Well I stole it from Roger Penrose (same Penrose as in the title article) which I think he presented in either The Emperor's New Mind or Shadows of the Mind. I think its the latter book which has more concrete arguments like this diagonalization argument. Also, the idea that collapse of the wave function could be mediated by virtual gravitons or something like that once the superposition state got sufficiently "massive" (which is the genesis of the ideas in the title article here).

Both are worthwhile if you like thinking about these kinds of things.

I've generally found nearly zero people who have read those books and of them most people poo-poo it all, and don't really engage with some of the issues presented in them.

He may be wrong. Einstein was also wrong about the EPR paper. But it seems like he's trying to engage with the problems and do science rather than just provide philosophical justification.