For example, assume two entities P and Q, where we are using 'proof-by-contradiction' to validate P (by using Q).
Something like:
IMO, one can only use such scenario, when P and Q are independent of each other existentially.
In other words, can one use proof-by-contradiction in cases where Q's existence is dependent on P's validity?
For example, Q exists if and only if P exists.
In such case does the proof-by-contradiction still hold valid?