Suppose that social and brain research provide us with this factual statement: ‘People have sex looking for pleasure’. In other words, there is a natural connexion between sex and pleasure (expressible, perhaps, in the form ‘pleasure is the natural purpose of sex’). It is a naturalistic fallacy to infer from this that people ought to have sex for pleasure.
Now, let’s take another factual statement: ‘Truth is a necessary condition for knowledge’. Let us assume that this statement is about a different fact – a semantic one, if you wish. The statement points out that there is a necessary connexion between the concept of knowledge and the concept of truth. From this a philosopher would infer that knowledge ought to be true.
Why don’t we speak of a naturalistic fallacy in the second case?
