According to George E. Moore, ethical claims all concern human conduct while philosophical ethics ultimately concerns itself with knowledge of what “good” is. Moore also believes philosophical ethics ought to concern itself with what is good instrumentally, or good as a means rather than good as an end, as a property. According to Moore, what is intrinsically good, or the property of “goodness” is not an analyzable property. For Moore, what “good” is, or “goodness”, as an individual property, is “unanalyzable”, or, undefinable. Therefore, any claim which gives a definition of “goodness” is attributing goodness to something, rather than identifying what goodness itself, as a property, is. Moore accuses those who make this error of committing the “naturalistic fallacy”. He believes that moral naturalists — philosophers who maintain that moral properties exist and can be objectively studied, through biology and sciences — are primarily responsible for this mistake. Moore thought philosophers committed the naturalistic fallacy when attempting to define “good” by moving from one claim that a thing is “good” to the claim that “good” is that thing. Moore thought one could not identify “good” with a thing one believes is “good”.

In order to test and determine whether an attempt at defining “good” is correct and not a concealed assignment is what Moore called the “open question argument.” Moore proposed that if “goodness” is a natural property, then there is some correct explanation of which natural property it is. For example, maybe “goodness” is the same property as “pleasantness”, or the same property as being “desirable”. Further, a correct property must be identified to fill in an identity statement of the form “goodness = __________”, or, “what is good is _________”. This kind of identity statement can be correct only if both terms on either side of the identity sign are synonyms for profici