arguments sound convincing. However, the philosopher G.E. Moore formulated what came to be known as the open question argument, which applied Hume's Law to moral judgements and made it easy to expose the error that is implicit in them.
If we defined good as meaning 'feeding hungry people' then we could always reasonably and meaningfully ask, 'If I feed hungry people is this good?' Having defined that good is 'feeding hungry people' and substituting it for the word good in our question we would get 'If I feed hungry people is this feeding hungry people?' which is a mere repetition of words and the sentence loses its meaning. The same would apply to any definition of good we chose to use because good is only an abstract name given to an action that we or society approve of.