Quantification over the empty set 
Possible Duplicate:
Why is predicate “all” as in all(SET) true if the SET is empty? 

In don't quite understand this quantification over the empty set:
$\forall y \in \emptyset: Q(y)$
The book says that this is always TRUE regardless of the value of the predicate $Q(y)$, and it explain that this is because this quantification adds no predicate at all, and therefore can be considered the weakest predicate possible, which is TRUE.
I know that TRUE is the weakest predicate because $ $P$ \Rightarrow$ TRUE is TRUE for every $P$.
I don't see what is the relationship between this weakest predicate and the quantification. 
 A: When you see a quantification like ‘$\forall \phi x : \psi x$’, this is shorthand for ‘$\forall x : \phi x \to \psi x$’. Since ‘$x \in \emptyset$’ is false for all ‘$x$’, the antecedent of ‘$x \in \emptyset \to \psi x$’ will always be false. Thus, the entire conditional statement is always true.
Edit:
In the case of existential quantification, the statement ‘$\exists \phi x : \psi x$’ is shorthand for ‘$\exists x : \phi x \land \psi x$’, so in this case, the conjunction ‘$x \in \emptyset \land \psi x$’ will always be false.
A: Well if $\forall y \in \emptyset : Q(y)$ were false, we would be able to find some $y \in \emptyset$ such that $Q(y)$ were false. However, there are no $y \in \emptyset$. So $\forall y \in \emptyset : Q(y)$ should be true.
A: A peculiar explanation. 
Whatever $Q(y)$ may be, it is true that for all $y\in \emptyset$, the sentence $Q(y)$ is true. For the empty set is $\dots$ empty.  Every unicorn likes wine. 
A: Because there are no members to the set, anything you say about a member can be considered trivially true. It's not really to say that there are actually members for which Q, but rather for all y, Q(y)--i.e. there are no circumstances where y and not Q(y). 
A: I think I got the idea, but it confuses me when I compare this reasoning with the existencial quantifier over the empty set
$ \exists x \in \emptyset : P(x) $
I think this is TRUE because I'm quantifying over the empty set too.
