Proving Vacuous Quantification in First-Order Logic

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Discussion Overview

The discussion revolves around proving the theorem related to the identity \(\exists x (P) \rightarrow P\) in the context of first-order logic, focusing on the rigorous justification of this statement. The scope includes theoretical aspects of first-order logic and the axioms that govern it.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant seeks a rigorous derivation of the identity \(\exists x (P) \rightarrow P\) from the axioms of first-order logic, expressing uncertainty about their understanding.
  • Another participant questions the nature of \(P\), suggesting that if \(P\) is a propositional symbol, the statement is trivially true.
  • A clarification is made that \(P\) is a predicate where \(x\) does not appear, indicating that the participant is aware of the tautological nature of the statement but desires a formal proof.
  • A later reply indicates that the original poster has resolved their query independently.

Areas of Agreement / Disagreement

Participants exhibit differing levels of understanding regarding the nature of \(P\) and the requirements for proving the identity. The discussion remains unresolved in terms of a formal proof, as the original poster finds a solution independently.

Contextual Notes

There is an assumption that participants share a common understanding of first-order logic axioms, but the specific steps or axioms used in the proof are not detailed in the discussion.

Manchot
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I'm trying to prove a theorem which makes use of the identity \exists x (P) \rightarrow P (where x is not a free variable of P). Intuitively, I want to believe it, but since I'm trying to do things rigorously, I'd like to be able to justify it to myself. Can anyone offer a suggestion as to how I'd derive the identifier from the usual axioms of first-order logic? (I'm sure that I'm missing something totally obvious). Thanks.
 
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I don't understand. P is a naked propositional symbol in FOL? If so, then your done
(regardless of whether it has any quantifiers attached to it, or not). It's a tautology.
 
Last edited:
P is a predicate in which x doesn't appear. Anyway, I know that it is a tautology, but I'm trying to prove it rigorously from FOL's axioms.
 
Never mind, I've got it. Thanks anyway.
 

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