  Reductionism has many meanings. For example, the phrase genetic reductionism concerns the idea that all biological phenomena are caused by genes, and hence presupposes an ontological sense of reductionism according to which one kind of micro-entity (in this case, gene) exclusively causes a variety of higher-level phenomena (in this case, biological features, cultural phenomena, and so forth). But this is not the meaning of reductionism at issue in the philosophical literature about the reduction of classical genetics. This literature is more concerned with epistemology than metaphysics. The concept of reductionism at issue is Nagel's concept of theoretical reduction. (See Sarkar 1992 and Schaffner 1993 for discussions of alternative conceptions of reduction.) According to Nagel's concept, the reduction of one science to another science entails the reduction of the central theory of one science to the central theory of the other. Nagel believed that this kind of theoretical reduction led to progressive changes in scientific knowledge. He formulated two formal requirements for theoretical reductions.