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Specializations => Semantics and Pragmatics => Topic started by: capjac145 on November 16, 2019, 12:59:33 PM

Title: Predicate Logic and Nobody
Post by: capjac145 on November 16, 2019, 12:59:33 PM
Hi!

Question about predicate logic.
I want to figure out the translation of a sentence like: Nobody is a college student and not poor. 
For now, I have something like this: ¬∃x(person'(x)∧student'(x)→¬poor'(x))
Would this be close to the meaning? Any input is awesome.

Title: Re: Predicate Logic and Nobody
Post by: Daniel on November 16, 2019, 10:03:47 PM
You seem to be mixing up two approaches: you're using both existential quantification and also material implication, when you only need one.

Is this homework? You should try to figure it out on your own (and also not be posting the questions online).

If not, if you just want to talk about the ideas behind predicate logic, we can discuss it here, but be aware this will NOT necessarily result in the "right" answers for your class which will use one specific approach that won't be the same as other classes or what people on the internet say.
Title: Re: Predicate Logic and Nobody
Post by: capjac145 on November 17, 2019, 12:39:44 PM
It isn't homework. I just heard the sentence and was trying to figure out exactly what it meant, so I thought to approach it that way. What is material implication exactly?

Thanks!
Title: Re: Predicate Logic and Nobody
Post by: Daniel on November 17, 2019, 01:16:35 PM
"→" is material implication. It's similar to English "if", but in a very specific sense. It's a relationship of truth conditions.
https://en.wikipedia.org/wiki/Material_implication_(rule_of_inference)

The sentence you're thinking about is complicated. It can be reduced to a simpler structure by substitution (eliminating some of the negation, for example), but I'd suggest starting with a basic sentence and working it out from there.

Notice also the relationship between material implication and negation mentioned at the start of the Wikipedia article:
Quote
In propositional logic, material implication[1][2] is a valid rule of replacement that allows for a conditional statement to be replaced by a disjunction in which the antecedent is negated. The rule states that P implies Q is logically equivalent to not-P or Q and that either form can replace the other in logical proofs.

To begin, I'd suggest trying a simpler sentence like "Nobody sleeps", and working up from there.
Title: Re: Predicate Logic and Nobody
Post by: mallu on December 08, 2019, 01:20:04 PM
For all X (student(x) ->poor(x) ) will do , isnt it?
Title: Re: Predicate Logic and Nobody
Post by: Daniel on December 08, 2019, 09:28:48 PM
That's one way to represent it (though you could replace 'for all X' with symbols), among several options, and assuming the most natural scope reading for the original sentence.
Title: Re: Predicate Logic and Nobody
Post by: mallu on December 08, 2019, 11:12:18 PM
That's one way to represent it (though you could replace 'for all X' with symbols), among several options, and assuming the most natural scope reading for the original sentence.
. I dont know how to type those symbols here, Could you please help me
Title: Re: Predicate Logic and Nobody
Post by: Daniel on December 09, 2019, 03:15:31 AM
There's actually a built-in addition to the forum for these symbols (at least a few: I could try to add more by request if needed):
http://linguistforum.com/phonetics-and-phonology/type-ipa-within-the-forum!-(instructions)/
(You'll need to be in the 'advanced' editor, rather than just the quick reply mode below a post.)

Alternatively there aren't many other options than just copying and pasting. I usually go to this page if I need to find something unusual:
https://en.wikipedia.org/wiki/List_of_logic_symbols