 Methodology. Unity has long been understood as a methodological principle, primarily, but not exclusively, in reductionist versions (Wimsatt 1976 and Wimsatt 2006 for the case of biology and Cat 1998 for physics). This is different from the case of unity through methodological prescriptions. One methodological criterion appeals to the epistemic virtues of simplicity or parsimony, whether epistemological or ontological (Sober 2003). As a formal probabilistic principle of curve-fitting or average predictive accuracy, the relevance of unity is objective. Unity plays the role of an empirical background theory.