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Cubitt_Sugden_baseline.lean

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@@ -156,7 +156,7 @@ Like `A1`, this cannot be derived from primitive operators.
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Vromen proves it as a theorem from the compositional structure of reason terms.
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-/
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variable (A6 : ∀ {i j : indiv} {u : Prop}, Ind A i (R j A) ∧ R i (Ind A j u) → Ind A i (R j u))
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variable (A6 : ∀ {i j : indiv} {p: Prop}, Ind A i (R j A) ∧ R i (Ind A j p) → Ind A i (R j p))
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/-!
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---
@@ -199,9 +199,9 @@ variable (C3 : ∀ i : indiv, Ind A i φ)
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/-!
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### Condition `C4`: Shared Inductive Standards
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If `A` indicates `u` to `i`, then `i` has reason to believe that `A` indicates `u` to `j`.
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If `A` indicates `p` to `i`, then `i` has reason to believe that `A` indicates `p` to `j`.
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> **Formula:** `Ind A i u → R i (Ind A j u)`
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> **Formula:** `Ind A i p → R i (Ind A j p)`
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This formalizes shared reasoning standards—agents know they reason alike.
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This formalizes Lewis' assumption about "common inductive standards and background information".
@@ -212,7 +212,7 @@ necessary - we only need it for certain types of propositions. But it
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simplifies the formalization.
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-/
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variable (C4 : ∀ {i j : indiv} {u : Prop}, Ind A i u → R i (Ind A j u))
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variable (C4 : ∀ {i j : indiv} {p : Prop}, Ind A i p → R i (Ind A j p))
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include A1 A6 C1 C2 C3 C4
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@@ -349,7 +349,7 @@ The R-closure of `φ` is the smallest set of propositions that:
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inductive RC (φ : Prop) : PropProp where
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| base : RC φ φ
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| step {u : Prop} (j : indiv) (hu : RC φ u) : RC φ (R j u)
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| step {p : Prop} (j : indiv) (hu : RC φ p) : RC φ (R j p)
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/-!
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### Key Lemma: Indication Propagates Through R-Closure
@@ -366,7 +366,7 @@ indicate not just `φ`, but all its higher-order iterations.
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-/
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omit A1 C1 in
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lemma rc_implies_indication {i : indiv} {q : Prop} (h : @RC indiv R φ q) : Ind A i q := by
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lemma indication_closure {i : indiv} {q : Prop} (h : @RC indiv R φ q) : Ind A i q := by
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induction h with
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| base => exact C3 i
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| @step v j _ ih =>
@@ -388,11 +388,12 @@ in `P` that `x`, then `r^P(x)` is true."
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**Significance:** Common knowledge is not an infinite regress (circular) but an infinite *consequence*of finite, non-circular conditions.
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-/
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lemma everyone_reason_of_rc (hp : @RC indiv R φ p) : ∀ i : indiv, R i p := by
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lemma lewis_theorem (hp : @RC indiv R φ p) : ∀ i : indiv, R i p := by
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intro i
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have hInd : Ind A i p := rc_implies_indication A φ A6 C2 C3 C4 hp
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have hInd : Ind A i p := indication_closure A φ A6 C2 C3 C4 hp
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exact A1 hInd (C1 i)
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end Lewis
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/-!

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