There is no mutually exclusive 'is – ought' distinction. The only mutually exclusive alternative to 'is' is 'is not'. This means that 'ought' either needs to find a comfortable home in the realm of 'is', or needs to be tossed into the realm of 'is not'.
A person comes to me and says, "Alonzo, you ought to do X."
I answer, "Prove it."
That person then says, "Well, as you know, an 'ought' statement cannot be derived from any set of 'is' statements . . . .”
"You can stop right there," I say. "We're done. You have just told me that your 'ought' statement is a work of fiction – an artifact of the realm of make-believe. If your claim that I ought to do X is false, then why are you telling me I ought to do X?"
-- Alonzo Fyfe
It doesn't seem very sensible to call a claim that someone "ought" to do something "false" if you're denying that an "ought" claim could ever be meaningful in the first place.
Anyway, it's a very annoying argument. It seems an awful lot like saying "You can't prove there's a such thing as value, therefore I refuse to take your money."
I'd be tempted to respond by hitting him with a stick until he conceded that stopping getting hit by a stick was a sufficient motivation to do X.
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