 A different way of approaching downward causation in biology is to explicate it in terms of higher-level constraints or boundary conditions. The idea is that the behavior of lower-level things is constrained by the higher-level whole that they are a part of, and therefore explaining the behavior of these lower-level things is not possible exclusively at the lower level, but requires appealing to higher-level factors (e.g., Bishop 2008; Campbell 1974; Emmeche et al. 1997, 2000; Kistler 2009; Noble 2012; Sperry 1969). In a classic article, Donald Campbell expressed this idea as follows: “processes at the lower levels of a hierarchy are restrained by and act in conformity to the laws of the higher level” (Campbell 1974: 180). For example, explaining protein folding (at the level of molecules) seems to require appealing to factors in the environment at the cellular level, such as the presence of chaperones (Love 2012).