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Now that Descartes has found a piece of certain knowledge—that he exists as a thinking thing—he starts to look around for more of these self- evident truths. He discovers that he has quite a few of them, prominent among these being the truths of mathematics and logic, and he is optimistic about his chances for developing a system of certain knowledge. Then he realizes a kink in his plan. These clear and distinct perceptions are only indubitable so long as he is attending to them. As soon as they fall out of awareness, the doubt can creep back in. Once again, he can begin to wonder whether it was an evil demon who caused him to believe in the certainty of these truths. Suddenly, things do not look too rosy for his system of certain knowledge; if he needs to keep every truth perpetually before his mind, then he cannot expect too make much headway in unraveling the facts of nature.
Descartes' solution is to bring God into the picture. By proving that God is the cause of our clear and distinct perception, and that, further, God is perfect in every way and thus no deceiver, he will be able to secure lasting certainty for clear and distinct perceptions. He, therefore, sets out to prove that God exists.
Descartes gives at least two arguments for God's existence. The first one, found in I.14, is a version of the ontological argument for God's existence. Descartes' ontological argument goes as follows: (1) Our idea of God is of a perfect being, (2) it is more perfect to exist than not to exist, (3) therefore, God must exist.
The second argument that Descartes gives for this conclusion is far more complex. This argument rests on the distinction between two sorts of reality. Formal reality is the reality that anything has in virtue of existing. It is just regular, garden-variety reality. Formal reality comes in three grades: infinite, finite, and mode. God is the only existing thing with infinite formal reality. Substances all have finite formal reality. Finally, modes have modal formal reality. An idea, insofar as it is considered as an occurent piece of thought, has modal formal reality (since any particular thought, as we will see later, is just a mode of mind).
Ideas, however, also have another kind of reality, unique to them. When considered in their relation to the objects they represent, ideas can be said to have objective reality. There are three grades of objective reality, precisely mirroring the three grades of formal reality. The amount of objective reality contained in an idea is determined solely on the basis of the amount of formal reality contained in the object represented by the idea.
Descartes begins the argument by making the controversial claim that we all have an idea of God as an infinite being. (He believes that we cannot fail to have this idea because he thinks it is innate.) Because our idea of God is of an infinite being, it must have infinite objective reality. Next, Descartes appeals to an innate logical principle: something cannot come from nothing. Reasoning from this principle he arrives at two other causal principles: (1) There must be as much reality in a cause as in an effect, and so, (2) there must be as much formal reality in a cause of an idea as there is objective reality in an idea. Since we have an idea with infinite objective reality (namely, the idea of God), Descartes is able to conclude that there is a being with infinite formal reality who caused this idea. In other words, God exists.
One of the most famous objections to Descartes' philosophy attacks his use of the proof of God in order to validate clear and distinct perceptions. The objection, often referred to as the "Cartesian Circle," is that Descartes uses God to prove the truth of clear and distinct perceptions and also uses clear and distinct perceptions to prove the existence of God. How can he use clear and distinct perceptions to prove God's existence, these critics ask, if he needs God in order to prove that clear and distinct perceptions to tell us the truth? This does, indeed, sound like circular reasoning.
Descartes, however, has not made this foolish mistake. God's existence does not prove that clear and distinct perceptions are true. We do not need any proof that clear and distinct perceptions are true. In fact, what it means for something to be a clear and distinct perception is that, so long as we are attending to it, we cannot possibly doubt its truth. God is only needed to ensure that doubt does not creep in after we stop attending to these perceptions. Descartes, then, can legitimately use clear and distinct perceptions to prove God's existence. In the proof of God's existence we are using clear and distinct perceptions that we are attending to, and so we cannot doubt their truth. After we prove God's existence, the only thing that changes is that now we do not have to keep attending to these perceptions to be certain that they are true.
There are, however, other problems with Descartes' arguments for the existence of God. The ontological argument is particularly faulty. Ontological arguments are common in the history of philosophy. The medieval philosopher St. Anselm gave a famous version of the ontological argument, and even Plato puts an ontological argument in Socrates' mouth in the Phaedo. Nicolas Malebranche, Baruch Spinoza, and G.W. Leibniz all have their own versions of the ontological argument.
In fact, in order to be a proper Cartesian rationalist (i.e. someone who believes that the entire world can be explained in terms of a chain of logical connections and that we have access to this explanation) you have to believe in the possibility of an ontological argument. Without an ontological argument, explanation must either end in some brute, unexplained fact, or turn into an infinite regress, where the there is no end to explanation. In order to ensure that explanation comes to a final halt (and a halt with no loose, unexplained end), it is necessary that there be some level of reality that causes itself, something that is its own explanation. The only plausible candidate for an entity that is its own explanation is God. And the only way for God to be his own explanation is for some version of the ontological argument to work.
To understand why a self-causing thing is necessary to bring explanation to a satisfying end, consider what would happen if there were no such self-causing thing (which, unfortunately, there probably is not): in order to explain any fact, you would have to appeal to another fact, and then, to explain that fact, to another, and, for that one, to another, and infinitely on. Unless, of course, you ended up at a fact that simply could not be explained, in which you would not have managed to give an explanation for everything in the world. Now imagine that there is something that is its own explanation: in order to explain a fact, you have to appeal to another fact, and to explain that fact, to another, and on and on, until, ultimately, you hit upon a final fact that explains itself. Everything has been explained. There are no loose ends. The rationalist's job is done.
Unfortunately, as appealing as this picture of explanation is, ontological arguments involve a severe logical fallacy. They simply do not work. Immanuel Kant was the first to point this problem out, although he himself had given his own version of the ontological argument years earlier. The reason that the ontological argument cannot work is because it treats the existential verb (i.e. to be) as a property like other properties, a property that something can either have or not have. Clearly, though, existence is not a property like other properties. It is not even logically coherent to say "God does not have existence." If God does not exist, he cannot have properties, and he also cannot not have properties. He simply is not. The rationalists and those before them, failed to notice this big difference separating existence from other properties.
The causal argument also has its fair share of problems. The strange notions of reality that Descartes introduces are easy prey to attack. Why claim, for instance, that there is any special kind of reality called "objective reality?" Why assume, for that matter, that reality comes in grades that are so metaphysically loaded? Even more fatal than these legitimate worries, however, is the fact that Descartes' central claim is demonstrably false. We do not all have a clear and distinct innate idea of God as a being of infinite perfection. The only people who have this idea are those who were raised in cultures where the notion of a single and perfect supreme being was prevalent.