GOD Aquinas’ proofs of God’s existence takes its start not from the Summa Theologiae but from the Summa contra Gentiles.11 The argument runs thus Every existing thing has a reason for its existence, either in the necessity of its own nature, or in the causal eYcacy of some other beings We would never, in the case of an ordinary existent, tolerate a blithe announcement that there was simply no reason for its existence; and it is irrational to abandon this principle when the existing thing in question is all-pervasive, like the universe Suppose that A is an existing natural thing, a member of a (perhaps beginningless) series of causes and eVects that in its own nature is disposed indiVerently to either existence or non-existence The reason for A’s existing must be in the causal eYcacy of other beings However many beings may be contributing to A’s present existence, they could not be the reason for it if there were not some Wrst cause at the head of the series— something such that everything other than it must be traced back to it as the cause of its being Persuasive as it is, this argument contains a key weakness What is meant by saying that A is ‘disposed indiVerently to either existence or nonexistence’? If it means ‘disposed indiVerently to going on existing or not’, then the contingent beings of the everyday world, from which the argument starts, not Wt the bill Contingent things aren’t of their nature equally disposed to exist or not: on the contrary, most things naturally tend to remain in existence On the other hand, if it means ‘disposed indiVerently to come into existence or not’, then we lapse into absurdity: before A exists there isn’t any such thing as a non-existing A to have, or to lack, a tendency to come into existence Duns Scotus’ Metaphysical Proof of an InWnite Being Flaws in Aquinas’ proofs of God’s existence were pointed out very shortly after his death Among his critics was Duns Scotus, who oVered his own proofs in their place The one closest to the argument of the Summa contra Gentiles makes use of the concept of causality to prove the existence of a Wrst cause Suppose that we have something capable of being brought into existence What could bring it into existence? It must be something, because 11 See Norman Kretzmann, The Metaphysics of Creation (Oxford: Clarendon Press, 1999), 84–138 304