The argument, replied DEMEA, which I would insist on, is the common one.  Whatever exists must have a cause or reason of its existence; it being absolutely impossible for any thing to produce itself, or be the cause of its own existence.  In mounting up, therefore, from effects to causes, we must either go on in tracing an infinite succession, without any ultimate cause at all; or must at last have recourse to some ultimate cause, that is necessarily existent:  Now, that the first supposition is absurd, may be thus proved.  In the infinite chain or succession of causes and effects, each single effect is determined to exist by the power and efficacy of that cause which immediately preceded; but the whole eternal chain or succession, taken together, is not determined or caused by any thing; and yet it is evident that it requires a cause or reason, as much as any particular object which begins to exist in time.  The question is still reasonable, why this particular succession of causes existed from eternity, and not any other succession, or no succession at all.  If there be no necessarily existent being, any supposition which can be formed is equally possible; nor is there any more absurdity in Nothing’s having existed from eternity, than there is in that succession of causes which constitutes the universe.  What was it, then, which determined Something to exist rather than Nothing, and bestowed being on a particular possibility, exclusive of the rest?  External causes, there are supposed to be none.  Chance is a word without a meaning.  Was it Nothing?  But that can never produce any thing.  We must, therefore, have recourse to a necessarily existent Being, who carries the reason of his existence in himself, and who cannot be supposed not to exist, without an express contradiction.  There is, consequently, such a Being; that is, there is a Deity.

I shall not leave it to Philo, said cleanthes, though I know that the starting objections is his chief delight, to point out the weakness of this metaphysical reasoning.  It seems to me so obviously ill-grounded, and at the same time of so little consequence to the cause of true piety and religion, that I shall myself venture to show the fallacy of it.

I shall begin with observing, that there is an evident absurdity in pretending to demonstrate a matter of fact, or to prove it by any arguments a priori.  Nothing is demonstrable, unless the contrary implies a contradiction.  Nothing, that is distinctly conceivable, implies a contradiction.  Whatever we conceive as existent, we can also conceive as non-existent.  There is no being, therefore, whose non-existence implies a contradiction.  Consequently there is no being, whose existence is demonstrable.  I propose this argument as entirely decisive, and am willing to rest the whole controversy upon it.