And playing chess relies on your opponent also following the rules of chess, so aren't we free to play chess either? (Or will the police arrest me if I castle while my king is in check?)
Chess can rely on reputation since games are short and someone who refuses to play by the rules will find no one to play against (also the stakes are small). (And in high states games, e.g., tournaments you will at least be escorted out for refusing to play by the rules.) Note, that since monogamy assumes someone will only have one spouse ever, reputation is less useful.
A better example is, are we free to engage in commerce if the police refuse to enforce either non-payment or non-delivery? (Especially if they do enforce anti-violence and anti-theft law...
Another month has passed and here is a new rationality quotes thread. The usual rules are: