> 
> **Possible Duplicate:**  
> 
> [How can we reason about “if P then Q” or “P only if Q” statements in propositional logic?](https://philosophy.stackexchange.com/questions/4089/how-can-we-reason-about-if-p-then-q-or-p-only-if-q-statements-in-proposition) 
> 
> 
> 

In a logic exercise, suppose this argument is given:

> 
> P1: If there's a God, then there's a world.  
> 
>  P2: If there's a God.  
> 
>  C : There's a world.
> 
> 
> 

This argument is valid. But if we put the first premise this way:

> 
> P1: If there's a world, then there's a God.  
> 
>  P2: If there's a God.  
> 
>  C : There's a world.
> 
> 
> 

Then, it won't be valid because we interchanged the parts of *if-then*. My question is: 

Doesn't "If there's a God, then there's a world" and "If there's a world, then there's a God." mean really the same thing?

If it doesn't mean the same thing in logic, then how do we know which one should we use? Is there a rule to how we should arrange the parts of if-then statement so that we know what we are doing? If we don't be careful with this, our logic can go wrong,