Leave it to the imagination and we’ll form only primitive and ridiculous ideas of a necessary being. When I catch my own imagination I’ll find it visualizing contingency by a circle popping onto a screen and then off again, which I guess makes necessity a sort of screen-burner existence. At other times necessity seems to be announcing itself as a sort of extremely durable stuff (diamond would be too distracting or flashy, so maybe something more like the gray plastic they used to make “Unbreakable” brand combs out of.)
All nonsense, of course. Necessity is first of all the rational, which Plato says explicitly in the Timaeus and which Bertrand Russell is assuming when he tells Copleston that necessity is a feature of propositions. This rationality is most evident in mathematics and logic where it is seen as belonging to them as formal (or abstract) systems, and it needs to be extended to some degree beyond these in order to allow for the axiomatic principles that structure various discourses, though we’ve taken to calling these sorts of things “logic”.
But this seems to lead to a crazy idea of a necessary being as a formal or abstract idea. But isn’t such a being supposed to be real and active, and even personal? We encounter the Euthyphro problem in a different application, confused over how the greatest possible being can be both an unchangeable ideal (a formal abstraction) and also a god (an intelligence or living).
But the answer, as Augustine figured out, was in ourselves, since man himself overcomes the opposition between the abstract and the concrete. My abstractions are concrete as mine and abstract as themselves. It is precisely this that allows you and I to share the same idea, for the same idea is both one between us and wholly contained in two. Human beings in fact have only the most primitive means of overcoming the abstract and concrete: angels can share the very act of their intelligence as such without having to transmute it into some sensible form first, and the divine, as it turns out, shares not just his intelligible act but even his act of existence among Trinitarian persons.
And so “necessary existence” is really communicable existence, i.e. existence that can be shared or taken part in. A “necessary being” is first of all only a logical abstraction which is communicable but does not exist of itself. The human person in which the abstraction exists transcends the opposition between the abstract and concrete by attaining to a mode of existence where his life itself becomes communicable. Human beings lack necessary existence to the extent that they are closed off from others, and greater degrees of necessity are indexed by transcendent degrees of shared life, at the limit of which we find the Trinity and the communion of saints.
(Q: If you run necessary being through the Fourth Way, and you assume the Trinity is possible, does it follow there is one?)