Natural theology and natural law › Possibility, necessity, and the third Way

The third Way begins with things that come into being and pass away. Aquinas calls them possible to be and not to be. He reasons that if everything were of this kind, there would have been a time when nothing existed. If nothing existed then, he argues, nothing could have begun to exist, which conflicts with the existence of things now.

This temporal step deserves scrutiny. From the fact that each member can be absent, it does not follow simply that all members are absent together. Imagine time extending indefinitely into both past and future, with lamps numbered by all integers, without a first or last. Each lamp numbered n is lit from time n until just before time n + 2. At every time at least one lamp is lit, although every individual lamp has times before and after its lit interval when it is dark. This idealized model challenges only the move from individual absence to universal absence. It does not establish a physically possible universe or disprove Aquinas's whole metaphysics.

The chapter should therefore preserve the argument's actual step rather than replacing it with the different claim that the universe needs an explanation. A modern reconstruction may defend a related dependence argument, but it must disclose the change. Understanding a historical argument includes identifying where its wording invites an objection.

Aquinas then distinguishes necessary beings whose necessity has a cause from a being whose necessity does not derive from another. He argues against an infinite regress of caused necessity and concludes that there is a source necessary through itself. This additional stage is needed because reaching something necessary is not yet, in his reasoning, reaching the ultimate source.