Throughout history philosophers have posited souls, vital spirits and other active principles in living beings in order to explain their differences from non-living beings. With the arrival of genetics and evolutionary biology, however, it now seems possible to account for these differences without assuming such principles. Living beings are henceforth to be understood mechanically, as products of self-replicating microscopic objects and selective environmental conditions, rather than pneumatically, in terms of active principles. It is very remarkable, therefore, that physics has since...