Mathematician Robert Rosen tackled this question but reframed it in the process. In his 1991 book, "Life Itself", Rosen suggests that a better question is, "Why are organisms different than machines?" His answer addresses the unfractionability, or self-causing holism, of life; he states "a material system is an organism if, and only if, it is closed to efficient causation." The supporting work behind this definition of life embodies his "relational theory of systems". The scientific paradigm behind this theory represents a radical departure from the mainstream mechanical and reductionist paradigm dating back to Newton and Descartes.