Carving Nature at its Joints? In order to map the future of biology we need to understand where we are and how we got there. Present day biology is the realization of the famous metaphor of the organism as a bete machine elaborated by Descartes in Part V of the Discours, a realization far beyond what anyone in the seventeenth century could have im- ined. Until the middle of the nineteenth century that machine was an articulated collection of macroscopic parts, a system of gears and levers moving gasses, solids, and liquids, and causing some parts of the machine to move in response to the force produced by others. Then, in the nineteenth century, two divergent changes occurred in the level at which the living machine came to be investigated. First, with the rise of chemistry and the particulate view of the composition of matter, the forces on macroscopic machine came to be understood as the ma- festation of molecular events, and functional biology became a study of molecular interactions. That is, the machine ceased to be a clock or a water pump and became an articulated network of chemical reactions. Until the ?rst third of the twentieth century this chemical view of life, as re?ected in the development of classical b- chemistry treated the chemistry of biological molecules in much the same way as for any organic chemical reaction, with reaction rates and side products that were the consequence of statistical properties of the concentrations of reactants."

Immunology is central to contemporary biology and medicine, but it also provides novel philosophical insights. Its most significant contribution to philosophy concerns the understanding of biological individuality: what a biological individual is, what makes it unique, how its boundaries are established and what ensures its identity through time. Immunology also offers answers to some of the most interesting philosophical questions. What is the definition of life? How are bodily systems delineated? How do the mind and the body interact? In this Element, Thomas Pradeu considers the ways in which immunology can shed light on these and other important philosophical issues. This title is also available as Open Access on Cambridge Core.

Carving Nature at its Joints? In order to map the future of biology we need to understand where we are and how we got there. Present day biology is the realization of the famous metaphor of the organism as a bete machine elaborated by Descartes in Part V of the Discours, a realization far beyond what anyone in the seventeenth century could have im- ined. Until the middle of the nineteenth century that machine was an articulated collection of macroscopic parts, a system of gears and levers moving gasses, solids, and liquids, and causing some parts of the machine to move in response to the force produced by others. Then, in the nineteenth century, two divergent changes occurred in the level at which the living machine came to be investigated. First, with the rise of chemistry and the particulate view of the composition of matter, the forces on macroscopic machine came to be understood as the ma- festation of molecular events, and functional biology became a study of molecular interactions. That is, the machine ceased to be a clock or a water pump and became an articulated network of chemical reactions. Until the ?rst third of the twentieth century this chemical view of life, as re?ected in the development of classical b- chemistry treated the chemistry of biological molecules in much the same way as for any organic chemical reaction, with reaction rates and side products that were the consequence of statistical properties of the concentrations of reactants."

What counts as an individual in the living world? What does it mean for a living thing to remain the same through time, while constantly changing? These questions are the province of immunology, one of the most dynamic fields in biology. Immunology answers these questions with its theory of "self" and "nonself" which has dominated the field since the 1940s. Thomas Pradeu argues that this theory is inadequate, because immune responses to self constituents and immune tolerance of foreign entities are the rule, not the exception. Instead Pradeu advances an alternative theory, the continuity theory, which offers a new way to answer the question of what triggers an immune response. It also echoes the recent realization that all organisms, and not only higher vertebrates, have an immune system. Pradeu's main thesis is that the self-nonself theory should be abandoned, but that immunology still proves to be decisive for delineating the boundaries of the organism. Articulating an evolutionary and an immunological perspective, he offers an original conception of the organism. Tolerance of the fetus by the mother and of countless bacteria on the body's surfaces proves that every organism is heterogeneous, that is, made of entities of different origins. In other words, every organism appears as a chimera , a mixed living thing-the cohesiveness of which is ensured by the constant action of its immune system. The Limits of the Self , will be essential reading for anyone interested in the definition of biological individuality and the understanding of the immune system.