 This is an instance of the famous imperfective paradox. Of course, taken literally this is not a paradox. Nevertheless the entailment patterns (12) and (13) pose a significant problem for any formal theory of tense and aspect. For instance, the first formal proposal for truth conditions of sentences in the past progressive stated that a sentence S in the past progressive is true if and only if there exists an open interval before the speech time at which the sentence without the progressive is true. According to definition 1.4 this immediately validates pattern (12) but without qualifications this truth definition does not account for pattern (13). Sections  6  and  7 contain extensive discussion of the imperfective paradox. 