In the philosophy of language, the distinction between sense and reference was an idea of the German philosopher and mathematician Gottlob Frege in 1892 (in his paper "On Sense and Reference"; German: "Über Sinn und Bedeutung"), reflecting the two ways he believed a singular term may have meaning. The reference (or "referent"; Bedeutung) of a proper name is the object it means or indicates (bedeuten), whereas its sense (Sinn) is what the name expresses. The reference of a sentence is its extension, whereas its sense is the thought that it expresses. The concept of sense can also be thought of as the "mode of presentation" of the referent. Frege justified the distinction in a number of ways.
Sense is something possessed by a name, whether or not it has a reference. For example, the description "the largest integer" is intelligible, and therefore has a sense, even though there is no individual object (its reference) to which the name corresponds. The sense of different names is different, even when their reference is the same. Frege argued that if an identity statement such as "Hesperus is the same planet as Phosphorus" is to be informative, the proper names flanking the identity sign must have a different meaning or sense. But clearly, if the statement is true, they must have the same reference. The sense is a 'mode of presentation', which serves to illuminate only a single aspect of the referent. Much of analytic philosophy is traceable to Frege's philosophy of language. Frege's views on logic (i.e., his idea that some parts of speech are complete by themselves, and are analogous to the arguments of a mathematical function) led to his views on a theory of reference.
Background Frege developed his original theory of meaning in early works like Begriffsschrift (concept paper) of 1879 and Grundlagen (Foundations of Arithmetic) of 1884. On this theory, the meaning of a complete sentence consists in its being true or false, and the meaning of each significant expression in the sentence is an extralinguistic entity which Frege called its Bedeutung, literally meaning or significance, but rendered by Frege's translators as reference, referent, 'Meaning', nominatum, etc. Frege supposed that some parts of speech are complete by themselves, and are analogous to the arguments of a mathematical function, but that other parts are incomplete, and contain an empty place, by analogy with the function itself. Thus "Caesar conquered Gaul" divides into the complete term "Caesar", whose reference is Caesar himself, and the incomplete term "—conquered Gaul", whose reference is a concept. Only when the empty place is filled by a proper name does the reference of the completed sentence – its truth value – appear. This early theory of meaning explains how the significance or reference of a sentence (its truth value) depends on the significance or reference of its parts.
Sense
Frege introduced the notion of "sense" (German: Sinn) to accommodate difficulties in his early theory of meaning. First, if the entire significance of a sentence consists of its truth value, it follows that the sentence will have the same significance if we replace a word of the sentence with one having an identical reference, as this will not change its truth value. The reference of the whole is determined by the reference of the parts. If the evening star has the same reference as the morning star, it follows that the evening star is a body illuminated by the Sun has the same truth value as the morning star is a body illuminated by the Sun. But it is possible for someone to think that the first sentence is true while also thinking that the second is false. Therefore, the thought corresponding to each sentence cannot be its reference, but something else, which Frege called its sense. Second, sentences that contain proper names with no reference cannot have a truth value at all. Yet the sentence 'Odysseus was set ashore at Ithaca while sound asleep' obviously has a sense, even though 'Odysseus' has no reference. The thought remains the same whether or not 'Odysseus' has a reference. Furthermore, a thought cannot contain the objects that it is about. For example, Mont Blanc, 'with its snowfields', cannot be a component of the thought that Mont Blanc is more than 4,000 metres high. Nor can a thought about Etna contain lumps of solidified lava. Frege's notion of sense is somewhat obscure, and neo-Fregeans have come up with different candidates for its role. Accounts based on the work of Carnap and Church treat sense as an intension, or a function from possible worlds to extensions. For example, the intension of ‘number of planets’ is a function that maps any possible world to the number of planets in that world. John McDowell supplies cognitive and reference-determining roles. Michael Devitt treats senses as causal-historical chains connecting names to referents, allowing that repeated "groundings" in an object account for reference change.
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