What we now call ‘deep venous thrombosis’ (DVT) has been studied in diverse ways during the last 200–300 years. Each of these approaches contributes to a full modern understanding of aetiology. Therefore, much of this book is a historical survey of the field. However, our remit is broader than the title might suggest: the evolution of ideas about DVT is typical in many ways of medical biology as a whole. Thus, although the aetiology of DVT may seem a narrow topic for a monograph – it implicitly excludes arterial thrombosis and marginalises prophylaxis, therapy, and even such clinically significant sequelae as pulmonary embolism – we hope to engage the reader in a much more general inquiry. Our historical investigation reveals a 160-year-old schism between two contrasting philosophies of medical and biological research, a schism that is particularly – but by no means uniquely – relevant to the study of DVT. In principle, these philosophies should be complementary rather than competing. So while we wish to elucidate the aetiology of DVT per se, we are also concerned with a more abstract and wide-ranging issue: the future accommodation or rapprochement between two conceptual and methodological traditions.