“Nature proves the being of God since she proclaims that she not only is, but is from another and could not be without another. For if it is certain and indubitable that out of nothing, nothing is made and that nothing can be the cause of itself (for then it would be before and after itself), it is also certain that we must grant some first and unproduced being from whom all things are, but who is himself from no one. For if every being is produced, it is produced either by itself or by some other; not by itself because (as was just said) nothing can be the cause of itself; not by another because then it would follow that there could be an infinite series in producing causes or that a circle would be made, both of which are untenable. For as to the circle, it evidently cannot be granted since in those things which are made there is always a last thing which has not made anything else. Besides such a circle is impossible; for suppose it were true, it would follow that the same thing was made by itself and was the cause (mediately at least) of itself. Nothing would be more absurd. Neither can an infinite series of producing causes be allowed because in causes there must necessarily be some order as to prior and posterior. But an infinite series of producing causes rejects all order, for then no cause would be first; rather all would be middle, having some preceding cause. Indeed there would be no cause which ought not to have infinite superior causes before itself (which is impossible [asystaton]). If there were infinite causes before each and every cause, before the whole multitude and collection of causes there would be infinite causes and thus that collection would not be total. Again, if the first cause can never be reached by ascending from the effects to the cause, so the ultimate effect can never be arrived at by descending from the causes to the effects. For the infinite can be gone through no more by ascending than by descending. Therefore we must necessarily stop in some cause which is so the first as to recognize no superior. Thus that series of causes ought not to be in infinitum, but ad infinitum in which it will be terminated.”
Francis Turretin, Institutes of Elenctic Theology, trans. George Musgrave Giger, ed. James T. Dennison, Vol. 1, Phillipsburg, New Jersey: Presbyterian and Reformed Publishing, 1992, p. 170.