Yes, we should trust our senses. That is the only way how we live and communicate with the world outside us and even inside our bodies. Descartes in his “Meditations” just pretends that he does not trust his senses to get a new insight in his mind, to make a switch in his custom form of thinking, to look at the world from the new perspective. Even though Descartes says he rejects everything he knew before, he conveniently retains memories of Classical and Medieval philosophy, which he periodically refers to.
Descartes thinks he accumulated a lot of questionable and dubious ideas through his life. He makes a conclusion that all those ideas come from senses which are not trustworthy. It is easy to doubt credibility of our senses when they work on the edge of their sensitivity. For example, recognition of small or far away objects. It is much harder to doubt bigger chunk of our senses especially if they work in their confidence interval because their correctness could be proved by experience. If we are going to insist that some of our senses are wrong, we risk being considered as mad. However, it is much easier to reject our senses as a whole. For example, in a dream we perceive all weird events of the dream as normal and real, but we can recognize the strangeness of the dream only outside of the dream “reality” when we wake up. That is why Descartes decides to question the reliability of the whole world of senses.
If we accept an idea that our real world, given to us through senses, is just another dream of a higher rank, we may want to find criteria which would allow us to find imperfections of the dreams (especially of the lower rank) comparing to the really real world. Importance of these criteria is stressed, for example, in the motion picture Inception. For a moment Descartes follows this path, suggesting that the human fantasy is impaired by its scantiness: fantastical creatures that the humans make up are just a combination of parts of real animals, or images in dreams are like bleak paintings of real things. Descartes implies that if we dream a dream impressed on us by some “Architect” (in terms of the Inception), we can use “simple and universal” invariants of the really real world (like mathematical concepts) as landmarks for detecting a dream. But Descartes quickly withdraws from this path, suggesting that the “Architect” may be an omnipotent God capable of creating a deceptive dream for us, which is as perfect as the really real world (i.e. “totem” from Inception would not work).
Descartes meets possible counter-arguments that God could be non-omnipotent or could not be possibly deceptive because deception is manifestation of imperfectness, by saying that he has no answers to these objections. Descartes started doubting senses for he wanted to leave only certain and distinct ideas in his understanding of the world, therefore he is willing to build it suitable for the worst case scenario that God’s task is to deceive Descartes with all his might. If Descartes is still able to infer anything about the world even in conditions of omnipotent deception, those inferences would be quite certain and unshakable.
Descartes is able to identify at least one thing which omnipotent evil deceiver cannot possibly trick him about. He cannot make Descartes believe that he does not exist, therefore the statement “I am, I exist” cannot be taken away from him. Descartes finds other certain qualities of his “I”, which is a “thinking thing”, and which exists only while he thinks. His “I” also has senses, which does not mean that these senses are somehow real, and, strictly speaking, sensing is a type of thinking. “I” can have mental images, which are modifications of thought as well.
Another classification of thought is aimed at identifying what type of thought is prone to errors. Descartes divides thoughts into three categories. Two of them, which are simple ideas (even unreal and imaginary) and emotions, cannot be judged as true or false. We can make an error only if we have thoughts of third category named judgments.
Descartes tries to analyze how the human mind works. He takes the example of the wax. Even when wax in a usual for human environment may appear to our senses in different states of matter, we perceive it as the same substance. He concludes that our mind does not comprehend things with senses, which give us information about appearance of substances, but our mind understands substances by essence. It is similar to Plato’s Forms. This conclusion is encouraging, because, to proceed further, Descartes has to take on the question of God’ existence. Because God could not be comprehended by senses, one must think about him in terms of the world of Forms.
Descartes attempts to find out which ideas could have originated from him and which could not. Based on the idea that the cause should be greater than the effect, he concludes that idea of the omnipotent, omniscient and perfect God may be originated only from the omnipotent, omniscient and perfect God himself. This proof effectively repeats after Aquinas’s proofs of God’s existence.
However, there are contradictions in Descartes syllogisms. He says that idea of physical objects, which are not intelligent and extended things, was created by him as opposition of the idea of a thinking and not-extended himself. Using the same logic, an idea of omnipotent and perfect God could have appeared in Descartes’ mind as an opposition to non-omnipotent and imperfect himself. On the other hand, using concept of the greater cause than its effect, Descartes could have said that it is not only impossible for an idea of God to originate from himself, but an idea of physical objects needs a greater cause in a form of the objective reality. In both cases: the personal dream world of Descartes whose ideas come only from himself, or an objective world created by perfect God, he could trust his senses, because they are either only his and not impressed by a foreign “Architect”, or are caused by the objective reality.
Perhaps feeling deficiency in his arguments Descartes makes a second attempt to justify the existence of God by asking the question whether he could be created by somebody else but God. His answer is “No”, based on the same medieval reasoning of necessity of the cause to be greater than the effect, but he introduces a new twist to this reasoning. Descartes says his continuous existence is of the same nature as his creation. Thus, his world, instead of being created once (long-long time ago and maybe left by God on its own), becomes a dream of God which requires God’s constant attention in actively redreaming it in the whole fullness every next moment.
Having proven the existence of the perfect God, Descartes proceeds to analyze a question where errors come from. He says that human’s free will is as great as God’s. Following Augustine’s thinking, he says his ability to understand is as good as God’s qualitatively, but quantitatively is less. When free will of judgment exceeds scope of understanding, an error appears. By his own standards Descartes’ will to question his senses and desire to prove God’s existence exceeded capacities of his contradictory understanding and made subsequent conclusions about credibility of senses shaky.