A careful examination of Cicero's logic, however, reveals that he is not actually stating anything in any sort of refutable or even affirmable manner. This clause is not a true part of Cicero's argument at all, but merely a definition of the term "miracle" as he is using it (or whatever other Latin equivalent he actually wrote). If "what was capable of happening is not a miracle," then the converse must also be true; that is, what is a miracle is incapable of happening. There is no reasoning required for this, it is simply the definition of "miracle" as Cicero defines it -- his statement simply defines it in the negative ("a miracle is not capable of happening). Cicero's response to our hypothetical theologian, them would simply be, "if it was capable of happening, then it was no miracle." There is no discussion or even consideration of the miraculous agent here; causation is not important to this argument of Cicero's. He merely defines a miracle as that which is incapable of happening, and then moves on to his conclusive point. The last clause of this statement is simply the logical culmination of the first two. If nothing that was incapable of happening has ever happened, and miracles are defined as that which is incapable of having happened, then, Cicero reasons, "there are no miracles." This wording is interesting (though admittedly it might be partially the responsibility of the translator,...

At first, this might seem grounds for refutation -- how can he extrapolate that because there haven't been any miracles that there aren't now or won't be later (as the argument implies)? However, the self-reflexive logic of the argument makes it true no matter what the scenario -- if God (should He exist) were to stomp his foot down in the middle of the Pacific and empty the ocean in a matter of seconds, then obviously such a thing, extraordinary though it may have been, was capable of happening, and is therefore no miracle according to Cicero's definition. The logic of this argument, though it eats its own tail, is perfectly sound.
That does not make it useful, however. If we use Cicero's definition of a miracle -- "that which is incapable of happening" -- and rewrite his statement using this, we end up with: "What was incapable of happening never happened, and what was capable of happening is not that which is incapable of happening...Consequently, there are no [things that have happened] which are incapable of happening. He has proved his point, but really it is the same self-evident point of the first clause with a newly -- or at lest freshly -- defined word. Being right is usually fun, but I hope Cicero didn't dine out too often on this one.