Descartes, however, also realizes that people might misunderstand him and claims that his ontological argument is ‘defining God into existence’. He acknowledges this point when he writes that ‘just because I can think of a mountain must exists with a valley does not mean that any mountain exists in this world; similarly simply because I conceive of a God as having existence does not mean that God actually exists.’ Descartes then refutes this point by saying that the relationship between God and existence is not like the relationship between mountain and existence, but rather the relationship between mountain and valley. Just as the idea of a valley is implied by the idea of a mountain, the idea of existence is implied by the idea of God. Therefore Descartes concludes by saying that ‘I cannot conceive of a God without existence’.
Descartes argument, however, is still open to challenges. Just because I cannot think of a God as not existing does not mean that God actually exists. There is no logical connection here. Descartes, however, makes it clear that the bounds of our thoughts are, at least in some occasions, indications of what is possible. This is not because our thoughts creates or influences reality, but because they reveal reality; and so Descartes argue that the connection between God and existence is not something we come up with, but rather something we discover. Descartes argues that as finite beings, we cannot think of a necessary being, i.e. God, on our own. It is the fact that God necessarily exists that makes us think that way. It is worth noting that Descartes argument here is quite similar to Calvin’s notion of ‘sensus divinitatis’. This again rests on the doctrine of clear and distinct ideas. Descartes argues that what one can perceive clearly and distinctively is true. Because we can clearly and distinctively perceive the connection between God and God’s existence, it follows that God must exist.
Even at this stage, Descartes’ ontological argument is open to other challenges. Philosopher Gassendi asks whether Descartes is right to say that existence is part of the idea of God as the supremely perfect being. Surely we can still think of a God who does not exist. Descartes responds by saying that we, as finite beings, do not always see that the attributes of God all entail each other. For example, we say that God is omnipotent. This means that God is more powerful than anything. Following on from that, God must not depend on anything. Consequentially, God cannot go into and out of existence and hence it would be absurd to say that God might exist. The conclusion of Descartes’ ontological argument is not that God exists, but that God exists necessarily. Here, Descartes’ ontological argument is very similar to St Anselm’s. St Anselm argues that because God is a being ‘that than which no greater can be conceived’, God must exist by necessity because otherwise it would not be the greatest being.
Aquinas and later, Caterus, argues that it still does not show that God exists in reality. At best, Descartes’ ontological argument shows that the concept of existence in inseparable from the concept of God, or in other words, if God exists, God exists necessarily. Descartes responds to this objection by saying that this overlooks two things: first, his doctrine that clear and distinct ideas are true; and second, necessary existence is part of the concept of God entails God’s actual existence. If it is part of the concept that God must exists, then God must exists. This, however, can be said to be an unsatisfactory response, since Caterus’ criticism is precisely that a conceptual connection does not point to a connection in reality.