In cases involving a posteriori necessities (such as identity discoveries), we are mistaken about essences because we identify entities only by their superficial, observable stereotypes.
Necessity (in philosophy)(describing what kind of claim is being made about supernatural things)
Something that must be true or must exist; it couldn't possibly be otherwise (as opposed to something that just happens to be true but could have been different).
Observable(in quantum mechanics and physics)
A property or measurement of something that can actually be detected or measured, like the position or spin of a particle.
Stereotype (in philosophical contexts)(used here to mean superficial or obvious characteristics)
A mental image or simple, familiar description of something based on common assumptions—not the sociological kind, but how we typically picture or describe something.
a posteriori(Used to classify the epistemic status of necessary statements post-Kripke)
Knowable, but not independently of empirical experience
a posteriori necessities(Used to illustrate cases where imagination can mislead us about what is genuinely possible.)
Necessary truths that are discoverable only through empirical investigation rather than a priori reasoning alone (e.g., the identity of Hesperus and Phosphorus).
A major rationale of those who think that imagination is not a safe indication of possibility, even when such possibility is not eliminable a priori, is that we can imagine that a posteriori necessities might be false – for example, that Hesperus might not be identical to Phosphorus. But if Kripke is correct, that is not a real possibility. Another way of putting this point is that there are many epistemic possibilities which are imaginable because they are epistemic possibilities, but which are