Aristotle's megarian manoeuvres

Towards the end of Theta.4 of the Metaphysics, Aristotle appears to endorse the obviously invalid modal principle that the truth of A will entail the truth of B if the possibility of A entails the possibility of B. I attempt to show how Aristotle's endorsement of the principle can be seen to arise from his accepting a non-standard interpretation of the modal operators and I indicate how the principle and its interpretation are of independent interest, quite apart from their role in understanding Aristotle.