 In the following section we will show that the imperfective paradox is an instance of the frame problem prominent in Artificial Intelligence (AI). To this end we will explain the event calculus, which is an extension of McCarthy’s situation calculus (McCarthy 1977) developed by Kowalski and Sergot (1986). Then we will indicate how the event calculus leads to a formalization of natural language tense and aspect by analysing the imperfective paradox. 