 Eells claims that general causal claims must be relativized to a population. A very heterogeneous population will include a great many different background conditions, while a homogeneous population will contain few. A heterogeneous population can always be subdivided into homogeneous subpopulations. It will often happen that C is a mixed cause of E relative to a population P, while being a positive cause, negative cause, or causally neutral for E in various subpopulations of P.