Description: The fourth of four axioms in the Russell-Bernays axiom system. (Contributed by Anthony Hart, 13-Aug-2011) (Proof modification is discouraged.) (New usage is discouraged.)
| Ref | Expression | ||
|---|---|---|---|
| Assertion | rb-ax4 | |- ( -. ( ph \/ ph ) \/ ph ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | pm1.2 | |- ( ( ph \/ ph ) -> ph ) |
|
| 2 | 1 | con3i | |- ( -. ph -> -. ( ph \/ ph ) ) |
| 3 | 2 | con1i | |- ( -. -. ( ph \/ ph ) -> ph ) |
| 4 | 3 | orri | |- ( -. ( ph \/ ph ) \/ ph ) |