Natural philosophy interacted with many other disciplines. The close relationship between natural philosophy and medicine had already been stressed by Aristotle himself at the beginning of On Sense and Sensible (436a19–436b2). Medicine often competed with natural philosophy within the universities: philosophy was a curricular requirement for those who wanted to study medicine in the Italian universities and many of the greatest Renaissance natural philosophers were also physicians (e.g., Alessandro Achillini (1463–1512) and Simone Porzio (1496–1554); there were also professional physicians who wrote on natural philosophy, such as Daniel Furlanus (d. 1600)). Ubi desinit philosophus (or physicus), incipit medicus (“where the philosopher ends, the physician begins”): so went a proverb which implied an ambiguous boundary between the two disciplines: on the one hand, it reflected the need to move beyond the theory represented by philosophy and into the actual practice of medicine; on the other, it affirmed the idea that natural philosophy was necessary in order to prepare for medical studies. From this perspective, natural philosophy represented either a mere preparatory stage on the way to the more perfect and concrete knowledge of medicine, or, alternatively, medicine was subordinate to natural philosophy (others, like the philosopher Jacopo Zabarella (1533–1589), preferred to distinguish natural philosophy from medicine because these two disciplines did not share subject and method).