**Statement:** Sign-flip requires operator external to the system.
Theorem ([[073_A9.1_External-Intervention-Required|A9.1]]): If sign-flip occurs and self-operations preserve sign, then external intervention occurs.
Proof:
1. Let S be a moral system with sign σ(S) ∈ {+1, -1}
2. Let F = {f : S → S | f is self-generated}
3. By [[068_A8.2_Sign-Conservation|A8.2]]: ∀f ∈ F, σ(f(S)) = σ(S)
4. Suppose σ(S_t1) = -1 and σ(S_t2) = +1 for t2 > t1
5. Then ∃ operation g: S_t1 → S_t2 with σ(g(S)) ≠ σ(S)
6. By 3: g ∉ F (g is not self-generated)
7. Definition: g ∉ F means g is external
8. Conclusion: External intervention occurred ∎