AIUI "invention" is simply a subset of "idea". In practice the notion of "subset" is becoming increasingly irrelevant, everywhere in the world. As for the quote, yes, it's a great one. In my opinion it is one of the best arguments against patents in the real world - they don't actually protect just the perspiration. If someone else has the same inspiration independently and also does all the perspiration, they're still legally prohibited from profiting from it.
Copyright is for protecting the perspiration - patents protect the inspiration. I'm not a big fan of copyright either, but I suppose I could conceive a copyright system that would at least be worth considering, but whilst a patent system satisfying my constraint a) might be conceivable, I cannot see one that satisfies b), not even with c).
Take medicines as an example - why should a company be barred from taking your patent, doing all the safety studies and so on (thereby doing the perspiration), and then selling the drug? Sure it'd be inefficient, but these kind of inefficiencies are a given in capitalism. Its point is competition - who can do the same task most efficiently.