Necessity with Gaps

In chapter A 13 of the Prior Analytics, Aristotle describes statements about what happens for the most part or by nature as contingency statements. He explains that in those cases the necessity has gaps, because natural processes may be interrupted. A few lines later he claims that there may be demonstrative knowledge of such cases, unlike mere coincidences. This clearly conflicts with his claim in the Posterior Analytics that only necessary truths can be known in the strict sense and that the premises of demonstrations must also be necessary. The apparent contradiction could be avoided if statements about natural events were understood as conditionals with an antecedent of the form ‘there is no obstacle’. But the limited syntax of Aristotle’s syllogistic did not allow him to see those statements as anything but contingently true.