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text from Terry Pratchett; art by Higgs who's tumblr i've linked
i can't rember which book this is from -- i wanna say it's Men at Arms but it could even be Guards Guards. Higgs seems to think it's Night Watch, but I'm pretty sure it happened before that...
in the post here
The theory isn't explaining why some people are rich, it's explaining how expensive it is to be poor by showing that the rich can end up spending less on things that everyone needs over time.