...whereas it has become a commonplace that one cannot legimitately deduce an 'ought' from an 'is', cannot prove a moral conclusion purely on the basis of some factual claim, it has been forgotten that this is only an instance of a wider principle. That wider principle is that one cannot legimtately deduce any claim simply from another: that Zebedee is married to Rahab does not, of itself, imply that Rahab is married to Zebedee nor that Zebedee is not married to Tamar. To reach such conclusions we need additional premisses, about the institution of marriage. If one claim were enough to establish what had seemed to be a different claim, that would be reason to consider that the second claim really was no other than the first, under some disguise.
[Stepen R. L. Clark, From Athens to Jerusalem, Clarendon Press (Oxford: 1984) p. 11.]
