open question argument

The argument used by Moore in Principia Ethica (1903) to show that you cannot define an evaluative term as equivalent to the features you use as a standard for applying it. Thus, suppose you value all and only things that create happiness, Moore still seeks to show that this is not what the word ‘good’ means in your mouth. The argument is that even if this is your standard, you will recognize that it makes sense to ask whether all and only things that create happiness are good. This is a substantial or ‘open’ question, which, according to Moore, it would not be if the two terms had come to mean the same.