  Frege notes that an expression might have a sense, but not a reference. An example he gives is, “the least rapidly convergent series.” Of course an object might have several signs that designate it, but with different senses. Frege extends the sense/reference dichotomy rather far. In particular, declarative sentences are said to have both a sense and a reference. The sense is the proposition it expresses, while the reference is its truth value. Then logically equivalent sentences have the same designation, but may have different senses. In indirect contexts sense, and not designation, matters and so we may know the well-ordering principle for natural numbers, but not know the principle of mathematical induction because, while they are equivalent in truth value, they have different senses.