 Cartwright explicitly calls Snell’s law a cp-law. Many laws we take to be strict universal laws are in fact cp-laws that hold in special circumstances only. According to Cartwright, the problem arises that these laws cover only special or ideal conditions (most media are anisotropic), whereas in scientific practice they are used to explain phenomena in non-ideal situations as well. Cp-laws seem to play an essential role in explanations but there is no account available of how cp-laws can do this work.