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Inverse View
It is not the case that Modal logic S4 is deductively embeddable into many-sorted logic: if Π ⊢_S4 φ then Trans(Π) ∪ ΔS4 ⊢ Trans(φ).
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Reasons For
2 perspectives
Reason for 1 of 2
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1.
Many-sorted logic is an extensional framework where sort-membership is a static, non-modal classification of objects.
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2.
S4's accessibility relation encodes iterated epistemic or metaphysical possibility, a hyperintensional structure that extensional sort distinctions cannot fully capture without circularity.
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3.
ΔS4 must therefore either beg the question by encoding modal facts as primitive sort axioms, or fail to derive all S4-valid sequents under the proposed embedding.
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Reason for 2 of 2
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1.
The translation function Trans presupposes a fixed domain across modal contexts, smuggling in a Barcan-style assumption that S4 itself does not require.
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2.
S4 permits variable domain semantics (as Kripke's 1963 semantics shows), so Trans(Π) ∪ ΔS4 may validate inferences S4 itself rejects under anti-Barcan conditions.
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Reasons Against
1 perspective
Reason against
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1.
Translations of axioms T and 4 are theorems of ΔS4
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2.
The translation preserves the deductive structure of S4
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