Descartes

Descartes writes the reply to prove to his objectors that the intellect corrects the errors of the senses. Descartes begins his reply by defining the way people use the word refraction to explain why a straight stick in a pool of water looks bent. By using the word refraction, the masses simply mean that any individual old enough to doubt their vision will know that the stick doesn't really bend. Children may be fooled by the appearance of the stick because they haven't learned that the eyes sometimes lie. Before viewing a mirage and finding no water or watching an uncle reattach a displaced thumb with a slight of hand, children may see their vision as a tool used to validate the truths of the world. Similarly, an adult, that still believes everything he sees, might think the stick bends before feeling it in the water and using the sense of touch to override the sense of sight.

Descartes feels that touching the stick lacks the power to correct the error seen with the eyes. One sense can't trump the influence of another sense and lead a man closer to truth. By trusting touch to lead to truth, people fall into the same trap as the child that uses vision for validation. Our sense of touch leads us in the wrong direction on many occasions. For example, a man may dip his hand into a vat of ice water. After leaving the hand in the freezing water for a few moments, the man thrusts his hand into a pot of warm water. The brain registers the sensation of burning in the hand even though the hand remains in a vat of warm water. The sense of touch, like the sense of vision, may be manipulated and deceive the man by causing him to believe the falsehood that his hand truly burns.

Since trusting vision, touch, or any other sense occasionally leads to error, Descartes argues that we need a certain degree of reason to believe in anything. Believing in something with a certain degree of reason proves to be a difficult task because we must doubt each one of our senses. All of our senses may be false because humans don't have any evidence that the world around us exists. We may be a ghost dreaming of life as a human or a brain hooked up to a computer program with electrodes. No one can prove the world you wake up to exists, so doubting a sense you can never be sure of seems logical. No child exits the womb and grows up doubting their senses, doubt is a learned behavior. Children enter the world believing all of their senses and only by