If the a priori status of premise (2) derives from its self-verifying character rather than from isolation from external facts, then presupposing premise (3) does not defeat its a priori warrant but merely reveals the scope of what is directly known.

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Premise: A premise is a statement or fact that you assume to be true as a starting point for reasoning or making an argument. Think of it as the foundation or building block you use to reach a conclusion—for example, "All dogs are animals" and "My pet is a dog" are premises that lead to the conclusion "My pet is an animal." Premises are essentially the evidence or claims you offer before drawing a final conclusion.
Presupposing(what the externalist secretly assumes in their reasoning): To assume something is already true without proving it, usually without realizing you're doing it.
a priori(Frege treats 'analytic' as entailing 'a priori' for arithmetic.): Knowable independently of empirical experience; here treated as a consequence of analyticity.
scope(formal semantics / generalized quantifier theory): The second argument of a type ⟨1,1⟩ determiner denotation

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