ABSTRACT
Since erroneous beliefs are often held just as strongly as true beliefs, it becomes a difficult question how they are to be distinguished from true beliefs. In philosophy, again, it seems not uncommon for two rival hypotheses to be both able to account for all the facts. Thus, for example, it is possible that life is one long dream, and that outer world has only that degree of reality that objects of dreams have; but although such view does not seem inconsistent with known facts, there is no reason to prefer it to the common-sense view, according to which other people and things do really exist. While it is not likely, it is at least possible that, instead of an all-powerful and all-good God, an evil demon exists who constantly causes him to fall into error. Descartes decides to adopt the hypothesis that such an evil demon exists in order to carry through to completion his process of systematic doubt.
