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Occam’s (or Ockham’s) Razor is a form of the principle of parsimony (broadly, that theories should be as simple as possible but not simpler.) It states: ‘Entities should not be multiplied without necessity.’ (In Latin, Entia non sunt multiplicanda praeter necessitatem.) However it seems that William of Occam never said it. As the Stanford Encyclopaedia of Philosophy reports:
Although the sentiment is certainly Ockham’s, that particular formulation is nowhere to be found in his texts. Moreover, as usually stated, it is a sentiment that virtually all philosophers, medieval or otherwise, would accept; no one wants a needlessly bloated ontology. The question, of course, is which entities are needed and which are not.
The other question is, who did originally say it? In 1918, William Thorburn published the result of his investigations into this question in Mind. The paper is available from Mind 27 (1918), 345-353; and on wikisource. That research suggested the origin lay with an Irish scholastic, John Punch.