JES
on that question, I think my answer would be "you can't".
I think that's part of my point, i.e. that the principles of gca --like those of playwriting etc etc -- have existed and have been tried and tested and proved sound for so long that they can be said now to exist independently of any particular course, as 'facts' in and of themselves.
Now, it wasn't ME who recognized/determined those principles, it was people who'd dedicated their lives to gca and its study and who came to understand them over decades and centuries; I'm just trying to learn them and, once learned, to accept them as such. (I didn't discover nor can I fully comprehend the law of gravity, so I can't explain it to you; but when an apple falls off a tree and smacks me on the head, I can tell you that gravity was at work).
This all might sound elitist or something, but I don't think it is. We live our lives everyday based in part of the 'authority' of others. We don't ask whether the principles of structural engineering are 'true' and reliable, we just assume that they are, and that the buildings we work in won't fall in around us. Basically, we're 'taking the word' of someone else; but that's all I've got to go on. And so when architectural experts for 7 decades have agreed that PV manifests the principles of great architecture (principles those experts didn't 'invent' either), well, that's enough for me.
Peter