Descartes' version of the Ontological Argument

For Descartes, anything that is perceived clearly and distinctly must be true. Therefore, when we know the essence of some idea, we have a clear and distinct idea of what that idea necessarily contains. In this ability to see as true or real any idea of a property contained in the essence of an idea Descartes sees another avenue for the demonstration of God’s existence.He claims to see clearly and distinctly that it is in the nature of God that he always exists. It is clear, he says, when he pays careful attention to the idea of God that God’s existence cannot be separated from his essence, any more than having three angles is separable from the idea of a triangle (CSM II 45-6). With this simple argument Descartes takes himself to have demonstrated the necessary existence of God.

Descartes anticipates an obvious objection: the fact that a property is part of the essence of an idea and cannot be separated from it doesn’t mean that entity that the idea represents actually exists.  Descartes responds that because the property in question contained in the essence of God is his existence, it does in fact entail his real existence.  It is not the thought or the definition that makes God necessary, rather it is his necessary existence that produces the necessity we clearly perceive in the idea of God. Descartes claims we are simply not free to think of God as not existing, for this would be to think of a supremely perfect being without a perfection (CSM II 46). He adds (without argument) that it is only in the case of God that existence is contained in the essence, so this argument could not be used to demonstrate the existence of anything else (CSM II 47).

One problem with this is the so-called "Cartesian Circle" in which Descartes may have employed a vicious circle in his defense of the veracity of clear and distinct ideas. According to some interpreters, Descartes needed the existence of God to provide the foundation for the reliability of clear and distinct ideas, which he then employed in his proof of the existence of God. Thus a vicious circle. This interpretation is contested, however (see here for more details). But even if we set that problem aside as unresolved there are major problems with this argument.

The notion that existence is a property contained in the essence of the idea of God is problematic. If Descartes means existence is a necessary condition (as part of a set of necessary and sufficient conditions) of being God, then the claim is uncontroversial but extremely uninformative. Also, the idea of God doesn't contain existence in any special way on this construal. In this sense existence is also a necessary condition of being a hammer (i.e., if something does not exist, then it isn't a hammer). But we can't infer anything at all about whether there actually are any hammers on the basis of this definition.

Alternatively Descartes could mean that there is a real metaphysical entity that is God's actual essence (like a Platonic form, perhaps), of which existence is a property in a way that existence is not a property of other real essences. Setting aside the profligate multiplication of entities this sort of metaphysics requires, Descartes gives us no reason for thinking that the essence of God is necessarily unique in this regard. Rather he repeatedly simply asserts that it is only the essence of the idea of God that has this necessary connection to existence; all other existences are contingent.

Ultimately, I think the only way to construe Descartes' argument without attributing a non sequitor to him is to read it as an argument for the psychological certainty that God exists. Unfortunately for Descartes, this means that he proves much less than he intended. The psychological version would go roughly like this:

1. We are psychologically unable to doubt the veracity of clear and distinct ideas 
2. We see clearly and distinctly that the essence of the idea of God contains existence as a property
3. We are unable to doubt that God's existence is entailed by the idea of God
4. We unable to doubt God exists

Of course, this argument merely points at a failure of human imagination rather than entailing anything at all about the existence or nonexistence of God.