"Sense" is the distinctive central notion in theories of thought and language inspired by the later work of Gottlob Frege ("sense" translates Frege's Sinn). For Frege what we think (not the act of thinking it) is a thought, an abstract object. Thoughts have quasi-syntactic structure. Any simple or complex constituent of a thought, even the thought itself, is a sense; thus, senses are abstract. Frege assumes that it is irrational to assent to a thought and simultaneously dissent from it. Since someone misled about astronomy may rationally combine assent to the thought that Hesperus is Hesperus with dissent from the thought that Hesperus is Phosphorus, the thoughts are distinct. Although the names "Hesperus" and "Phosphorus" have the same reference, they express different sense, two modes of presentation of one planet. The role of a sense is to present the thinker with a reference—that is, something on which...