“And here it is convenient to begin from the moment when G.E. Moore made a certain distinction. Moore said we should distinguish between the question, what things are good? and the question, what does the word ‘good’ mean? On this second question Moore had important things to say. He claimed that ‘good’ was indefinable and that previous moral philosophers, because they had failed to distinguish those two questions from each other, had fallen into the error of defining ‘good’ or valuable, in terms of some other non valuable entity, whether a natural entity, such as pleasure, or a metaphysical entity such as rationality. If asked, what things are good, one might indeed answer this question by pointing to pleasure or rationality -but one could not answer the question what is good itself in this way. Moore convinced his readers of this very simply that it made sense always, given any proposition of the form ‘X is good’ to withdraw thoughtfully and ask – ‘But is X really good?’ That is, the notion of ‘good’ could significantly be attached to or withdrawn from anything whatever, and the things to which it happened to be attached did not form part of its meaning.
This simple argument of Moore’s produced a complete change of perspective in moral philosophy. It transformed the central question of ethics from the question, ‘What is goodness?’ – where an answer was expected in terms of the revelation of some real and eternally present structure of the universe – into the question ‘What is the activity of “valuing” ( or “commending”)? where what is required is to see what is in common to people of all ages and societies when they attach value to something. This phrase ‘attach value’ is itself significant of the change of attitude. The philosopher is now to speak no longer of the Good, as something real or transcendent, but to analyze the familiar human activity of endowing things with value. If we want to place the definitive breach with metaphysical ethics at any point, we can place it here”
Iris Murdoch, Existentialists and Mystics