I think he meant that he tried to trisect an angle in general, by construction; this has been proven impossible.
Yes, but there are some constructions like Archimedes's use of a marked ruler (which is covered, actually, in the 'Means to trisect angles by going outside the Greek framework' section) which work correctly & are not immediately obviously breaking the rules. So I had to ask before I could know whether he had broken the rules or broken his proof (if you follow me).
A monthly thread for posting rationality-related quotes you've seen recently (or had stored in your quotesfile for ages).