Some being is not eternal, and therefore it does not exist of itself, neither is it caused by nothing, because nothing produces itself. Hence, it is from some other being. The latter either gives existence in virtue of something other than itself or not. And its existence, too, it either gets from another or not. If neither be true—i.e., if it neither imparts existence in virtue of another nor receives its own existence from another—then this is the first efficient cause, for such is the meaning of the term. But if either of the above alternatives holds [viz. if it receives existence, or imparts it to others only in virtue of another], then I inquire about the latter as I did before. One cannot go on this way ad infinitum. Hence, we end up with some first efficient cause, which neither imparts existence in virtue of another nor receives its own existence from another.
John Duns Scotus, De Primo Principio (sect. 41). He expands elsewhere in the treatise on the various parts of the argument, of course.
And today is his feast day, so toast the Scotists! I would say to hug them, but that wouldn't be subtle enough.
