I’m trying to articulate why a possible being needs a necessary being here and now for it to exist. Something pithy if possible. So A exists but does not have to exist. There is no point throughout the duration of the existence of A that is must exist. Therefore it must be given existence at all times. – Matt
“A” cannot give to itself what it does not have. And what it did not have prior to existing, was existence. So something else must have caused it to exist. But you’re interested in why, after it first begins to exist, that it still needs a necessary being at every moment of its existence.
The pithy answer is because the cause is simultaneous with the effect. Put another way the effect only exists for as long as the cause is present. For example, daytime (as opposed to night time) only exists for as long as the sunlight illuminates the air. So, in the third way, if the Cause upon which those beings are dependent for existence (i.e., God) were ever removed then those things would instantly vanish into nothingness.
Here is how Aquinas explains it in Summa theologiae, I, q. 8, a.1, reply:
“I answer that, God is in all things; not, indeed, as part of their essence, nor as an accident, but as an agent is present to that upon which it works. For an agent must be joined to that wherein it acts immediately and touch it by its power; hence it is proved in Phys. vii that the thing moved and the mover must be joined together. Now since God is very being by His own essence, created being must be His proper effect; as to ignite is the proper effect of fire. Now God causes this effect in things not only when they first begin to be, but as long as they are preserved in being; as light is caused in the air by the sun as long as the air remains illuminated. Therefore as long as a thing has being, God must be present to it, according to its mode of being. But being is innermost in each thing and most fundamentally inherent in all things since it is formal in respect of everything found in a thing, as was shown above (Question , Article ). Hence it must be that God is in all things, and innermostly.”
Hope that helps!
Robert A. Delfino is Associate Professor of Philosophy at St. John’s University in New York City. He received his PhD from the State University of New York at Buffalo, where he specialized in metaphysics, medieval philosophy, and Thomas Aquinas, studying under Professor Jorge J. E. Gracia. His current research interests include metaphysics, ethics, and the relationship between science, philosophy, and religion. He lives in New York with his wife and two sons.