To study physiology with a good conscience, we must insist that the sense organs are not appearances in the way idealist philosophy uses that term: as such, they certainly could not be causes! Sensualism, therefore, at least as a regulative principle, if not as a heuristic principle. – What? and other people even say that the external world is the product of our org ans? But then our body, as a piece of this external world, would really be the product of our organs! But then our organs themselves would really be – the prod- uct of our organs! This looks to me like a thorough reductio ad absurdum: given that the co ncept of a causa sui is something thoroughly absurd. So does it follow that the external world is not the product of our org ans –?