THE RELIABILITY OF SENSORY KNOWLEDGE
Perspective on Descartes' Sixth Meditation
A fundamental question in philosophy is whether we can really know anything about the external world around us, including whether it exists. We can, after all, conceive that everything we think we see, hear, or feel around us is a hallucination or illusion, having no correspondence to any objective reality. Dreams seem real at the time, and people with mental disorders may hear voices that no one around them hears.
Descartes addresses this question in his Sixth Meditation. He starts by distinguishing between imagination and pure knowledge. We can not only conceive of the possibility of a triangle or pentagon, we can "see" it in our mind's eye, thus imagine it. In contrast, we cannot imagine a thousand-sided polygon in this way; we can conceive of it existing, but we cannot "see" it as any different from a 10,000-sided polygon (Descartes 20). Imagination thus draws at least indirectly on the senses. I can imagine a starship, though I have never seen one; I cannot imagine four dimensions, even though I can conceive intellectually that they exist.
Descartes then argues that this capacity for vivid imagination relies on some distinctive features recognizable by our senses, even if not accurately so. He then proceeds through three stages of analysis of our sense perceptions. Since they are vivid and distinctive, it is natural to suppose that they are real. He then, however, registers causes for doubt. Dreams seem vivid and "real" when we are dreaming them. Amputees sometimes feel pain in their absent limbs. Could all our sensory experiences be equally illusory?
Descartes acknowledges that our senses can often be mistaken as to specifics, but argues that this is no grounds for dismissing sensory impressions entirely. God, he says, is not a deceiver (Descartes 22). He does not explain in the Sixth Meditation why we should presume ...