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?
Let’s take a third example. ‘Bachelors are unmarried.’ is also stating that something – i.e. a semantic fact – is the case. However, it would be weird to infer from it that bachelors ought to be unmarried. Even if you speak about how something is necessarily, you still cannot deduce an ‘ought’ from an ‘is’.
I think there is something wrong about the third example, but I cannot say as yet what it is.
In the second example I imagine the following reply: “The conclusion is that we should not call something ‘knowledge’ if we wouldn’t call it ‘true’. This is justified by the semantic rule ‘Truth is a necessary condition for knowledge.’ The conclusion says that we should obey the rule and nothing else.”
I’ll skip the part where I wonder how do we get from ‘X is a rule’ to ‘One ought to obey X’. It was, perhaps, unclear in the second example if ‘Truth is a necessary condition for knowledge.’ should have a de re reading or a de dicto one.
And now is perhaps clear that this was the problem with my third example. Its weirdness came from the fact the we got from a semantic fact (de dicto) to a norm about how things ought to be (de re).
There are still two problems left. The first concerns the justification for getting from ‘X, in a de dicto reading, expresses a semantic fact’ to ‘X is a rule’. The second is how can we get from a de re reading of a statement expressing a necessary connexion (e.g., “Sincerity is a necessary condition for romantic love.”) to a norm (“One ought to be sincere with the person(s) she/he is in love with.”).