In a couple of papers, Chisholm tried to show how the central concepts of axiology and deontology can be defined by appeal to this one fundamental concept of requirement. These papers provide beautiful illustrations of one of Chisholm’s most characteristic philosophical procedures — the clear definition of a tangle of related concepts by appeal to a small set of undefined primitives. In this instance, he tries to explain confusing and problematic concepts in the realm of ethics. He makes use of just a few undefined concepts. One of them —  requirement — seems to carry all of the distinctively normative burden. (It would be interesting to compare Chisholm’s efforts on behalf of requirement in ethics with his efforts on behalf of more reasonable than in epistemology. The projects seem to illustrate the same methodological propensity.)