Clarke's Cosmological Argument
From Samuel Clarke, A Demonstration of the being and Attributes of God (1705)
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First, then, it is absolutely and undeniably certain that something has existed from all eternity.  This is so evident and undeniable a proposition, that no atheist in any age has ever presumed to assert the contrary, and therefore there is little need of being particular in the proof of it.  For since something now is, it is evident that something always was, otherwise the things that now are must have been produced out of nothing, absolutely and without a cause, which is a plain contradiction in terms.  For, to say a thing is produced and yet that there is no cause at all for that production, is to say that something is effected when it is effected by nothing, that is, at the same when it is not effected at all.   Whatever exists has a cause, a reason, a ground of its existence, a foundation on which its existence relies, a ground or reason why it does exist rather than not exist, either in the necessity of its own nature (and then it must have been of itself eternal), or in the will of some other being (and then that other being must, at least in the order of nature and causality, have existed before it).
    That something, therefore, has really existed from eternity, is one of the most certain and evident truths in the world, acknowledged by all men and disputed by no one.  Yet, as to the manner how it can be, there is nothing in nature more difficult for the mind of men to conceive than this very first plain and self-evident truth.  For how anything can have existed eternally, that is, how an eternal duration can be now actually past, is a thing utterly as impossible for our narrow understandings to comprehend, as anything that is not an express contradiction can be imagined to be.  And yet, to deny the truth of the proposition, that an eternal duration is now actually past, would be to assert something still far more unintelligible, even a real and express contradiction.
    The use I would make of this observation is this: that since in all questions concerning the nature and perfections of God (or concerning anything to which the idea of eternity or infinity is joined), though we can indeed demonstrate certain propositions to be true, yet it is impossible for us to comprehend or frame any adequate or complete ideas of the manner how the things so demonstrated can be.  Therefore, when once any proposition is clearly demonstrated to be true, it ought not to disturb us that there be perhaps perplexing difficulties on the other side which, merely for want of adequate ideas of the manner of the existence of the things demonstrated, are not easy to be cleared.  Indeed, were it possible there should be any proposition which could equally be demonstrated on both sides of the question, or which could on both sides be reduced to imply a contradiction (as some have very inconsiderately asserted), this, it must be confessed, would alter the case.   Upon this absurd supposition, all difference of true and false, all thinking and reasoning, and the use of all our faculties, would be entirely at an end.   But when to demonstration on the one side there are proposed on the other only difficulties raised from our want of having adequate ideas of the things themselves, this ought not to be esteemed an objection of any real weight...

II

    There has existed from eternity some one unchangeable and independent being.   For since something must needs have been from eternity, as has been already proved and is granted on all hands, either there has always existed some one unchangeable and independent being from which all other beings that are or ever were in the universe have received their original, or else there has been an infinite succession of changeable and dependent beings produced one from another in an endless progression without any original cause at all.  Now this latter supposition is so very absurd that, though all atheism must in its accounts of most things (as shall be shown hereafter) terminate in it, yet I think very few atheists ever were so weak as openly and directly to defend it.  For it is plainly impossible and contradictory to itself.  I shall not argue against it from the supposed impossibility of infinite succession, barely and absolutely considered in itself, for a reason which shall be mentioned hereafter.  But, if we consider such an infinite progression as one entire endless series of dependent beings, it is plain this whole series of beings can have no cause from without of its existence because in it are supposed to be included all things that are, or ever were, in the universe.  And it is plain it can have no reason within itself for its existence because no one being in this infinite succession is supposed to be self-existent or necessary (which is the only ground or reason of existence of anything that can be imagined within the thing itself, as will presently more fully appear), but every one dependent on the foregoing.  And where no part is necessary, it is manifest the whole cannot be necessary-- absolute necessity of existence not being an extrinsic, relative, and accidental denomination but an inward and essential property of the nature of the thing which so exists.
     An infinite succession, therefore, of merely dependent beings without any original independent cause is a series of beings that has neither necessity, nor cause, nor any reason or ground at all of its existence either within itself or from without.  That is, it is an express contradiction and impossibility.  It is a supposing something to be caused (because it is granted in every one of its stages of succession not to be necessarily and of itself), and yet that, in the whole, it is caused absolutely by nothing, which every man knows is a contradiction to imagine done in time; and because duration in this case makes no difference, it is equally a contradiction to suppose it done from eternity.  And consequently there must, on the contrary, of necessity have existed from eternity some one immutable and independent being...
    Otherwise, thus:  either there has always existed some unchangeable and independent being from which all other beings have received their original, or else there has been an infinite succession of changeable and dependent beings, produced one from another in an endless progression without any original cause at all.  According to this latter supposition, there is nothing in the universe self-existent or necessarily existing.  And if so, then it was originally equally possible that from eternity there should never have existed anything at all, as that there should from eternity have existed a succession of changeable and dependent beings.   Which being supposed, then, what is it that has from eternity determined such a succession of beings to exist, rather than that from eternity there should never have existed anything at all?  Necessity it was not because it was equally possible, in this supposition, that they should not have existed at all.  Chance is nothing but a mere word, without any signification.  And other being it is supposed there was none, to determine the existence of these.  Their existence, therefore, was determined by nothing; neither by any necessity in the nature of the things themselves, because it is supposed that none of them are self-existent, nor by any other being, because no other is supposed to exist.  That is to say, of two equally possible things, viz., whether anything or nothing should from eternity have existed, the one is determined rather than the other absolutely by nothing, which is an express contradiction.  And consequently, as before, there must on the contrary of necessity have existed from eternity some one immutable and independent being.  Which, what it is, remains in the next place to be inquired.

III

    That unchangeable and independent being which has existed from eternity, without any external cause of its existence, must be self-existent, that is, necessarily existing.  For whatever exists must either have come into being out of nothing, absolutely without cause, or it must have been produced by some external cause, or it must be self-existent.  Now to arise out of nothing absolutely without any cause has been already shown to be a plain contradiction.  To have been produced by some external cause cannot possibly be true of everything, but something must have existed eternally and independently, as has likewise been shown already.  Which remains, therefore, [is] that that being which has existed independently from eternity must of necessity be self-existent.  Now to be self-existent is not to be produced by itself, for that is an express contradiction, but it is (which is the only idea we can frame of self-existence, and without which the word seems to have no signification at all)-- it is, I say, to exist by an absolute necessity originally in the nature of the thing itself.