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Related theorems GIF version |
| Description: Principle of identity. Theorem *2.08 of [WhiteheadRussell] p. 101. For another version of the proof directly from axioms, see id1 60. (The proof was shortened by Stefan Allan, 20-Mar-06.) |
| Ref | Expression |
|---|---|
| id | ⊢ (φ → φ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | ax-1 4 | . 2 ⊢ (φ → (φ → φ)) | |
| 2 | ax-1 4 | . 2 ⊢ (φ → ((φ → φ) → φ)) | |
| 3 | 1, 2 | mpd 26 | 1 ⊢ (φ → φ) |