Of all the recurring questions of Man, one of the most persistent is the question of our origins. Specifically the question of what, if anything, caused us to exist. It has been argued by generations of minds, all seeking the definitive explanation of our existence. One such mind was that of Rene Descartes, a brilliant philosopher of his time, throughout and beyond ours. His ideas on geometry and metaphysics, among others, remain influential upon the thinkers of today.
In Meditations, Descartes formulates the framework and guidelines of his First Philosophy or metaphysics, where methodic doubt is used to discern the nature of being and the world. Here he describes how we can derive a reliable method that can definitively determine what is certain and what is suspect, and further apply that method to prove the existence of absolute ideas such as God or mathematics. The method's ground-up approach is supposed to provide for the foundations of certain knowledge, and so it does.
Descartes believes that after I call into doubt everything that can easily be relegated as uncertain (such as sense dependent data), I am left with mental ideas of things that I once experienced through the senses. Given that a chair's physical existence may be suspect, my idea of a chair may also be suspect in regard of some aspects such as appearance, yet I cannot suspect the fact that I am thinking of scale, quantity, measurement, space, etc. in providing for my mental image of the chair. Hence for Descartes, there are things that are certain regardless of sense experience and it seems mentally impossibly to conceive of them as false.
Given this, Descartes develops his reasoning for the existence of God as an all-perfect being. He does this by beginning with an idea that is considered certain and attributing what makes us feel as if something is certain. In this case he considers something certain as something so clearly and vividly perceived that it cannot be untrue. Here he implies then that for something to be true, I just have to have a clear and vivid idea of it and that alone is enough antecedent for its truth-value. He clarifies this by describing the nature of the mind to have clear and distinct knowledge of certain thing to be true because of its basis in something true that is external from the senses. So, when I distinctly and vividly perceive of something I do so in such intensity because of my recall with the higher form of the idea. Descartes then says that one can know that some properties of these higher idea forms can be known to be true because if I know of ultimate idea of a triangle, when I perceive its three angles to be equal to two right angles, it must be true because I cannot so vividly perceive it as a triangle unless the ultimate nature of a triangle did not contain the predicate of its three angles being equal to two right angles. So then, a property of a clear and distinct object must be true by the basis of its being perceived as also being clear and distinct. Since the concept of a perfect being implies that it contains its own necessary existence, that is, for it to be perfect it must in itself contain all perfections and by extension all expressions of such. For Descartes, because it is clear and distinct that the idea of a perfect God must hold that God has necessary existence, and that if I have an idea of something and I clearly perceive it to have a property then that thing really has that property, then God must exist because God's existence naturally follows from God's conception.
The problem with this is that because of Descartes logical framework, all it takes for something to exist in the world is to somehow incorporate the idea of existence into the nature of the concept. Also, because all it takes for it to be true is that that I perceive that existence is part of the concept just enough to be vivid and clear. Such that I could conceive of a chair and it would not exist, but the chair were somehow ascribed with a nature of existence such that the chair has the property of existence, and that I clearly and so distinctly perceived it as such, then it would exist. So then, all it takes to create a chair into existence is to somehow be able to build it to the point that I clearly perceive it to be an "existing" chair. Clearly, we cannot just go around creating chairs out of thin air, so this must not be the case. So then, it should not be so that I can conclude that a thing is existing in the world just by clearly and distinctly perceiving that existence is part of the things nature. While there must be something that carries its own necessary existence within itself, it should not be so that I am able to ascribe self-necessity to something, as that defeats its purpose.
Another similar problem with the Cartesian ontological argument is that just because one knows a perfect Gods nature as having necessary existence, it does not follow that God is in a state of existence. This is because something that is existing, can not necessarily be in existence as a thing. Given that I were to think of the concept of God, and God's properties, it follows by Cartesian logic that the only thing I can know to be true is that the existence of the concept God, rather than that God is existing in the world. Therefore, when I think of something, I regard it as existing just by the conception of it, regardless of whether the thing I am thinking of exists as such. So existence does not really add to the idea of something because it already exists as one conceptualizes the idea. It can be seen then that Descartes is sneaking that God is existing in the world, when he can only actually observe that there is a subject such as God, and that God-concept is omnipotent, omniscient, self-necessary, etc. in the world. For it would be different if there were ways we could empirically observe that God existed in the world. If God existed in the world, then we would be able to observe that a part of God's essence is that God is existing since it would be obvious to our senses. Given that God was existing, then there would be more perceivable signs of God interacting with matter that are present, even to the point of having an experience of personally watching God affect matter as it is happening. However this is clearly not the case, for we come to our knowledge of God only through the mind, and our current understanding of the physical world. We know that God, being wholly perfect, must then contain all necessary existence within itself, otherwise it would be dependent on another for its own existence and hence not perfect. Clearly this is a logical deduction, which comes from premises that show our collective conception of what an all perfect being is. In other words, all we know is that there is a concept of omnipotence- but not perceive omnipotence in reality, there is such a thing as omniscience-but not perceive of omniscience in reality, etc, and that there is such a concept of God who contains all these things by virtue of conceiving God's God-ness- but no perceive of God in reality. As shown, this is the case in the world, where we can have no relevant or reliable sensory experiences that can show God's existing in the real world.
So then, Descartes is making a logical leap of assumption when he shows that the basis for God's existing as the fact that God has necessary existence. In making that assumption, he assumes that when I have the full concept of God in my head, as in the idea of God fully and purely, then I clearly perceive God's necessary existence, and therefore I am supposed to realize that God exists-just because it seems so real to me. Descartes attributes much weight to the feeling that something is clear and distinct, when as it can be seen that it is not enough to perceive a concept as clear and distinct and immediately assume that the concept exists. After all, one can be clear on a concept, and yet that concept could exist in reality or not. In the same way, from being clear that God as existence per se, we can be clear that the concept God as such exists, not that God is existing. The all-perfect being may in fact exist then, but Descartes ontological argument is not enough to be able to explain how such a being exists in the world.