Descartes cant trust the senses relating to the welfare of the body because of the following cases: First, after a man's leg is amputated, he still sometimes fells pain on the lost leg. Second, sometimes when a man is suffering dropsy, he still gets the sensation of thirsty. He, then, drinks water and worsens his illness. Third, when a man feels hunger, he eats the food with poison and gets injured. Since these senses are all taught by the "nature of man, Descartes wonders " how goodness of God doesnt prevent nature, this consider, from being deceptive?" (56).
After Descartes carefully observes about how mind be affected the sensations from the brain, which get message from the nerves of the body, he now realizes that most of the sensations are conductive to the maintenance of the healthy a man. "The nerves in the foot provokes the mind to do its outmost to move away from the cause of the pain, since it is seen as harmful to the foot."(57) When we nee something to drink the mind will affect by the brain with a sensation of thirst "because in this entire affair nothing is more useful for us to know than that we need something to drink in order to maintain our health."(58) The same hold for the case of hunger. From these cases, " here is absolute nothing to be found in them that does not bear witness to God's power and goodness." (57) .
Then, even though nature of man sometimes makes mistakes on these sensations, such as in the three cases that make Descartes doubt these sensation's credibility, these sensations are right most of the time because God makes us with these sensations. God is not a deceiver, so the senses he gives us wont deceive us and make us die for inability of maintaining our health. Therefore, Descartes says, " To be sure, I know that all the senses set forth what is true more frequently than what is false regarding what concerns the welfare of the body." (58) He now trusts them.