Why are living things alive? As a theoretical biologist, Robert Rosen saw this as the most fundamental of all questions-and yet it had never been answered satisfactorily by science. The answers to this question would allow humanity to make an enormous leap forward in our understanding of the principles at work in our world. For centuries, it was believed that the only scientific approach to the question 'What is life?' must proceed from the Cartesian metaphor (organism as machine). Classical approaches in science, which also borrow heavily from Newtonian mechanics, are based on a process called 'reductionism.' The thinking was that we can better learn about an intricate, complicated system (like an organism) if we take it apart, study the components, and then reconstruct the system-thereby gaining an understanding of the whole. However, Rosen argues that reductionism does not work in biology and ignores the complexity of organisms. Life Itself, a landmark work, represents the