... lives could be an imaginary part of a dream. On this point, it is possible to doubt that any physical thing really exists or if there is an external world at all. 'Since my most vivid dreams are internally indistinguishable 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.' (First Meditation) However, Descartes also states that this level of doubt is not utterly comprehensive, as the truths of mathematics and simple concepts remain unaffected: "For whether I am awake or asleep, two and three added together are five, and a square has no more than four sides. It seems impossible that such transparent truths ...

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