Entia non sunt multiplicanda praeter necessitatem
A year or so a ago I was being deposed by a hostile Lawyer near Boston. I was "The Expert Witness" and my stated belief was that certain webpages showed something was known at such a time in the past. These pages were dated, they also showed up in www.archive.org, they were totally consistent with other evidence and the author and his assistant had made sworn affidavits that they were of the date stated. The East Coast lawyer tried hard to shake my conviction that they were really of that time. I had to admit that with the right resources and a conspiracy of people they could have been faked. I turned the table on him by taking one of his printed case record books and asking if he would rely on a page chosen at random. He said he could because they were printed and the book had been bought by his firm when it was printed. But he had to agree with me that he had never actually seen that page before. I then said it would be a much easier job to have slipped a new page into the book than it was to fake the pages we were talking about. I then brought up the principle of Occam's Razor as the basis for my belief. He had genuinely never heard of it (after the hostilities had ended we discussed it).
The Bush memo saga has brought the memories back because it is the same argument - maybe it is an East Coast Lawyer thing. For a full dissection of it see:
For more on Occam's razor see below:
The principle is most often expressed as Entia non sunt multiplicanda praeter necessitatem, or "Entities should not be multiplied beyond necessity", but this sentence was written by later authors and is not found in Occam's surviving writings. William wrote, in Latin, Pluralitas non est ponenda sine neccesitate, which translates literally into English as "Plurality should not be posited without necessity".
Dave Beckett of the University of Kent at Canterbury writes: "The medieval rule of parsimony, or principle of economy, frequently used by Ockham came to be known as Ockham's razor."  (http://wotug.ukc.ac.uk/parallel/www/occam/occam-bio.html)
The principle of Occam's Razor has inspired numerous expressions including: "parsimony of postulates", the "principle of simplicity", the "K.I.S.S." (keep it simple, stupid), in some medical schools "When you hear hoofbeats, think horses, not zebras", and "brevity is the soul of wit".
A re-statement of Occam's Razor, in more formal terms, is provided by information theory in the form of minimum message length.
Another variant of this law is Thargola's Sword from Nightfall, (originally a short story by Isaac Asimov and later expanded to a novel in conjunction with Robert Silverberg): "We must drive a sword through any hypothesis th