This is a silly request, but it has been my white whale for a while. For the past few years I've set up every proof by contradiction with "Presume, foolishly, .

This is a silly request, but it has been my white whale for a while. For the past few years I've set up every proof by contradiction with "Presume, foolishly, ...". I thought I got it from this series of stories, but every once in a while I give them a close read and fail to find anything like it. Maybe I saw it in an HN comment? Anyone remember something like that?