He proves that nature is our teacher because it tells us what is right and wrong. Our intellect, understanding, and free will help us act upon what we have learnt through nature and obtained through our senses. We then make judgments in life even though we don't have complete knowledge of anything and everything is doubtful and believe that God will pass the best judgment because God is the all knowing.
Descartes created his Meditations to unlearn everything and build stronger foundations for all the ideas that exist within humans. He said that often human beings make errors of judgment habitually and believe things without reason. He wanted to prove to the world that God exists not through religion or science but through a rationalized argument. In his meditations Descartes discusses how we know that we exist as human beings, how the senses work, how we gain knowledge, make judgments and that God exists and is the best judge of all.
"Nevertheless, the belief that there is a God who is all powerful, and who created me, such as I am, has, for a long time, obtained steady possession of my mind. How, then, do I know that he has not arranged that there should be neither earth, nor sky, nor any extended thing, nor figure, nor magnitude, nor place, providing at the same time, however, for [the rise in me of the perceptions of all these objects, and] the persuasion that these do not exist otherwise than as I perceive them (Descartes, Meditation 1, section 9) Descartes asks a critical question in the first meditation. He wonders if our mind is in God's control as is everything else in the universe how do we know if they universe really exists. The mind thinks and that is its only purpose. All the information gathered by our five senses is registered with our brain for us to know what we felt, saw, smelt, heard or tasted. We lose one of our senses if we lose that part of our body. For example, if a person loses his eyes he loses his sight and if a person loses his ears he loses his hearing. Everything in the body is connected to the brain, therefore, if one part of the brain is damaged, everything connected to it gets affected. The mind is the sensory treasure box. To think and to act are controlled by the mind. The mind is controlled by God as long as a person believes that there is a God and has created us. Therefore, it is absolutely possible that anything that mind observes and thinks maybe an illusion. We know there is a sky because we can see a sky; we know there is an Earth because we feel it under our feet. We know all of this because all signals we sense are shot up to our brain and processed so we know what we have sensed. Therefore, anything that we think we know is because of our mind. Our mind has been in 'steady possession' of God so anything that we perceive or think we know can easily by an illusion and not exist outside of what we perceive through my mind.