When is a Game a Clone?

Raph Koster has some musings on the topic of originality in games on his blog this week. He explores basic notions of games and their variants, looking at them from the point of view of rules and variants. This is kind of amusingly academic to someone like me, who is a major gamer, but remember that games and game clones have led more than a few people to be up in arms over intellectual property. (h/t Boingboing for the original pointer)