John Duns Scotus

“Also, I say that although things other than God are actually contingent as regards their actual existence, this is not true with regard to potential existence. Wherefore, those things which are said to be contingent with reference to actual existence are necessary with respect to potential existence. Thus, though "Man exists" is contingent, "It is possible for man to exist" is necessary, because it does not include a contradiction as regards existence. For, for something other than God to be possible, then, is necessary. Being is divided into what must exist and what can but need not be. And just as necessity is of the very essence or constitution of what must be, so possibility is of the very essence of what can but need not be. Therefore, let the former argument be couched in terms of possible being and the propositions will become necessary. Thus: It is possible that something other than God exist which neither exists of itself (for then it would not be possible being) nor exists by reason of nothing. Therefore, it can exist by reason of another. Either this other can both exist and act in virtue of itself and not in virtue of another, or it cannot do so. If it can, then it can be the first cause, and if it can exist, it does exist—as was proved above. If it cannot [both be and act independently of every other thing] and there is no infinite regress, then at some point we end up [with a first cause].”
― John Duns Scotus, A Treatise on God as First Principle