It is immediately obvious that he is attempting proof by contradiction, because the first line supposes the opposite of the title of the paper. As in the proof

It is immediately obvious that he is attempting proof by contradiction, because the first line supposes the opposite of the title of the paper. As in the proof of the irrationality of the square root of 2, he sets up his assumption that pi is rational by defining it as a/b. In a proof, we don't have to know why something is done, just that it logically follows from our assumptions and is used as a step in the proof. Explaining how the author came up with the proof should be left to an educational textbook or an "Author's Notes" if the proof is several pages long and would help readers be guided along.