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Saul Kripke's argument, in his seminal 'Naming and Necessity', that Hesperus (the Evening Star) is necessarily Phosphorus (the Morning Star), has become one of the canonical examples of a posteriori necessity. A key element of that argument is the claim (or observation) that Hesperus is identical with Phosphorus, as both these names designate the same thing - the planet Venus.

On March 21, 2021, Venus will be at superior conjunction - directly behind the sun, with respect to earth - and on October 29, it will have reached its maximum elongation east (appearing in the sky to the east of the sun, and therefore following it across the sky, being visible after sunset.) By January 9, 2022, it will be at inferior conjunction, directly between the earth and the sun.

Given the identity (necessary identity, indeed) of Hesperus, Phosphorus and Venus, if someone were to utter, on March 21, 2021, the phrase "when Phosphorus is next visible", there should be no ambiguity over the month and year being picked out here, right?

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    Ambiguity of speech (semantics) has little to do with the necessity of identity (metaphysics). People can still use "Phosphorus" ambiguously, as relating to the "morning star" description or as relating to Venus the planet.
    – Conifold
    Oct 15, 2020 at 0:43
  • @Conifold And yet are not these metaphysical arguments, concerning the necessity of identity, essentially from semantics? You seemed to be not quite square with them back in '15 ('Why is Hesperus necessarily Phosphorus?'); did the answers there resolve your concerns? More generally, a lot of philosophy seems to be concerned with language and its meaning, and I am struck by how rarely it is acknowledged that people are often ambiguous and irrational (see for example the IEP entry on Modal Illusions)
    – A Raybould
    Oct 15, 2020 at 1:45
  • @Conifold I'm actually ambivalent over whether the above phrase is ambiguous: if someone chose to use "Phosphorus" rather than "Venus", I would expect that they were referring to the phenomenon known as the morning star.
    – A Raybould
    Oct 15, 2020 at 1:59
  • They are more from "metaphysical intuitions" as detected in common use. One of philosophy's tasks is to disambiguate, but it does not mean that common use does not remain ambiguous. And there are reasons for it to stay so, its ends and philosophy's are quite different, so it is not a sign of irrationality either.
    – Conifold
    Oct 15, 2020 at 4:27
  • @conifold On reflection, I am no longer ambivalent over the common use: "when the morning star is next visible" clearly indicates a date in January 2022, and as Phosphorus is a simple synonym for 'morning star', there is no ambiguity there either, and not even with Venus, except the date picked out by the corresponding phrase is then in May 2021. Ambiguity arises only if we insist that these three names are all rigid designators of the same thing, even though they are not exactly synonyms. For what it is worth, I do not see a similar issue with water/H2O or Cicero/Tully.
    – A Raybould
    Oct 16, 2020 at 12:33

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