- The fallacious belief that something is automatically good because it is natural or automatically bad because it is unnatural; an appeal to nature.
- Any attempt to define "good" verbally, instead of treating it as an undefined term, in terms of which other terms are defined.
- 1903, G. E. Moore, Principia Ethica:
- Yet a mistake of this simple kind has commonly been made about good. It may be true that all things which are good are also something else, just as it is true that all things which are yellow produce a certain kind of vibration in the light. And it is a fact, that Ethics aims at discovering what are those other properties belonging to all things which are good. But far too many philosophers have thought that when they named those other properties they were actually defining good; that these properties, in fact, were simply not other, but absolutely and entirely the same with goodness. This view I propose to call the naturalistic fallacy and of it I shall now endeavour to dispose.
- Whether this second usage is a fallacy is disputed.