The problematic of this chapter involves the search for a criterion of the identity of sense of expressions and for the very "nature" of the sense of expressions. In a sense the first question is subordinate to the second; for, if the sense of every expression in a language were clearly given and able to be determined, there could be no doubt as to the identity or nonidentity of the sense of two expressions. It would be immediately evident whether or not two expressions were synonymous in the way that we understand identity of sense here (not as mere identity of reference). But, at least in the natural languages, we do not find this to be the case. In fact, the situation is so discouraging that one is tempted to drop the second question for a while and deal with the first question by itself. This is possible. Just as two mathematicians can agree that two expressions designate the same number without agreeing as to what a number "really is", so two semanticists can agree on a concept of synonymity without being in agreement on the "essence" of sense.