I had a flashback scene of Diotima's past which sounded very pointedly like it would apply to Youmu's argument. I believe it even said I got that exact Idea. But when I went to present it to her, and the Idea just wasn't there. Which is frustrating, because the game was enjoyable up to that point...
...aside from the times that I presented seemingly applicable arguments and lost Credibility because Diotima couldn't figure out the connection there, and the fact that this is a jam game with no save slots, meaning that running out of Credibility meant I had to start over from the beginning.
I really want to see how the game ends, but I'm not sure I want to replay most of the game over and over until I figure out what arguments are right and how to not lose the important ideas.