 If \(S\) and \(S'\) are specific universals and \(G\) is a determinable or generic universal under which they both fall, then the causal relations between \(G\) and other universals are a subset of the causal relations between \(S\) and other universals; and also a subset of the causal relations between \(S'\) and other universals. For \(S\) and \(S'\) to be species of a common genus is for them to share some subset of causal relations, which subset can itself constitute the causal essence of a universal, such as \(G\). For \(S\) and \(S'\) to be distinct species is for this subset to be a proper subset of the causal relations which characterize \(S\), and also of the causal relations which characterize \(S'\): there must be some causal relation which \(S\) has, but not \(S'\), and some causal relation which \(S'\) has, but not \(S\). (1990: 175)