According to Philoponus, the activity of drawing syllogisms is a dynamic operation. Following the classical idea that actions are specified by their objects and habitual powers by their actions, Philoponus concludes that only a dynamic power can elicit the act of syllogizing. This power is identified with discursive reasoning (dianoia). Imagination, on the contrary, is a static power, that cannot elicit that particular motion of drawing a syllogistic inference. The issue, however, is not entirely uncontroversial, because Ammonius maintains that sophistical syllogisms can only be formed by imagination, since they involve “empty concepts” as terms and only imagination can form such concepts. In this paper I will reconstruct Philoponus’ and Ammonius’ theories about the “activity” of syllogizing, and I shall explain how Philoponus can deal with sophistical syllogisms in a consistent way.