That the existence of God may be rightly demonstrated from the fact that the necessity of His existence is comprehended in the conception which we have of him.
Rene Descartes

The (ontological) argument does not, to a modern mind, seem very convincing, but it is easier to feel convinced that it must be fallacious than it is to find out precisely where the fallacy lies.
Bertrand Russell

In Meditation II, Descartes presented the reader with a detailed explanation of the human Cogito Sum along with the method the reader could use to realize it. He claimed that a person attempting to doubt its own existence, even under the most extreme (hyperbolic) of scenarios (the dreaming doubt and the malicious demon doubt), would ultimately and inevitably realize, or intuit, during its doubting activity in the first person, present tense mode, that its existence was an indubitably certain existence. A simultaneous intuition or realization would occur that not existing while doubting or thinking was impossible for the thinker. Or, phrasing it positively, a simultaneous intuition or realization would occur that existing while doubting or thinking was indubitably certain for the thinker. As Descartes put it: "I am, I exist. This is certain. How often? As often as I think."

However, Descartes did not say that his existence was necessary in itself. He said only if, and when, he doubted, only if, and when, he thought, only then, during the time of their occurrence, did he simultaneously intuit his existence to be indubitably certain. If he ceased to think for an instant of time, then Descartes claimed that he would have no ground for believing that he could have existed during that instant. As Descartes cautioned: "For it might indeed be that if I entirely ceased to think, I should thereupon altogether cease to exist."

So then, according to Descartes, a person's thinking activity is CONTINGENT in the specific sense that it is experienced by the person as always being open to the possibility of complete cessation and non-existence. In other words, the Cogito portion of the Cogito Sum is experienced by the person, in the first person, present tense mode, to be contingent thinking activity (a contingent Cogito), since it is experienced as always being open to the possibility of complete cessation and non-existence. Search as one will, there is no separate or concomitant intuition available which would also assure the person, beyond all reasonable and hyperbolic doubt, that its thinking or doubting is an activity impervious to the possibility of complete cessation and non-existence. And the force of this realization would apply equally to all the many different modes of the person's thinking.

Descartes showed how the performance of a human Cogito Sum did, in fact, yield the intuition of an indubitably certain contingent personal existence (the contingent human Sum) based upon, emerging from, and restricted to the human person's simultaneous experience of the occurrence of its contingent thinking activity (the contingent human Cogito). Or, stating it more succinctly, a person's contingent thinking activity (the human Cogito), during the time that it is experienced by the person, always provides the person with a simultaneous intuition of the indubitable certainty of that person's contingent personal existence (the human Sum).

Surprisingly, in none of his subsequent meditations did Descartes attempt to present the reader with a detailed explanation of the nature of the divine Cogito Sum which would have paralleled nicely the detailed explanation of the nature of the human Cogito Sum he offered in Meditation II. Preoccupied as he was with the urgent need to provide a divine guarantee for his clear and distinct criterion of truth, in Meditation III Descartes decided to present the reader with a series of more, or less, traditional a posteriori arguments for the existence of God and, in Meditation V, he decided to present the reader with his a priori ontological argument for the existence of God based, curiously enough, upon his clear and distinct perception criterion of truth.

Nevertheless, had he wanted to, we suspect Descartes could have provided a detailed explanation of the nature of the divine Cogito Sum along the following lines.

If one assumes the divinity thinks, then its thinking activity (the divine Cogito) would be NECESSARY in the specific sense that it would be experienced by the divinity as always being closed to the possibility of complete cessation and non-existence and, as such, it would always provide the divinity with an intuitiion of its indubitably certain necessary personal existence (the divine Sum). In other words, he could have explained how the performance of a divine Cogito Sum would have provided an intuition of indubitably certain necessary personal existence (the divine Sum) based upon, emerging from, and restricted to the divine person's experience of the occurrence of its necessary thinking activity (the divine Cogito). The divine person's necessary thinking activity (the divine Cogito) would provide the divine person with an eternal intuition of the indubitable certainty of the divine person's necessary personal existence (the divine Sum).

He could then have gone on to explain that IF the human person were also able to experience the occurrence of such necessary thinking activity (the divine Cogito), then the human person, too, would be able able to experience it as always being closed to the possibility of complete cessation and non-existence. But that since the human person is, in fact, simply not able to experience the occurrence of necessary thinking activity (the divine Cogito) in the same way as the human person is able to experience the occurrence of contingent thinking activity (the human Cogito), the human person is, therefore, prohibited from ever having direct access to an intuition of indubitably certain necessary personal existence (the divine Sum).

From a Cartesian-based perspective, the central issue is the possibility of having a personal experience of thinking activity that can cease to occur and can go out of existence versus the impossibility of having a personal experience of thinking activity that can never cease to occur and can never go out of existence. Human thinkling activity is contingent being because the human person experiences that, at any given moment, the human person's thinking activity can cease to occur and can go out of existence - nothing more, nothing less. The human person's, alone, is the I think contingently, I exist contingently (Cogito contingenter, Sum contingenter). By contrast, divine thinking activity is necessary being because the divine person experiences that its thinking activity can never cease to occur and can never go out of existence - nothing more, nothing less. God's, alone, is the I think necessarily, I exist necessarily (Cogito necessario, Sum necessario). It is simply impossible for a human being to have a personal experience of thinking activity that can never cease to occur and can never go out of existence (the divine Cogito).

However, from a Cartesian-based perspective, it is precisely this impossible experience which is the indispensable prerequisite that would enable a human being to have a performative intuition, in the first person, present tense mode, of the indubitable certainty of necessary personal existence (the divine Sum); i.e., the existence of God.

But, unfortunately, Descartes' ontological argument lacks this indispensable experiential prerequisite. And, in response to Bertrand Russell, I submit that this is PRECISELY where the fallacy of the ontological argument lies!