Jack - I seems to me that 'slippery slope' may have been a sloppy use on my part. What I meant was that 'I think therefore I am' so implies dualism that it would be difficult to avoid it once you accepted the statement. It is a statement that starts with ideas and goes on from there. On the other hand 'I exist therefore I think', starts with materialism. The question is not whether we exist or not but whether we know of our existence because of mental thoughts or because of physical reality. I agree that we cannot be mistaken about our existence. Descartes' method also implies that in introspection we gain direct knowledge of something. I believe that this is an untenable idea in light of neuroscience. When we see a tree, there is no actual tree inside our skulls, there is a model of a tree. When we experience our thoughts we are likewise experiencing a model of our thoughts. Consciousness is highly processed and in no sense that I know of is it direct knowledge.