Is Every Concrete Being Contingent?

A reader experiences intellectual discomfort at the idea of a being that is both concrete and necessary.  He maintains that included in the very concept concrete being is that every such being is concrete.  To put it another way, his claim is that it is an analytic or conceptual truth that every concrete being is contingent.  But I wonder what arguments he could have for such a view.  I also wonder if there are any positive arguments against it. 

1. We must first agree on some terminology.  I suggest the following definitions:

D1.  X is concrete =df x is possibly such that it is causally active/passive.  A concretum is thus any item of any category that can enter into causal relations broadly construed. 

D2. X is abstract =df X is not concrete.  An abstractum is thus any item that is causally inert.

D3. X is necessary =df X exists in all possible worlds. 

D4. X is contingent =df X exists in some but not all possible worlds.

The modality in question is broadly logical.

2. Now if this is what we mean by the relevant terms, then I do not see how it could be an analytic or conceptual  truth that every concrete being is contingent.   No amount of analysis of the definiens of (D1) yields the idea that a concrete being must be contingent.  God is concrete by (D1), but nothing in (D1) rules out God's being necessary.

3.  Off the top of my head, I can think of three arguments to the conclusion that everything concrete is contingent, none of which I consider compelling.

Everything concrete is physical
Nothing physical is necessary
Ergo
Nothing concrete is necessary
Ergo
Everything concrete is contingent.

The second premise is true, but what reason do we have to accept the first premise? 

Whatever we can conceive of as existent we can conceive of as nonexistent
Whatever we can conceive of is possible
Ergo
Everything is such that its nonexistence is possible
Ergo
Everything is contingent
Ergo
Everything concrete is contingent.

One can find the first premise in Hume.  I believe it is correct.  Everything, or at least everything concrete, is such that its nonexistence is thinkable, including God.  By 'thinkable' I mean 'thinkable without logical contradiction.' But what reason do I have to accept the second premise?  Why should my ability to conceive something determine what is possible in reality apart from me, my mind, and its conceptual powers?    If God is necessary, and exists, then he exists even if I can conceive him as not existing.

Nothing is such that its concept C entails C's being instantiated
A necessary being is one the concept C of which entails C's being instantiated
Ergo
Nothing  is necessary.

The first premise is true, or at least it is true for concrete beings.  But what reason do we have to accept the second premise?  I reject that definition.  A necessary being is one the nonexistence of which is possible.  The existence of God is not a Fregean mark (Merkmal) of the concept God. 

Is there some other argument? I would like to know about it.

Comments

[Steven]

Contingent existence (that is, the existing of contingent beings) is a phenomenon in need of explanation. For contingent existence is certainly a 'fact', a reality, but it does not explain itself, being contingent: if it explained itself, if anything could explain itself at all, then it would not be contingent but necessary. On the other hand, it cannot be entirely unexplained, for then there would be being from non-being (i.e., some reality which cannot account for itself and yet is unaccounted for by anything else, a sort of "incompleteness" of reality). Therefore there must be something necessary.

[Jonah]

For the second argument that everything concrete is contingent, I'm unsure about the first premise. It seems to me that the conceiving of the nonexistence of a necessary being _is_ a contradiction, since that would be holding that a necessary being could be nonexistent. But that might already be skipping ahead to endorsing the second premise, which you deny. It still, however, appears to me that (1) is not thinkable without contradiction (a necessary being not being necessary, that is, nonexistent?), otherwise (1) and (2) might be equivocating on 'conceive'.

I can also 'conceive' of the possibility that I've totally butchered the argument. I hope that's not the case.

[chris wash]

Pardon what i suspect is obtuseness on my part, but how can we be sure that no physical being is necessary?
Why could not the physical universe as whole be both eternal and necessary?

regards

Chris