Some historians of philosophy have insisted on the fact that most medieval philosophers are profoundly realistic. This means that: 1) they accept the existence of an extra-mental world and, 2) they believe that sensory perception presents the world as it is itself. Based on the results of historical-philosophical research, this paper seeks to systematically develop the nature of realism, adopted by most of philosophers of thirteenth and fourteenth century, in order to explain its own characteristics. Comparing the latter with some contemporary versions of realism, I show that its main characteristic is being the starting point of the philosophical reflection.