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For example, All that glitters is not gold is sort of a fixed term, and people often use the same “all . . . not” form when talking about things. See also the question “Is it wrong to use ‘not’ in sentences that have an ‘all . . . not’ form?”, which concludes that it is illogical and actually ambiguous.

The proverb implies that everything that shines cannot be gold, which is in stark contrast to its meaning that not everything that shines must inevitably be gold — logically Not all that glitters is gold, which seems rather uncommon.

Why is it that people prefer the “illogical” construct over the “logical” one, at least in English?

tchrist
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    I don't know. A few of us, who have studied logic, prefer the logical one. – Brian Hitchcock Feb 14 '15 at 09:58
  • English is not the only place where this happens. I have noticed, for example, a tendency in recent years for Danes (in Danish, that is) to say necessarily not when they mean not necessarily; e.g., “That's necessarily not a bad thing.” – Janus Bahs Jacquet Feb 14 '15 at 10:29
  • "All that glitters..." is poetic, as such it claims "license" to use what are otherwise illogical structures, for effect. –  Feb 14 '15 at 11:38
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    Any proposition, in any language, that contains both a quantifier (e.g, all) and a negative like not, is potentially ambiguous. This is because both quantifiers and negatives have foci -- words they focus on -- and they can appear in a lot of places, quite often in places where the operator focussing on them can't get close enough to clarify it. Every boy didn't leave means either they all stayed, or some but not all of them left. Logic provides two ways to state the proposition, one for each reading. Language just hasta cope; ambiguity is a feature of all language. – John Lawler Feb 14 '15 at 16:14
  • @JohnLawler The difference between that and the case in point here is that I doubt most people prefer “Every boy didn’t leave” over more logically unambiguous (and mellifluous, if you ask me) versions like “None of the boys left” and “Not all the boys left”. The same ought (in my head, which seems to be decently aligned with Brian’s here) to apply to all … not, but it doesn’t—quite the opposite, in fact. – Janus Bahs Jacquet Feb 14 '15 at 18:34
  • @JanusBahsJacquet: Same phenomenon, different words, therefore different syntax; but same logic. (∀ x: Glitter (x)) ¬ Gold (x) versus ¬ ((∀ x: Glitter (x)) Gold (x)). Pronounced, respectively, as "For every x such that x glitters, it is not true that x is gold", equivalent to "nothing that glitters is gold". Versus "It is not true that, for every x such that x glitters, x is gold", equivalent to "not everything that glitters is gold." – John Lawler Feb 14 '15 at 19:00
  • My version claims All that is gold does not glitter http://www.goodreads.com/quotes/229-all-that-is-gold-does-not-glitter-not-all-those – anemone Feb 14 '15 at 22:41
  • In general, natural language uses logical terms and constructs very sloppily compared to their use in boolean math. We depend on context and common sense to clear up the meaning. E.g. or can mean either inclusive or exclusive or, and implications are often reversed from the mathematical form. People don't think mathematically, they think in generalities, and language reflects that. – Barmar Feb 15 '15 at 06:45

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As Ngram shows,, it appears that "not all of X are" has been more common than "all of X are not" since the 1880s. "All that glitters is not gold" is a somewhat archaic phrasing; as Wikipedia notes, the popular form of this proverb today comes from a line by Shakespeare, when this "all...not" form was presumably more common.

See also the question “Is it wrong to use ‘not’ in sentences that have an ‘all . . . not’ form?”, which concludes that it is illogical and actually ambiguous.

John Lawler's answer demonstrates that, though potentially ambiguous, there's nothing "illogical" about this construction.

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