The point is that in order to prove something by contradiction, you must assume something that turns out to be false and see what you can derive from that assum

The point is that in order to prove something by contradiction, you must assume something that turns out to be false and see what you can derive from that assumption (ideally a contradiction). The last link in the sibling comments does indeed contain proofs that the given premise is impossible. Think about how you know that the square root of 2 is irrational.