Theorem mt2d 109

Description: Modus tollens deduction. (Contributed by NM, 4-Jul-1994.)

Hypotheses

Ref	Expression
mt2d.1	|- (ph -> ch)
mt2d.2	|- (ph -> (ps -> -. ch))

Assertion

Ref	Expression
mt2d	|- (ph -> -. ps)

Proof of Theorem mt2d

Step	Hyp	Ref	Expression
1		mt2d.1	. 2 |- (ph -> ch)
2		mt2d.2	. . 3 |- (ph -> (ps -> -. ch))
3	2	con2d 107	. 2 |- (ph -> (ch -> -. ps))
4	1, 3	mpd 14	1 |- (ph -> -. ps)

This proof depends on syntax axioms:  -. wn 3   -> wi 4

This proof depends on axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8

This theorem is used by:  mt2i  110  nsyl3  111