I really believe that one cannot derive " ought" from "is".
But the case of linguistic usage causes me some intellectual trouble.
(1) Linguistic usage is a simple fact.
(2) Linguistic usage often deviates from linguistic norm, from correct speech ( from a phonetic, semantic, syntactic aspect...)
(3) Incorrect linguistic usage often becomes correct, by the simple fact that it is the most common usage, and/or the fact that it is deep-rooted.
(4) So, assuming one wants to speak correctly, one ought to conform to linguistic usage. In language, the fact of usage has a genuine normative value.
Hence the question : is the case of linguistic usage a genuine couterexample to the rule " ought cannot be derived from is"?
If usage were not normative, the following extract from Through the looking glass would not be so funny I think :
"I don't know what you mean by 'glory,' " Alice said.
Humpty Dumpty smiled contemptuously. "Of course you don't—till I tell you. I meant 'there's a nice knock-down argument for you!' "
"But 'glory' doesn't mean 'a nice knock-down argument'," Alice objected.
"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means
just what I choose it to mean—neither more nor less."
"The question is," said Alice, "whether you can make words mean so many different things."
"The question is," said Humpty Dumpty, "which is to be master—that's all."
Alice was too much puzzled to say anything, so after a minute Humpty Dumpty began again. "They've a temper, some of them—particularly verbs, they're the proudest—adjectives you can do anything with, but not verbs—however, I can manage the whole lot! Impenetrability! That's what I say