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Everyone depends on senses for information and awareness. When we want to know what the world is like, we look around us, listen, taste, smell, touch. Even scientific experiments depend on the senses. We mix two chemicals and observe what results, or we let some ball bearings drop and observe how they behave.
Descartes would like this heavy reliance on sensory observation to stop. He admits that for some knowledge the senses are required. For example, I could not know what books were on my desk if I did not use my eyes to check. However, he does not believe that we need sensory input when doing science. In fact, he is convinced that the senses only mislead us in scientific endeavors. Science, he feels, should proceed strictly by tracing logical connections between ideas of the intellect, not by observation. The senses do not even initially provide us with the ideas that we use in this reasoning. We are born with them already in our intellect.
Descartes, therefore, begins Principles of Philosophy with two skeptical worries meant to undermine our faith in the senses. First, he points out that our senses systematically mislead us. For example, when we view a straight stick through water, it looks bent; when we view things from a distance we tend to see them as much smaller than they are, or even as a different shape.
Not only are the senses periodically unreliable, however, but they are also constantly and stubbornly unbelievable. When we sleep, we often have sensations indistinguishable from those that we have when we are awake. We admit that those dreaming sensations do not correspond to reality, so why are we any more certain of our waking sensations? How do we know that any particular sensation we have is not a dream? We cannot. Therefore, Descartes concludes, better not to rely on the sensations at all, at least not when you are after certain knowledge (such as in science).