I know this statement is not always true, but I'm having a hard time proving it.

I'm also wondering what the difference between:

$[\exists x \in U, P(x)] \implies [\forall x \in U, P(x)]$


$[\exists x \in U, P(x)] \implies [\forall y \in U, P(y)]$

would be?

For the first one, when I try to negate it and show that the negation is true, I end up with something like:

$[\exists x \in U, P(x)] \land [\exists x \in U, \lnot P(x)]$,

but if it was written as the second one I could just provide some counterexample y...?

Or am I just not parsing the statement correctly?

Thanks in advance!

  • $\begingroup$ It is only true if $U$ is non-empty. $\endgroup$ – user99680 Jun 18 '14 at 6:47
  • $\begingroup$ There is no difference between the two statements - the choice of letter is not significant. To disprove it, you need to show a single counterexample to the statement. (Actually, the negation isn't always false, just like the original isn't always true). $\endgroup$ – user61527 Jun 18 '14 at 6:48
  • $\begingroup$ To prove that your statement is not a logical truth, demonstrate a domain with two elements such that $P$ holds for one and not the other. $\endgroup$ – Robert Wolfe Jun 18 '14 at 6:48
  • $\begingroup$ My bad, I flipped the two premises. $\endgroup$ – user99680 Jun 18 '14 at 6:51
  • $\begingroup$ @Bryan: I was about to do just that to review my logic: are you going to do it yourself? $\endgroup$ – user99680 Jun 18 '14 at 6:53

Unless you restrict the set $U$ to having one element, then your statement is not a theorem: you basically state: if there is one element that satisfies property $P$, then all elements satisfy property $P$. Like Bryan said, a counterexample, i.e., a world/interpretation in which your premises are not mapped to truths. Consider a world with two elements {$a,b$}, and just declare $P(a) $ $\land \neg P(B) $ . For a more substantial example, let $P(x):=$ x is even, be a property defined on the universe of all numbers, then your statement would be interpreted as: "if there is an even number, then all numbers are even." Or consider the example with $P(x)$:= "x is male" , defined in the world of all people. Then, you interpret this as: If there is a male, all people are males.

And, actually $ A \implies B $ is equivalent to $\neg A \lor B$, so that its negation is $A \land \neg B $, i.e., your statement $$[\exists x \in U, P(x)] \implies [\forall x \in U, P(x)]$$ negates to:

$$ [\exists x \in U, P(x)] \land \neg [\forall x \in U, P(x)]$$ negates to:

  • $\begingroup$ Thank you for your answer! but doesnt $\lnot [\forall x \in U, P(x)]$ negate to what I had in the second part before? thanks again $\endgroup$ – user157742 Jun 18 '14 at 7:30
  • $\begingroup$ @user157742: It negates to " There is x in U with $\neg P(x) $, which is indeed the same as what you wrote; sorry if I misread before, you were correct. $\endgroup$ – user99680 Jun 18 '14 at 7:33
  • $\begingroup$ ahh. Thanks! I've got it now! $\endgroup$ – user157742 Jun 18 '14 at 7:36
  • $\begingroup$ Good going, user 157742. $\endgroup$ – user99680 Jun 18 '14 at 7:38

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