Simplification gives .
But gives .
Establish the validity of this argument by means of an unmixed indirect derivation.
~(P-->Q)-->Q
P-->Q
Attempt:
1. Show P --> Q
2. ~ (P -->Q) Assumption( Indirect Derivation)
3. ~ (P -->Q) --> Q
4. Q
Remark: I cannot find the contradiction, and am not sure whether line 4 can be used for Indirect Derivation.