Ancient philosophical worldviews commonly included the idea of an eternal structure of reality which was taken to be manifested in the invariant system of natural kinds, unchangeable natural tendencies, and circular operations of the cosmos. The philosophers thought, since Aristotle, that the terms "possibly" and "the possible" can be used of that which, being assumed, results in nothing contradictory. They also seem to have shared the view that possibilities are assumed to be realized in our actual history. Hence the denials of natural invariances, static or dynamic, were impossible and the statements asserting them necessary. The same was claimed about statements concerning the past and the present. One feature of this picture is the special status attributed to the so-called future contingents in ancient thought. It could be asked whether they specified an area where possibility and reality did not overlap. Anyway, the common metaphysical tenet was that the invariant and eternal structures and operations fixed the generic necessities and possibilities or, what was taken to be the same, what can or cannot be justifiedly assumed to be realized.¹