"Hesperus Is Phosphorus"
"Hesperus is Phosphorus" is a famous sentence in the philosophy of language (see, e.g., proper name). Gottlob Frege used the terms "the evening star" (der Abendstern) and "the morning star" (der Morgenstern) to illustrate his distinction between sense and reference, and subsequent philosophers changed the example to "Hesperus is Phosphorus" so that it utilized proper names. Saul Kripke used the sentence to demonstrate that the knowledge of something necessary (in this case the identity of Hesperus and Phosphorus) could be discoverable rather than known a priori.
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