http://www.youtube.com/watch?v=lwaNZgY9PCQ&feature=player_embeddedThe Tragedy of the Commons is a (false) ASSUMPTION that EVERYONE is GREEDY and will exploit nature to the point of exhaustion even though it will ultimately destroy nature AND bring about starvation of the "greedballs". It's the old, "EVERYBODY is going to do it so I might as well do it before they do!" predatory capitalist RESPONSIBILITY DODGE.
The TRUTH about the ACTUAL COMMONS in England was QUITE different. When viability, NOT MAXIMUM EXPLIOTATION (as in modern predatory psychopathic capitalism) is the ruling principle, the COMMONS works quite well as it did in England for centuries until the land owners got super greedy with the dawn of the industrial revolution and DELIBERATELY began to overgraze the land (that had hitherto been shared by the poor commoners) with backing by the bought-and-paid for parliament that invented the land grab called the enclosure laws.
The video explains all this better than I do but the main thing for you to remember is to yell BULLSHIT the next time you hear some libertarian or predatory capitalist cry crocodile tears about the "Tragedy" of the Commons.
