According to Descartes, the essence of material substance is simply extension, the property of filling up space. (Med. V) So solid geometry, which describes the possibility of dividing an otherwise uniform space into distinct parts, is a complete guide to the essence of body. It follows that there can be in reality only one extended substance, comprising all matter in a single spatial whole. From this, Descartes concluded that individual bodies are merely modes of the one extended being, that there can be no space void of extension, and that all motion must proceed by circular vortex. Thus, again, the true nature of bodies is understood by pure thought, without any information from the senses. 1.The Dream Problem Second, Descartes raised a more systematic method for doubting the legitimacy of all sensory perception. Since my most vivid dreams are internally indistinguishible from waking experience, he argued, it is possible that everything I now "perceive" to be part of the physical world outside me is in fact nothing more than a fanciful fabrication of my own imagination. On this supposition, it is possible to doubt that any physical thing really exists, that there is an external world at all. (Med. I) Severe as it is, this level of doubt is not utterly comprehensive, since the truths of mathematics and the content of simple natures remain unaffected. Even if there is no material world (and thus, even in my dreams) two plus three makes five and red looks red to me. In order to doubt the veracity of such fundamental beliefs, I must extend the method of doubting even more hyperbolically.In the Sixth Meditation, Descartes finally tried to eliminate the dream problem by proving that there is a material world and that bodies do really exist. His argument derives from the supposition that divinely-bestowed human faculties of cognition must always be regarded as adequately designed for some specific purpose. Since three of our faculties inv...
