Biologists studying complex causal systems typically identify some factors as the causes and treat other factors as background conditions.  When geneticists explain biological phenomena, they often foreground genes as causes and the cellular milieu as background.  But factors in the milieu are as causally necessary as genes for the production of phenotypic traits, even traits at the molecular level such as amino acid sequences.  Gene-centered biology has been criticized on the grounds that because there is a parity among causes, the privileging of genes reflects a reductionist bias, not a difference based in reality.  The idea that there is an   ontological parity among causes is related to a philosophical puzzle identified by John Stuart Mill:  what, other than our interests or biases, could possibly justify identifying some causes as the actual or operative ones, and others causes as mere background?  The aim of this paper is to solve this conceptual puzzle.  It turns out that solving that puzzle helps answer a seemingly unrelated philosophical question:  what kind of causal generality matters in biology?