MHB Question on the concept of " Identity "

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Van Dalen's approach to axiomatizing 'Identity' aims to provide a more rigorous framework than simply viewing it as a binary predicate. By establishing axioms for identity, he enhances the understanding of its properties and implications within logical systems. The discussion highlights that while identity is represented as a binary predicate, the axioms serve to clarify its role in derivability rather than merely its semantic interpretation. Axioms I2 and I3 can be derived from I1 and I4, emphasizing the interconnectedness of these axioms. Ultimately, the axiomatization strengthens the foundational understanding of identity in logic.
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Hi all;
Look at the attached part from Van Dalen's Logic and structure.
What is he doing exactly?
In axiomatizing 'Identity' as he does, what is gained rather than what we had before (i.e., looking at 'Identity' as a binary predicate)?!
Even in the axioms, he is again using a symbol in the language for identity as a binary predicate (i.e., = ) and then he proves the axioms (or says they are provable) in the language. [note that he also proves I3 and I4 that i haven't shown.]
Thanks.
 

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Mathelogician said:
Hi all;
Look at the attached part from Van Dalen's Logic and structure.
What is he doing exactly?
In axiomatizing 'Identity' as he does, what is gained rather than what we had before (i.e., looking at 'Identity' as a binary predicate)?!
Even in the axioms, he is again using a symbol in the language for identity as a binary predicate (i.e., = ) and then he proves the axioms (or says they are provable) in the language. [note that he also proves I3 and I4 that i haven't shown.]
Thanks.

The axiom I4 is rather an axiom sceme ,because for each "t" and for each "φ" we a corresponding axiom.

Axioms I2 and I3 can be proved using axioms I1 and I4
 
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Of course they are axiom schemes; but I'm afraid my question is something else!
Thanks.
 
When we are talking about semantics (i.e., $\models$), this section may be considered just as an observation that identity satisfies these axioms. The importance of the axioms comes when we consider the derivability relation $\vdash$ (Definition 1.4.2). Then we have to use special axioms or inference rules to say that $=$ is not just an arbitrary predicate symbol.

solakis said:
Axioms I2 and I3 can be proved using axioms I1 and I4
This is true. It is given as an exercise later in the text. The term version of $I_4$ can also be proved from the formula version.
 
I was reading documentation about the soundness and completeness of logic formal systems. Consider the following $$\vdash_S \phi$$ where ##S## is the proof-system making part the formal system and ##\phi## is a wff (well formed formula) of the formal language. Note the blank on left of the turnstile symbol ##\vdash_S##, as far as I can tell it actually represents the empty set. So what does it mean ? I guess it actually means ##\phi## is a theorem of the formal system, i.e. there is a...
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