Suppose Curley is deciding whether to accept a $1,000,000 bribe and lose his soul, or remain honest. It seems quite possible to have situations like this where neither option is preferable to Curley. Of course, prima facie that need not be a case of incommensurability--it might be a case of equal preference. But we cannot say that all similar cases like this are cases of equal preference. For in most cases like this, Curley also wouldn't have a preference between a $1,010,000 bribe and honesty. If that too has to be a case of equal preference rather than incommensurability, then by the transitivity of equal preference, we would have to say in such cases that Curley has equal preference as to a $1,000,000 bribe and a $1,010,000 bribe. But of course that's false: in most cases like this, he prefers the larger bribe.