Fa is a concept in Chinese philosophy that concerns aspects of ethics, logic, and law. Although it can be accurately translated as 'law' in some contexts, especially modern Chinese, it refers to a 'model' or 'standard' for the performance of behavior in most ancient texts, namely the Mozi, with a prominent example including the performance of carpentry. Although theoretically earlier, Fa comes to prominence in the Mohist school of thought. An administrative use of fa standards is prominently elaborated in Legalism, but the school of names also used fa (models) for litigation. Given its broadness, the term fa even included medical models (theories).

Fa was still considered important by Warring States period Confucians. Xunzi, whose work would ultimately be foundational to Confucian philosophy during the Han dynasty, took up fa, suggesting that standards could only be properly assessed by the Confucian sage (聖; shèng), and that the most important fa were the very rituals that Mozi had ridiculed for their ostentatious waste and lack of benefit for the people at large.

In Han Fei's philosophy, the king is the sole source of fa (including 'law'), taught to the common people so that there would be a harmonious society free of chance occurrences, disorder, and "appeal to privilege". High officials were not to be held above the fa, nor were they allowed to independently create their own fa, uniting both executive fiat and rule of law.

Despite a usage by Shang Yang including penal law, in Imperial China fa more commonly referred to government institutions, such as for agricultural loans, than to law per se. The Qin dynasty differentiated the body of statues under the term lu 律, though lu does include a similar root meaning of measurement (like Metre). Originally meaning pitch pipe, it referred to a chromatic scale subdividing four solar seasons as twelve, with much of Qin law being administrative.

The concept of fa first gained importance in the Mohist school of thought. To Mozi, a standard must stand "three tests" in order to determine its efficacy and morality. The first of these tests was its origin, as to whether the standard had precedence in the actions or thought of the mythological sage kings of the Xia dynasty, whose examples are frequently cited in classical Chinese philosophy. The second test was one of validity; does the model stand up to evidence in the estimation of the people? The third and final test was one of applicability; this final one is a utilitarian estimation of the net good that, if implemented, the standard would have on both the people and the state.

The third test speaks to the fact that to the Mohists, a fa was not simply an abstract model, but an active tool. The real-world use and practical application of fa were vital. Yet fa as models were also used in later Mohist logic as principles used in deductive reasoning. As classical Chinese philosophical logic was based on analogy rather than syllogism, fa were used as benchmarks to determine the validity of logical claims through comparison. There were three fa in particular that were used by these later Mohists to assess such claims, which were mentioned earlier. The first was considered a "root" standard, a concern for precedence and origin. The second, a "source", a concern for empiricism. The third, a "use", a concern for the consequence and pragmatic utility of a standard. These three fa were used by the Mohists to both promote social welfare and denounce ostentation or wasteful spending.

Described with reference to the square, compass or plumb-line, Mozi used fa in the sense of models and standards for copy and imitation in action. As in Confucianism, Mozi's ruler is intended to act as the fa (or example) for the nobles and officials. From this, the concept of fa develops towards political technique.

Illustrated by the scale, grain-leveler and ink and line, together with a benevolent heart, Mencius's ruler will not achieve effective results without fa. A late contemporary of Shen Buhai and Shang Yang, Mencius's fa more broadly represents models, exemplars and names. Amongst other categories, including techniques of the heart-mind, Mencius's fa includes more specific examples of physical statistics such as temperatures, volumes, consistencies, weights, sizes, densities, distances, and quantities.