# $(\forall x \in D) (P(x)) \Leftrightarrow (\forall x)(x\in D \implies P(x))$. how are two the same?

Assuming all the variables are naturals, why are those two equal? I don't get how $\implies$ is introduced in the latter equation: $(∀x)(x∈D \implies P(x))$

Thanks.

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In the latter equation, you're looking at all $x$, but all you want is that $P(x)$ occurs whenever $x\in D$. I think that's where $\implies$ comes in. – Ian Coley Sep 17 '13 at 20:50

## 3 Answers

As Peter Smith notes, most commonly

$$\forall x \in D.\ P(x) \quad \text{ is defined as }\quad\forall x.\ x \in D \implies P(x).$$

However, should this be unsatisfactory explanation for you, there is alternative argument below.

First \begin{align} \mathtt{true} \implies P(x) \quad \text{ is equivalent to } \quad P(x), \\ \mathtt{false} \implies P(x) \quad \text{ is equivalent to } \quad \mathtt{true}. \end{align} Hence,

\begin{align} \forall x \in D.\ P(x) &\quad\text{ is equivalent to }\quad \forall x \in D.\ x \in D \implies P(x), \\ \forall x \notin D.\ \mathtt{true} &\quad\text{ is equivalent to }\quad \forall x \notin D.\ x \in D \implies P(x). \end{align} Then, if we join them with $$P'(x) = \begin{cases}P(x) & \text{for }x \in D \\ \mathtt{true} & \text{for }x \notin D\end{cases}$$

we get

$$\forall x.\ P'(x) \quad\text{ is equivalent to }\quad \forall x.\ x \in D \implies P(x).$$

I hope this helps $\ddot\smile$

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No, it doesn't help. The straightforward answer is that $(\forall x \in D)(P(x)$ is usually just defined as a slick abbreviation for $(\forall x)(x\in D \implies P(x))$. – Peter Smith Sep 17 '13 at 22:35
@PeterSmith Usually it is defined this way, but it does not have to be, e.g. if the universe is not a set, perhaps there is some specific semantic to it. Of course, point taken, this is beyond the scope of this question. – dtldarek Sep 17 '13 at 22:45

Because that's what the $\forall x\in D. P(x)$ notation is defined to mean.

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This is, of course, exactly right! – Peter Smith Sep 17 '13 at 22:32

If you interpret both D and P as sets, both of them say precisely that D is a subset of P.

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