Theorem mt3d

Description: Modus tollens deduction. (Contributed by NM, 26-Mar-1995)

	Ref	Expression
Hypotheses	mt3d.1	|- ( ph -> -. ch )
	mt3d.2	|- ( ph -> ( -. ps -> ch ) )
Assertion	mt3d	|- ( ph -> ps )

Proof

Step	Hyp	Ref	Expression
1		mt3d.1	|- ( ph -> -. ch )
2		mt3d.2	|- ( ph -> ( -. ps -> ch ) )
3	2	con1d	|- ( ph -> ( -. ch -> ps ) )
4	1 3	mpd	|- ( ph -> ps )