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Logic reprised

Posted by TheFallen on March 16, 2003

In Reply to: So, dear friends, in modern english posted by R. Berg on March 16, 2003

: : are the structures of 'Not be (or Do )sth else'
: : and 'All sth to be (or do) not sth else' still in used side by side ? coz too me they are mean two difference things. And I even don't know if the way to understand 'All that glitters is not gold' as 'Not all that glitters is gold' still conventional in modern english. If it is , then how should we differenciate the meanings of those two structures.

: : By the way, I have a wild guess that 'Not all that glitters is gold'could be rephrased in full as 'it is not that all that glitters is gold'.

: : Thank you all

: Your last statement is correct. Logically, "It is not that all that glitters is gold" says the same thing as "Not all that glitters is gold." We could also say "It is not true that all that glitters is gold."

: I can't answer the first question because I don't know what you mean by "sth else."

If you want to avoid confusion, do not use constructions in English such as "all X are not Y", because they are unclear. The expression "all that glitters is not gold" is only permissible because it's traditional - not because it's well-expressed or clear. (In fact, it's quite the opposite, as you yourself found out).

I reproduce Fred's post (fractionally edited) from the earler thread on this subject, because he explained it far better than I could.

*** start snip ***

The sentence 'All that glitters is not gold' is used in logic text books. It often occurs in the
section on Aristotle's logic. Sentences of the form 'All X are not Y' are eschewed since they are ambiguous, since the word 'not' may apply solely to 'Y' or to the whole sentence 'All X are Y.'

If the former, we get 'All X are non-Y' or 'No X are Y.'

If the latter we get 'It is not true that all X are Y' or 'Some X are *not* Y.'

*** end snip ***
Basically, no native English speaker would ever use any other phrase formed in a "all X are not Y" fashion. To use an example, we'd either say "all politicans are dishonest", if we wanted to damn politicians in their entirety for untruthfulness, or we'd say "not all politicians are honest", if we wanted to admit that a few politicians are liars. We would absolutely NEVER say "all politicians are not honest" - it's just not English.