 This idea was taken up by Cartwright and others. Cartwright’s discussion of cp-laws focused on the question why cp-laws are of interest in non-ideal situations. If what cp-laws say is confined to special, namely ideal, circumstances it seems prima facie that these laws are irrelevant for all other, non-ideal, situations. That, however, is not the case in scientific practice. Cp-laws are used to explain phenomena outside the ideal circumstances. Cartwright argues that this practice requires the postulation of capacities or tendencies: