After all, it is not as though people would stop believing each other simply because it is known that people lie when doing so will save lives. For one thing, that situation rarely comes up—people could still be telling the truth almost all of the time. Even the taking of human life could be justified under certain circumstances. Take self-defense, for example. There appears to be nothing problematic with the rule “It is permissible to kill when doing so is the only available means of defense against an attacker”.
After all, it is not as though people would stop believing each other simply because it is known that people lie when doing so will save lives. For one thing, that situation rarely comes up—people could still be telling the truth almost all of the time. Even the taking of human life could be justified under certain circumstances. Take self-defense, for example. There appears to be nothing problematic with the rule “It is permissible to kill when doing so is the only available means of defense against an attacker”.