Naturalistic Fallacy

The naturalistic fallacy is the logical error of deriving a prescriptive claim ('ought') from a purely descriptive one ('is') — inferring what should be the case from what is the case in nature. The term was introduced by G.E. Moore in Principia Ethica (1903), though Moore's specific target was the identification of 'good' with any natural property (pleasure, evolutionary fitness, social harmony). In its broader usage, the fallacy appears whenever the natural or evolved status of a trait is cited as evidence of its moral acceptability or optimality: 'this behavior is natural, therefore it is good,' or 'this is how organisms evolved, therefore this is how humans should live.' The fallacy is ubiquitous in popular discussions of ecology, evolutionary psychology, and nutrition — and its ubiquity in precisely these domains should prompt a rationalist investigation into why biological descriptors carry such persistent normative freight. The fallacy's converse, the moralistic fallacy — inferring facts from values ('this would be bad, therefore it cannot be true') — is equally common in biology, where distasteful evolutionary hypotheses are rejected on motivational rather than empirical grounds. Both errors share the same structure: a confusion between what is and what ought to be. See also: Is-Ought Problem, Moral Realism.