Can two individuals, each of whom needs a certain resource for his survival, have equal and conflicting rights to that resource? If so, is each entitled to try to exclude the other from its use? An old chestnut of moral and legal philosophy raises the problem. Following a shipwreck, two men converge simultaneously on a plank floating in the sea. There is no other plank available and no immediate hope of rescue. Unfortunately the plank can support only one; it sinks if two try to cling to it. Is it permissible for each to attempt to secure his own survival by pushing the other off the plank?