Full Idea
The naturalistic fallacy ..consists in the contention that good means nothing but some simple or complex notion, that can be defined in terms of natural qualities.
Gist of Idea
The naturalistic fallacy claims that natural qualties can define 'good'
Source
G.E. Moore (Principia Ethica [1903], §044)
Book Reference
Moore,G.E.: 'Principia Ethica' [CUP 1980], p.73