  The term fa denotes any guide, aid, or tool for following a norm or making a judgment. Explicit rules, laws, and definitions are all fa. But fa may also be role models, such as a virtuous parent, teacher, or ruler. They may be prototypes, exemplars, analogies, or pictures of some kind of thing, or they may be tools or measuring devices, such as a yardstick or a carpenter’s compass and set square. They may even be concepts, such as “the benefit (lì) of the world,” which the Mohists take to be co-extensive with and thus a criterion for identifying what is yi (morally righteous). In short, any criterion or paradigm that helps us to make correct judgments or to act correctly can be a fa.