Boasian anthropology was based on the four-field model of anthropology uniting the fields of cultural anthropology, linguistic anthropology, physical anthropology, and archeology under the umbrella of anthropology. It was based on an understanding of human cultures as malleable and perpetuated through social learning, and understood behavioral differences between peoples as largely separate from and unaffected by innate predispositions stemming from human biology—in this way it rejected the view that cultural differences were essentially biologically based. It also rejected ideas of cultural evolution which ranked societies and cultures according to their degree of "evolution", assuming a single evolutionary path along which cultures can be ranked hierarchically, rather Boas considered societies varying complexities to be the outcome of particular historical processes and circumstances—a perspective described as historical particularism.