Many political libertarians argue, or assume, that negative moral duties (duties not to harm others) prevail over positive moral duties (duties to aid others), and that the legal system ought to reflect such pre-eminence. I call into question this strategy for defending a libertarian order. I start by arguing that a successful account of the well-known case of a runaway trolley that is about to kill five innocents unless a passer-by diverts it onto one innocent, killing him, should point to (...) (i) the ex ante advantage to all six of being subject to a policy of redirection of runaway trolleys, and (ii) the causal structure of killing vs. letting-die choices. I then argue that this account of the trolley case entails that legal systems reflecting the relative stringency of negative and positive moral duties should uphold redistributive measures at odds with libertarianism. The assumption that the legal system ought to reflect, through non-causal routes, moral principles and their relative weights leads to either an ideal-theory (in Rawls's sense) assessment of libertarianism or a symbolic account of the relationships between morality and law. Libertarians should undermine this assumption if they hope to offer an all-things-considered case for free markets. (shrink)