But if so many difficulties attend the argument a posteriori, said DEMEA, had we not better adhere to that simple and sublime argument a priori, which, by offering to us infallible demonstration, cuts off at once all doubt and difficulty?
By this argument, too, we may prove the infinity of the Divine attributes, which, I am afraid, can never be ascertained with certainty from any other topic.

For how can an effect, which either is finite, or, for aught we know, may be so; how can such an effect, I say, prove an infinite cause?
The unity too of the Divine Nature, it is very difficult, if not absolutely impossible, to deduce merely from contemplating the works of nature; nor will the uniformity alone of the plan, even were it allowed, give us any assurance of that attribute.
Whereas the argument a priori ...
You seem to reason, DEMEA, interposed CLEANTHES, as if those advantages and conveniences in the abstract argument were full proofs of its solidity.

But it is first proper, in my opinion, to determine what argument of this nature you choose to insist on; and we shall afterwards, from itself, better than from its useful consequences, endeavour to determine what value we ought to put upon it.
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.