This chapter opens with the discussion of vital unity, a problem at the intersection of medicine and philosophy. Broadly speaking, medicine is concerned with the preservation of a living whole, but for many ancient thinkers, like Galen, the practice of medicine was informed by a highly theoretical understanding of the relationship between the parts and the whole. The first section of the Introduction sets out the key preoccupation of the study: Galen’s understanding of the role that different body parts and systems have in maintaining the functioning of the living whole. Subsequent sections contextualise Galen’s work within the phusiologia tradition, as well the debate between empiricism and rationalism, and briefly outline key classifications to be discussed in later chapters, before turning to the content of individual chapters, situating the present study within the existing scholarship and, finally, briefly explaining how this work approaches the much-debated problem of the substance of the soul.