  Because they play such a central role in the Mohist conception of cognition and reasoning, fa can be treated as a key to understanding the Mohists’ approach not only to ethics, but to epistemology, semantics, and logic or argumentation. Action, linguistic communication, knowledge, and reasoning are all explained by appeal to the practical activity of drawing distinctions according to public norms. Fa are models or standards that aid us in following such norms reliably. In ethics, they take the place of universal ethical principles in moral reasoning and in guiding action. In epistemology, they are public criteria that guide distinction drawing and thus justify claims to knowledge. In semantic theory, they are part of the explanation of why words refer to what they do: a word is associated with a fa (exemplar) by convention and then refers to everything deemed relevantly similar to the fa. In reasoning or argument, a judgment is established or refuted by citing one or more fa and showing that the object of the judgment does or does not conform to the fa.