  The commentary on Aristotle’s On the Soul is apparently the earliest to contain passages in which Philoponus  at times abandons conventional exegesis in order to criticize an Aristotelian doctrine, a tendency which becomes more and more conspicuous in the Physics commentary, dated to 517.  Scholars now believe that at least some of these deviations from commentary proper derive from later revisions of earlier expositions in the classroom.  One can distinguish between two kinds of criticism: substantive modification of Aristotelian ideas on the one hand, and outright rejection on the other. Philoponus’ commentary On the Soul presents a good example of the former kind of criticism. In On the Soul II 7 Aristotle takes light to be incorporeal, and he describes its appearance as an instantaneous transition from the potentiality (dúnamis) of a medium to be transparent to the actuality (enérgeia) of transparency. At pp. 330ff of his commentary, Philoponus offers a constructive kind of criticism that seems to have had momentous consequences for his eventual conceptualization of the theory of the impetus (see below). Philoponus contends that Aristotle’s view fails to account both for the laws of optics and for the plain phenomenon that the region below the moon is warmed by the light of the sun, a celestial body. In an Aristotelian spirit of modifying the theory so as to save the phenomena, he proceeds to re-interpret the term enérgeia not as a state of actuality, but rather as an ‘incorporeal activity’ which, besides constituting the transparency of the medium, is also capable of warming bodies. Philoponus argues that in the same way soul, as an incorporeal activity in the body, causes the animal to be warm. Due to this novel interpretation of Aristotle’s terminology, light is now understood not statically but as something dynamic. And it is possible to trace Philoponus’ further development of the idea. In the Meteorology commentary, which belongs to a stage when Philoponus had already abandoned the Aristotelian assumption of an immutable celestial fifth element (ether), he argues that light and heat may be explained best as consequences of the nature of the sun, which is fire (In Meteor. 49). Heat, so he explains, is generated when the rays emanating from the sun are refracted and warm the air through friction.