In “On the Ultimate Origination of Things”, we get one of Leibniz’s presentations of the cosmological argument. One difference between Leibniz’s argument and the two versions of the argument we discussed in connection with Aquinas should stand out: Leibniz’s argument is not meant to rely on any premise ruling out the possibility of an infinite series. He says, for example:
“I certainly grant that you can imagine that the world is eternal.”
He means something like this: it is possible, for all we know, that the world is eternal. But if the world could be eternal, then it could contain an infinite series of temporal causes. But then Leibniz is committed to the claim that, for all we know, the world contains an infinite series of temporal causes. But then we surely cannot argue for the existence of God (as the kalam argument does) on the basis of the impossibility of such an infinite series.
So how is the argument supposed to work? It seems that a key part of the contrast is the distinction between things which are the case only contingently, or as a matter of physical necessity, and things which are the case necessarily, or as a matter of metaphysical necessity. For example, Leibniz says
“...in a series of changeable things ...the reason would be the superior strength of certain inclinations ...where the reasons don’t necessitate ...but incline. From this it follows that even if we assume the eternity of the world, we cannot escape the ultimate and extramundane reason for things, God.”
What is the contrast between reasons which necessitate and reasons which incline? We can understand this in terms of the distinction between two classes of facts: those which are contingent and those which are metaphysically necessary. Leibniz’s argument seems to have something to do with the idea that ultimately we need to explain facts which are contingent in terms of facts which are necessary.
At this point, you might object: aren’t ordinary scientific explanations of facts explanations in terms of something necessary? After all, isn’t it true that, given the laws of nature, the state of the world at one time is a necessary consequence of the state of the world at an earlier time?
Leibniz would say that this reply depends on a confusion of necessary and physical necessity:
“And so we must pass from physical or hypothetical necessity, which determines the later things in the world from the earlier, to something which is of absolute or metaphysical necessity, something for which a reason cannot be given. For the present world is physically or hypothetically necessary, but not absolutely or metaphysically necessary. That is, given that it once was such and such, it follows that such and such things will arise in the future. Therefore, since the ultimate ground must be in something which is of metaphysical necessity, and since the reason for an existing thing must come from something that actually exists, it follows that there must exist some one entity of metaphysical necessity, that is, there must be an entity whose essence is existence, and therefore something must exist which differs from the plurality of things, which differs from the world, which we have granted and shown is not of metaphysical necessity.”
One way to reconstruct the argument of this passage is as follows:
No facts ‘in the world’ are metaphysically necessary.
The ultimate ground of all things which are not metaphysically necessary must be in something which is metaphysically necessary.
Observed facts have as their ultimate ground something which is metaphysically necessary and exists outside of the world (‘is extramundane’).
Why does Leibniz think that premise (1) is true?
The key element in this argument clearly seems to be premise (2). Why can’t it be the case that contingent facts are always explained by other contingent facts? This question seems especially pressing for Leibniz, since he, unlike Aquinas and the defender of the kalam argument, does not see any impossibility in an infinite series of causes (or, presumably, an infinite series of explanations). So what is supposed to stop there being an infinite series of contingent facts, each of which is explained by the one before it, but none of which need explanation in terms of something which is metaphysically necessary?
Leibniz thought that this was not possible because of his allegiance to what is sometimes called ‘the principle of sufficient reason’. There are several different versions of this principle. Let’s consider the following one:
Every contingent fact must have a sufficient reason (explanation).
Can this be used in an argument of the above sort, or as a way to support premise (2) of that argument? What we would need to do is find some contingent fact which is such that, in principle, it cannot be explained by other contingent facts. Then it seems that, if we accept the principle of sufficient reason, we’d have no choice but to accept that it is explained by something which exists of necessity, rather than something which exists only contingently. And if we accept Leibniz’s claim that all things of the world exist only contingently, we’d have established (C) above.
Two possible examples of contingent facts which cannot be explained in terms of other contingent facts:
There are contingent facts.
There are contingent beings.
Why the first of these seems necessary rather than contingent, and why the second seems plausible.
A regress argument against the idea that there must be something metaphysically necessary which can explain the fact that there are contingent beings. Is ‘being a sufficient reason’ a relation which holds of necessity, or only contingently? If the former, then it seems that something necessary can only explain necessary truths. If the latter, then it seems that we have another contingent fact which needs explanation.