Unit iv lecture vi march 15, 2012. After descartes wrote his meditations, he sent them to various philosophers, theologians, and scientists to promote his views and to ask for possible objections. Serious problem of skepticism is not that the world might be an illusion. It"s a problem about what we know whether or not it is in principle possible to tell the difference between our appearances and reality. According to skeptics, if we have no criterion (no rule against which we can measure what appears to us in order to determine if its real), then it seems we can"t ever come to have true knowledge. Candidates for sources of knowledge: memory. Much of what i know is based on memory, but memory is often false. So i should not trust any knowledge based on memory: sense perception. There are sensory illusions, madness, and dreams. It seems that all sensory knowledge of the material world should be doubted: reason.