A robot may not injure humanity or, through inaction, allow humanity to come to harm.

In Robots and Empire (pg. 63), the "Zeroth Law" is extrapolated, and the other Three Laws modified accordingly. Unlike the Three Laws, however, the Zeroth Law is not a fundamental part of positronic robotic engineering, is not part of all positronic robots, and, in fact, requires a very sophisticated robot to even accept it.