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Full Idea
From sameness of meaning there does not follow sameness of thought expressed. A fact about the Morning Star may express something different from a fact about the Evening Star, as someone may regard one as true and the other false.
Clarification
Hesperus (the evening star) and Phosphorus (the morning star) turned out to be the planet Venus
Gist of Idea
I may regard a thought about Phosphorus as true, and the same thought about Hesperus as false
Source
Gottlob Frege (Function and Concept [1891], p.14)
Book Ref
Frege,Gottlob: 'Translations from the Writings of Gottlob Frege', ed/tr. Geach,P/Black,M [Blackwell 1980], p.29
A Reaction
This all gets clearer if we distinguish internalist and externalist theories of content. Why take sides on this? Why not just ask 'what is in the speaker's head?', 'what does the sentence mean in the community?', and 'what is the corresponding situation?'
| 18806 | Frege thought traditional categories had psychological and linguistic impurities [Frege, by Rumfitt] |
| 18899 | Frege takes the existence of horses to be part of their concept [Frege, by Sommers] |
| 4028 | Frege allows either too few properties (as extensions) or too many (as predicates) [Mellor/Oliver on Frege] |
| 9947 | Concepts are the ontological counterparts of predicative expressions [Frege, by George/Velleman] |
| 9948 | Unlike objects, concepts are inherently incomplete [Frege, by George/Velleman] |
| 10319 | An assertion about the concept 'horse' must indirectly speak of an object [Frege, by Hale] |
| 4972 | I may regard a thought about Phosphorus as true, and the same thought about Hesperus as false [Frege] |
| 8488 | A concept is a function whose value is always a truth-value [Frege] |
| 8487 | Arithmetic is a development of logic, so arithmetical symbolism must expand into logical symbolism [Frege] |
| 8489 | The concept 'object' is too simple for analysis; unlike a function, it is an expression with no empty place [Frege] |
| 8490 | First-level functions have objects as arguments; second-level functions take functions as arguments [Frege] |
| 8491 | The Ontological Argument fallaciously treats existence as a first-level concept [Frege] |
| 8492 | Relations are functions with two arguments [Frege] |