Essence Implies Existence
When we think of a mountain, we necessarily think of a valley — they are inseparable concepts. Similarly, Descartes argues, when we think of God as a supremely perfect being, we necessarily think of God as existing. Existence is itself a perfection, and a being lacking it would not be supremely perfect. Therefore God must exist.
I cannot think of God but as Existing, it follows that Existence is
Inseperable from God, and therefore that he Really Exists; Not because
my Thought does all this, or Imposes any necessity on any Thing, but
contrarily, because the necessity of the thing it self (viz. of Gods
Existence) Determines me to think thus
The Objection
The obvious reply: we cannot think a thing into existence. Descartes anticipates this. The mountain-valley analogy shows that the necessity comes from the nature of the things, not from our thought imposing it. What distinguishes the God case from a perfect island — or any other invented perfection — is that God's essence uniquely requires existence in a way a perfect island's does not.
Why It Matters for the System
The argument is not merely a proof of God. It is structurally essential to Descartes's project. Once God's existence is certain, God's non-deceptive nature follows. And once God's non-deception is established, the whole guarantee of clear and distinct ideas is in place — and the edifice of knowledge can be rebuilt on secure foundations.
And Thus I Plainly see, that the Certainty and Truth of all Science
Depends on the Knowledge of the True God, so that before I had Known
Him, I did Know nothing; But now many things both of God himself, and
of other Intellectual Things... may be Plainly Known and Certain to me.
Kant's is the most famous objection: existence is not a predicate. Adding 'exists' to the concept of God adds nothing to the concept — it only asserts that the concept is instantiated. The argument was first put in this form by Anselm in the eleventh century. Contemporary philosophers including Plantinga and Malcolm have defended modal versions of it.