Descartes’ Proof of God and the Material World
The Existence of God
Descartes believes that the thinking self is not perfect, and that it has been established [through] methodical doubt to correct its mistakes, illusions, and inaccuracies. Despite that, the thinking ‘I’ has the idea of perfection. If we are aware that our nature is imperfect, it is because we know what a perfect nature is and we compare ourselves to it. Thus, the idea of perfection is innate in us; the idea of a perfect being: the idea of God.
According to Descartes, this idea of perfection, or God, cannot come from us, imperfect beings. Therefore, it must have been a divine reality that has given rise to it in our minds. Only a truly infinite substance can be the cause of the idea of an infinite being. Thus, of the set of ideas that the thinking self has, one stands out as very privileged, an idea that can go beyond its own subjectivity. It is an idea that allows me to state clearly and distinctly that, outside of my mind, there actually exists an extra-mental reality. And this privileged idea that I discover within me, and that in turn allows me to go beyond myself, is the innate idea of God.
The proof of the existence of God is a fundamental part of Cartesian metaphysics. God is the reality that overcomes my subjectivity. Now I know that outside of myself there is another reality, the perfect substance, a being who cannot let my clear and distinct ideas be a hoax. Thus, Descartes goes a step further: God becomes a guarantee of knowledge.
In God, there are great truths eternally set by Him. All mathematical truths are discovered by God; the laws of nature are ordered by God in the same way that a king enacts laws in his kingdom.
In his arguments for the existence of God, Descartes incorporates the earlier ontological argument of St. Anselm, but strengthened, as for Descartes, ideas have an undeniable reality. Just as the idea of a triangle is inseparable from its properties (for example, that the sum of its angles equals 180 degrees), the essence of God is inseparable from His greatest attribute: existence. For Him to be perfect, He cannot lack a perfection such as existence.
Existence of Material Things: The World
Doubt has enabled Descartes to affirm the existence of a first substance, the thinking self. In turn, the thinking ‘I’ discovered a second substance, God, a being with all perfections, including truth. What about the outside world, and my own body? Can we talk about them with certainty? Let’s see.
My self is fully aware of the difference between the idea of the thinking self and the idea of the extended body. It has the clear and distinct idea of the thinking self as *not* extended and, moreover, it has the clear and distinct idea of the extended body as *not* thinking. Of the thinking ‘I’, I cannot doubt; of the body itself, [I can]. But if I have a clear and distinct idea of my body as extended, and there exists a perfect God, the God who created me as rational, He cannot afford to fool me when I make proper use of my reason. Thus, the goodness of God guarantees that the very great inclination, or natural human tendency, to believe in the existence of extended things is not misleading.