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Every truth which is not an absolutely first truth can be demonstrated from an absolutely first truth. Every truth can be demonstrated either from absolutely first truths (and it is demonstrable that these are indemonstrable), or is itself an absolutely first truth. And this is what I usually say, nothing must be asserted without a reason, or rather nothing happens without a reason. 2. On the reason why these things exist rather than other things (March-August 1689)7 The same reason that brings it about that these things exist rather than other things, also brings it about that something exists rather than nothing.

Therefore this truth, and others like it, cannot be grounded in something contingent. But as necessary truths have a kind of reality they must be grounded in something that exists, and as that something cannot be contingent it must be necessary. This necessary something Leibniz takes to be God, the necessary being. The third proof of God's existence is a form of what Kant was later to call the cosmological argument. In this argument Leibniz takes as his starting point the existence of contingent things.

How was this to be explained? To the modern mind the obvious explanation is that the creatures in question had become extinct, which Leibniz rejected as 'not likely' (p. 139). However he did allow that there might have been what we would today call local extinctions, that is, the extinction of a species from a certain geographical area: I even believe that kinds of terrestrial animals that formerly lived on our shores have today perished, whether on account of a climatic change, or on account of other reasons that would be difficult to guess, because of the obscurity of remote matters (p.