"To the Reverend Dr***
"Your objection against arguing at all a priori, concerning the existence and perfections of the first cause, is what many learned men have indeed stuck at. And it being evident that nothing can be prior to the first cause, they have therefore thought it sufficient to say that the first cause exists "absolutely without cause;" and that therefore there can be no such thing, as reasoning or arguing about it a priori at all. But if you attend carefully you will find this way of speaking to be by no means satisfactory. For though it is indeed most evident, that no thing, no being, can be prior to that being which is the first cause and original of all things, yet there must be in nature a ground or reason, a permanent ground or reason of the existence of the first cause: Otherwise its existence would be owing to, and depend upon mere chance. And all that could be said upon this head would amount to this only; that it exists, because it exists; that it therefore does and always did exist, because it does and always did exist: Which the followers of Spinoza will, with equal strength of reason, affirm concerning every substance that exists at all.
"If the idea of an eternal and infinite nothing were a possible idea, and not contradictory in itself; the existence of the first cause would not be necessary:  And if the existence of the first cause was not necessary, it would be no contradiction to suppose it either not to have existed in time past, or to cease to exist at any time to come. The existence therefore of the first cause is necessary; necessary absolutely and in itself. And therefore that necessity is, a priori, and in the order of nature, the ground or reason of its existence. For that which exists necessarily, or in the idea of which existence and necessity are inseparably and necessarily connected, must either therefore be necessary, because it exists, or else it must therefore exist because its existence is necessary. If it was therefore necessary, because it existed, then, for the same reason, every thing that exists would exist necessarily; and either every thing or nothing would be the first cause. On the contrary, if the first cause does therefore exist, because its existence is necessary, then necessity is the ground or reason or foundation of that existence; and the existence does not infer, (that is a priori, or in the order of nature and consequence, antecede) the necessity of existing; but the necessity of existing does on the contrary infer, (that is, a priori, or in the order of nature, antecede) the supposition of the existence; which is what I proposed to prove.
"The argument a posteriori is indeed by far the most generally useful argument; most easy to be understood, and in some degree suited to all capacities; and therefore it ought always to be distinctly insisted upon. But forasmuch as atheistical writers have sometimes opposed the being and attributes of God by such metaphysical reasonings as can no otherwise be obviated than by arguing a priori; therefore this manner of arguing also, is useful, and necessary in its proper place.
The eternity of God can no otherwise be proved, than by considering, a priori, the nature of a necessary or self-existent cause. The temporary phenomena of nature prove indeed demonstrably, a posteriori, that there is, and has been from the beginning of those phenomena, a being of power and wisdom sufficient to produce and preserve those phenomena. But that this first cause has existed from eternity, and shall exist to eternity, cannot be proved from those temporary phenomena; but must be demonstrated from the intrinsic nature of necessary-existence. If the first cause exists "absolutely without any ground or reason of existence;" it might as possibly in times past, without any reason, have not existed; and may as possibly in times to come, without any reason, cease to exist. Can it be proved, a posteriori, that the first cause of all things will exist to-morrow? Or can it be proved any otherwise, than by showing that necessity is a certain ground of future as well as of present existence? And if so, then the ground, or reason, upon which the first cause now does, and hereafter always will, and cannot but exist, is the very same ground or reason upon which he always did exist: And, consequently, it cannot with truth be affirmed that the first cause exists "absolutely without any ground or reason of existence." It is true, indeed, there is no antecedent reason why necessity is necessity. It is in itself essentially immediate; and it is absurd to suppose that it can be perceived otherwise than immediately and intuitively. Yet, I think, it is not an absurd question to ask, why that which is now a necessary being must equally in all past time have been, and in all future time continue to be, a necessary being? And the answer to that question will express fully all that I mean, by affirming the necessity to be the ground or reason of the existence. When atheistical writers affirm that the material universe, and every existing substance in particular, was eternal "absolutely without any ground or reason of existence;" can this assertion be confuted by him who shall himself affirm that God was eternal absolutely without any ground or reason of existence? Or can it be any other way confuted at all, than by showing that something must be necessarily-existent, (else nothing would ever have existed;) and that that which is necessarily-existent, cannot possibly be either finite or moveable, or at any time capable of any alterations, limitations, variations, inequalities, or diversifications whatsoever, either in whole, or in part, or in different parts, either of space or time?
In like manner, the infinity or immensity or omnipresence of God, can no otherwise be proved than by considering, a priori, the nature of a necessary or self-existent cause. The finite phenomena of nature prove indeed demonstrably, a posteriori, that there is a being which has extent of power and wisdom sufficient to produce and preserve all these phenomena. But that this author of nature is himself absolutely immense or infinite, cannot be proved from these finite phenomena, but must be demonstrated from the intrinsic nature of necessary existence. If the first cause exists "absolutely without any ground or reason of existence," it may as possibly be finite as infinite; it may as possibly be limited as be immense. It may as possibly, in other places, without any reason, not exist, as it does, without any reason, exist in those places where the phenomena of nature prove that it does exist. Can it be proved, a posteriori, that that governing wisdom and power, which the phenomena of nature in this material world demonstrate to be present here, must therefore be immense, infinite, or omnipresent? Must be present likewise in those boundless spaces, where we know of no phenomena or effects to prove its existence? Or can the immensity and omnipresence of the first cause be at all proved any other way than by showing that necessity of existence is capable of no limitation; but must for the same reason be the ground of immense or omnipresent existence, as it is the ground or foundation of any existence at all?
Again; the unity of God, (which, I think has always been allowed to be a principle of natural religion, otherwise St Paul could not justly have blamed the heathen as inexcusable, in that they did not like to retain God in their knowledge, and that, when they knew God, they glorified him not as God: The unity of God, I say,) can no otherwise be demonstrated, than by considering, a priori, the nature of a necessary or self-existent cause. The phenomena of nature which come within the reach of our observation, prove indeed demonstrably that there is a supreme author and director of that nature, or of those phenomena, whereof we have any knowledge. But that this supreme author and governor of nature, or of these phenomena, is likewise the supreme author and governor of universal nature; cannot be entirely proved by our partial and imperfect knowledge of a few phenomena in that small part of the universe which comes within the reach of our senses; but must be demonstrated from the intrinsic nature of necessary existence. If the first cause exists "absolutely without any ground or reason of existence," it is altogether as possible, and as probable, and as reasonable, to suppose that there may, without any reason, exist numberless finite independent coexistent first causes (either of like nature and substance to each other, or of different nature and substance from each other,) in different parts of the immense universe; as that there should, without any reason, exist one only infinite, immense, omnipresent, first cause, author and governor of the whole.
That there is, and cannot but be one, and one only, such first cause, author and governor of the universe; is (I conceive) capable of strict demonstration, including that part of the argument which is deduced a priori The subject of the question is no trifle. If any sober-minded man is persuaded, he can find any flaw in that demonstration, or cares not to examine it, lest any of its consequences should prove inconsistent with some other notions he may perhaps through prejudice have imbibed, I should be very thankful to him to show how the unity of God (the first principle of natural religion) can at all be proved by reason a posteriori only.
Some such considerations as these, (I suppose) they were, or others of the like nature, which moved Mr Limborch to write thus to Mr Locke: "Argumentum desiderat vir magnificus, quo probetur ens, cujus existentia est necessaria, tantum posse esse unum, et quidem ut id argumentum à necessitate existentia desumatur, et a priori (ut in scholis loquuntur,) non a posteriori concludat; hoc est, ex natura necessariæ existentiæ probatur eam pluribus non posse esse communem." To which Mr Locke replies; "Les theologiens, les philosophes, et Descartes luy-meme, supposent l'unité de Dieu, sans la prouver." After which, having suggested his own thoughts, he thus concludes. "C'est la, selon moy, une preuve a priori, que l'Etre eternel independent n'est qu'un."
"To argue, therefore, a priori concerning the existence and attributes of the first cause, is no absurdity. For though no thing, no being, can indeed be a priori to the first cause; yet arguments may, and must be drawn from the nature and consequences of that necessity, by which the first cause exists. Mathematical necessary truths are usually demonstrated a prioriand yet nothing is prior to truths eternally necessary. To confine, therefore, the use of term, argumentations above such things only as have other things prior to them in time, is on y quibbling about the signification of words.
"To the objection, that an attribute cannot be the ground or reason of the existence of the substance itself, which is always on the contrary the support of the attributes, I answer; that, in strictness of speech, necessity of existence is not an attribute, in the sense that attributes are properly so styled; but it is, [sui generis,] the ground or foundation of existence, both of the substance and of all the attributes. Thus, in other instances, immensity is not an attribute, in the sense that wisdom, power, and the like, are strictly so called; but it is [sui generis,] a mode of existence both of the substance and of all the attributes. In like manner; eternity, is not an attribute or property in the sense that other attributes, inhering in the substance, and supported by it are properly so called; but it is [sui generis,] the duration of existence, both of the substance and of all the attributes. Attributes or properties, strictly so called, cannot be predicated one of another. Wisdom cannot properly be said to be powerful; or power to be wise. But immensity is a mode of existence, both of the divine substance and of all the attributes. Eternity is the duration of existence, both of the divine substance, and of all the attributes. And necessity is the ground, or reason, or foundation of existence both of the divine substance, and of all the attributes.
"I am, Sir,
"Your very humble Servant, &c."
Edinburgh: Printed by A. Allardice.
 Nothing, is that of which every thing can truly be denied and no thing can truly be affirmed. So that the idea of nothing, (if I may so speak,) is absolutely the negation of all ideas. The idea therefore either of a finite or infinite nothing is a contradiction in terms.) (For necessity of being, and possibility of not being, are contradictory ideas.