 One worry about this formulation concerns the apparent existence of pairs of sentences, like “If Obama exists, then Obama=Obama” and “If McCain exists, McCain=McCain” which are such that any rational person who understands both will take both to be true. These sentences seem intuitively to differ in content—but this is ruled out by the criterion above. One idea for getting around this problem would be to state our criterion of difference for senses of expressions in terms of differences which result from substituting one expression for another: