Plato's Theory of Forms and Descartes' Proof of God
Plato's Theory of Ideas
Plato inherited from his teacher Socrates the certainty that truth exists. To convey this truth, there must be a language: words, which often have a conventional meaning. Therefore, because there is a true meaning to words, a single idea is necessary. These ideas must be categories; that is, they must meet Parmenides' three principles of being.
The world in which we live is sensible—that is, accessed through the senses. If everything that exists in the world we live in (the sensible world) is changing (not categorized as "being"), then ideas must reside in another world: the world of ideas or the suprasensible. Ideas are consistent and objective realities; they do not contradict each other and possess their own objective existence. Thus, an idea is the same for everyone.
Descartes on the Existence of God
Arguments, Consequences, and Criticism
Descartes asserts the existence of God through innate ideas. God represents perfection and infinity; however, as we are imperfect and finite, we cannot represent perfection and infinity ourselves. The idea of perfection and infinity must come from God; if He is the producer of the idea, He exists—this is known as the ontological argument.
If God exists, He must be good, because goodness defines Him; therefore, there is no "evil genius," and consequently, our perceptions are reliable. The second consequence is that the world exists because God is good. The suspicion that life might be a dream is dismissed because God, being good and humane, would not allow such deception.
Descartes, as a rationalist, does not resolve the reliability of the senses through the senses themselves, but through reason; he knows the world exists because he understands it through reason. It is argued that if we are imperfect and finite, and we represent the infinite and perfect only finitely and imperfectly, then the content is a finite representation of infinity that we might have created ourselves. This makes it likely that the idea of God could be factitious, meaning God's existence is not guaranteed.
Solipsism and Reality
If God's existence is not guaranteed, neither is the existence of the world or the reliability of our senses. Our only guarantee would be the "I am" (the Cogito), leading to solipsism. However, Descartes is not a solipsist because, for him, God and the world exist. He also notes that other things are objects of interest, as relationships involve power and domain between people.