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This paper investigates what conditions are to be met for sensory perception to occur. It introduces two different theories of perception that were held by two medieval Franciscan thinkers — namely, Roger Bacon (1214/1220–1292) and Peter Olivi (ca. 1248–1298). Bacon analyses especially the causal relation between the object and the sensory organ in his doctrine of the multiplication of species. In his view, a necessary condition of perception is the reception of the species in a fully disposed sensory organ. On the contrary, Olivi stresses the active role of the sensory power. A necessary condition of sensation is the aspectus — i.e. the focus of our power’s attention on the object. Furthermore, the paper investigates whether and how each of the two thinkers can deal with the arguments proposed by his opponent — namely whether Bacon’s theory is able to explain attention and what the causal role of the object in Olivi’s theory is.