This rule can be used forwards to insert a formula and prove it by contradiction or used backwards on a goal line to prove the goal line by contradiction (i.e. assume the negation of the goal formula and prove that you can reach a contradiction (bottom) inside a box).

1. select both the empty line and PC.
2. then enter the formula you would like to prove by contradiction when prompted. This formula must not contain any predicate symbols, function symbols or constants that are not in the signature of the box you are working in.
3. a new box will then be added to your proof which assumes the negation of the formula to wish to insert and has a conclusion of bottom.

1. select both the goal line you wish to prove by contradiction and PC.
2. a new box will then be added to your proof which assumes the negation of the goal formula and has a conclusion of bottom.