The view that so-called neutral bodies (neutrum), neither healthy nor ill, exist was first introduced in the Galenic Ars medica at the turn of the third century. Since this book became a medical university textbook in the thirteenth century, the idea of neutral bodies was systematically studied in medicine up to the early seventeenth century. This study, which introduces their various definitions and relation to the concepts of health and illness in scholastic and Renaissance medicine, is the first systematic investigation of the conceptual history of a neutral body. The main focus of this study is on the Renaissance with the special interest in the humanist impact on medicine. Scholastic and humanist ideas of neutrum also interestingly resemble some modern views of the nature of health.