Among the other divine attributes, God is typically defined by classical theists as being a necessary being. As I pointed out in my last entry, there are two types of necessity: metaphysical and logical. Thomas' arguments (TCA) conclude with a metaphysically, but not necessarily logically, necessary being. To put it succinctly, God exists and must exist in the actual world, given the reality of dependent beings and their corresponding need (in the actual world) for some type of ultimate cause. The Leibnizian argument (LCA), on the other hand, concludes that a logically necessary being (one that exists in all possible worlds, not just our own) exists.
Now, my preference for the TCA may be related to my own personal subscription to a very general Aristotelian philosophy, of which Thomism is an offshoot. One advantage of the TCA is that it starts out with a goal that is, in my opinion, less bold. Further, the TCA is more accommodating to inductive arguments, or arguments to the best explanation. The LCA, on the other hand, requires that God be a logically necessary being; so it isn't even possible for God to not exist. Before I am accused of compromising God's sovereignty and logical necessity, I need to point out that I do believe in both. It's just that the TCA may be more likely to persuade those who are skeptical of talk about "necessary beings" to begin with.
The reason I want to introduce these two seemingly different arguments (which is a bit poetic, given that the two are often confused with one another), is that I think they are capable of complimenting each other quite well. If, for example, we take seriously Thomas' claim that God is not only the First Cause, but also that His existence and essence are identical, we should arrive at the conclusion that God is a logically necessary being. As noted before, this is something the LCA (and not the TCA) sets out to do. Let's think a bit more about what Thomas' claim about God's existence/essence really implies.
A being whose existence and essence are identical must, by definition, be a being whose essence is to exist. This is the type of being that cannot not-be. If, then, there were some possible world in which God does not exist, then His essence would not require His existing. Hence, God must exist in all possible worlds, and therefore be logically necessary.
Now, to make this whole situation even more complicated than it already is, I should note that I find it strange how a number of proponents of the LCA criticize Thomas' view that God's existence and essence are identical. Craig expresses some reservation about this in his contribution to The Rationality of Theism (an excellent short anthology, if I might add). What I want to argue is that if God's existence and essence are not identical, then the LCA is left unintelligible.
Since the LCA concludes that a logically necessary being exists, such a being must exist by its very essence. But, if its existence and essence were not identical, then it could possibly not-be. Indeed, could a being that does not exist by the nature of its very essence truly be necessary? Although I support the LCA, I'd like to hear what more knowledgeable proponents think of this. If my analysis is correct, is the LCA not simply an offshot of the TCA?
Further, it seems to me that both arguments result in the existence of a single necessary being. After all, two beings whose existence and essence are identical would actually be identical beings! God is, as Thomas notes, ipsum esse subsistens: "being itself subsisting."