Description: Existential quantification of negation is equivalent to negation of universal quantification. Dual of alnex . See also the dual pair df-ex / alex . Theorem 19.14 of Margaris p. 90. (Contributed by NM, 12-Mar-1993)
| Ref | Expression | ||
|---|---|---|---|
| Assertion | exnal | |- ( E. x -. ph <-> -. A. x ph ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | alex | |- ( A. x ph <-> -. E. x -. ph ) |
|
| 2 | 1 | con2bii | |- ( E. x -. ph <-> -. A. x ph ) |