The main objective of this book is to develop a theory of functional biology incorporating the notion of the unity of living organisms. This approach is meant to establish the principles underlying the functioning of living organisms, and to explain how life arose from nonliving matter. The theory is based on a new representation in terms of the novel concept of functional interactions that serve as elementary building blocks to construct the entire edifice, and a new formalism, the S-propagator. Although the heuristic method used calls for highly abstract, mathematical concepts, let the reader have no fear - scarcely any mathematics will appear in what follows. Just as physics uses mathematics to provide a general view of the nonliving world, biology has to rely on mathematical formalism to obtain an integrated vision of living organisms. Furthermore, it is shown how the use of mathematical tools helps to deduce the evolutionary possibilities of a given species from available physiological knowledge. The determination of the characteristics of life in nonliving matter requires that biology be separated from physics, while still recognizing that biological phenomena are based on physical substrates.