I can't figure out how the model solution of tut01 question 2.3 follows the principles of proof by contradiction. I have the idea that one starts with p and introduce a contradiction to derive not p.
In the solution, q is derived seemingly from thin air.
What am I missing?
q is derived from thin air due to the "bottom elimation" rule (see page 20 in the textbook) and the discussion of why this is valid on page 21, on page 7 of the study guide and in an earlier thread on contradiction