According to Faye, there are various explanations for why Bohr believed that classical concepts were necessary for describing quantum phenomena. Faye groups explanations into five frameworks: empiricism (i.e. logical positivism); Kantianism (or Neo-Kantian models of epistemology); Pragmatism (which focus on how human beings experientially interact with atomic systems according to their needs and interests); Darwinianism (i.e. we are adapted to use classical type concepts, which Léon Rosenfeld said that we evolved to use); and Experimentalism (which focuses strictly on the function and outcome of experiments that thus must be described classically). These explanations are not mutually exclusive, and at times Bohr seems to emphasize some of these aspects while at other times he focuses on other elements.