Evelyn de Morgan, Phosphorus and Hesperus.
In Greek mythology, Hesperus is the Evening Star, the planet Venus in the evening. He is the son of the dawn goddess Eos and is the half-brother of her other son, Phosphorus. Hesperus’ Roman equivalent is Vesper. Hesperus’ father was Cephalus, a mortal, while Phosphorus’ was the star god Astraios.
In the philosophy of language, “Hesperus is Phosphorus” is a famous sentence in relation to the semantics of proper names. 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 empirical rather than knowable a priori.