The subject of the article is the interpretation of a series of experiments proving the existence of vacuum. This was performed by the Capuchin Valerian Magni in 1647 and described in the treatise Demonstratio ocularis, which is the first printed text referring to successful experiments with vacuum. The work generated great controversy at the time, not only with opponents of void, but also with French scholars, who accused Magni of plagiarism. The article reconstructs both the situation around the work’s publication and the reaction to it, with an aim of presenting the philosophical background behind Magni’s experiments. Magni understood the experiments as confirmation of his anti-Aristotelianism, and placed them among his metaphysics and natural philosophy, in which he attributes the key epistemological and ontological function to light.