that all of his former opinions were false:
But since reason has already persuaded me that I ought to withhold belief no less carefully from things not entirely certain and indubitable than from those which appear to me manifestly false, I shall be justified in setting all of them aside, if in each case I can find any ground whatsoever for regarding them as dubitable (599).
Descartes notes a number of reasons why we have to doubt our senses. For one thing, the senses sometimes deceive us with regard to minute objects or objects that are at a great distance from us. Our senses have limitations when it comes to things that are too small or too far away. There are other things which are self-evident and not to be doubted, and Descartes cites the fact that he is sitting in his
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