This notion of the body as a machine would clear away centuries of intellectual detritus: By arguing that the body was the sum of its interacting parts, and, more importantly, by suggesting that the study of those parts would reveal the workings of the body, Descartes shifted centuries of scientific and philosophical discussion about imponderable life forces and inexplicable animist vapors.

The number of elements that compose any living system—an ecosystem, an organism, an organ or a cell—is enormous. In living systems, the specific identities of these component parts matter. Unlike chemistry, for instance, in which an electron in a lithium atom is identical to an electron in a gold atom, all proteins in a cell are not equivalent or interchangeable. Each protein is the result of its own evolutionary trajectory.