[S]ince I am a thinking thing, and have in me an idea of God, whatever finally the cause may be to which my nature is attributed, it must necessarily be admitted that the cause must equally be a thinking thing, and possess within it the idea of all the perfections that I attribute to the divine nature.
In book 3 of his Meditations, Descartes puts forward an argument for God's existence. This argues that God exists from the evidence of the mark of the maker that can be found within us, as Descartes describes it: 'the artisans trademark imprinted on his work'.
This mark is the idea of God's perfection. Descartes argues:
Given that I am imperfect (3) I cannot be responsible for the idea of perfection that I hold (1)
Therefore, given that every cause must be at least as great as its effect (2), whatever caused my idea of perfection (1) must be perfect. Therefore a perfect being exists and this is God who created me.
The reasons behind this argument being put forward (aside from the historical context of the time in which Descartes worked) can be seen from the placing of this argument in the overall structure of the Meditations. In book one Descartes has shown that it is possible to doubt everything (demolishing knowledge as it was understood at the time). In book two Descartes, as part of his attempts to build a new system of knowledge, starts on the foundations, and demonstrates an argument for the existence of his mind (the Cogito).
As Descartes is a creationist Christian, before he can move on to proving the existence of the objective world he must first prove the existence of the creator. This fits with his concept of the cause having at least as much reality as the effect. As he follows back the chain of cause and effect Descartes ultimately reaches the point of creation and thus the ultimate cause. This cause is the most real, he argues, as it has no cause and thus cannot be denied or doubted.
Although this argument relies heavily on the thoughts of medieval philosophers such as Thomas Aquinas, the clarity of logic of the argument must be admired, and indeed this is one of the arguments strongest points.