In his Tertia Via Thomas Aquinas argues that, since some things are generated and corrupted, they possibly exist and possibly do not exist. 'But,' he continues, 'it is impossible for these always to exist, for that which is possible not to exist at some time does not exist. Therefore, if all things possibly do not exist, then at some time no thing existed.' These inferences can be justified by construing the Third Way as a modal argument relying on two principles: (...) one denying any temporal realization for the impossible and another expressing the nature of a necessary causal condition in modal terms. (shrink)