# Thread: Need Help showing S can not be a propsition??

1. ## Need Help showing S can not be a propsition??

Show that if S is a proposition, where S is the conditional statement “If S is true, then unicorns live," then“Unicorns live” is true. Show that it follows that S can not be a proposition.

2. Hello Grillakis,

Here is my guess

<---> means equivalent
---> means implies

1. “If S is true, then unicorns live," then“Unicorns live” is true <---> S
2. Let: unicorns live <--> P
3. “If S is true, then unicorns live," <---> S ---> P

4. From 1. ( S ---> P) ---> P <--->
S
5. But from 3. S--->P is true
6. from 5 & 4 P is true
7. But since S <---> P therefore S is true
8. So S is an assertion and can't be a proposition, which is required to proof

Hope this helps

Hello Grillakis,

Here is my guess

<---> means equivalent
---> means implies

1. “If S is true, then unicorns live," then“Unicorns live” is true <---> S
2. Let: unicorns live <--> P
3. “If S is true, then unicorns live," <---> S ---> P

4. From 1. ( S ---> P) ---> P <--->
S
5. But from 3. S--->P is true
6. from 5 & 4 P is true
7. But since S <---> P therefore S is true
8. So S is an assertion and can't be a proposition, which is required to proof

Hope this helps

4. I, on the other hand, don't understand what amrmuhammed said!

He seems to be saying S is an "assertion" (alway true? I would call that a "tautology") and so not a proposition. My understanding is that a "proposition" is any statement that is either true of false (regardless of whether we know which). A statement that is always true is certainly a proposition. I think, rather, that we need to look at it like this:

If S is true, then the statement "if S is true, then unicorns live" which IS S is a true statement. But then since whenever "if P then Q " is true and "P" is true, it follows that Q is true, we must have "unicorns live".

What about if S is false? The only way a conditional statement can be false is if the hypothesis is true and the conclusion false. But since the hypothesis is "S is true", that does not hold here. S cannot be false!

So far that is what amrmuhammed said. But the fact that S must be true does not mean it cannot be proposition!

But it does follow that, since S is true, the statement "unicorns live" is also true. And now we need one more fact: there are NO living unicorns! For that reason, the statement S cannot be true and since it also cannot be false, it cannot be a proposition at all.

5. Originally Posted by HallsofIvy
I, on the other hand, don't understand what amrmuhammed said!

He seems to be saying S is an "assertion" (alway true? I would call that a "tautology") and so not a proposition. My understanding is that a "proposition" is any statement that is either true of false (regardless of whether we know which). A statement that is always true is certainly a proposition. I think, rather, that we need to look at it like this:

If S is true, then the statement "if S is true, then unicorns live" which IS S is a true statement. But then since whenever "if P then Q " is true and "P" is true, it follows that Q is true, we must have "unicorns live".

What about if S is false? The only way a conditional statement can be false is if the hypothesis is true and the conclusion false. But since the hypothesis is "S is true", that does not hold here. S cannot be false!

So far that is what amrmuhammed said. But the fact that S must be true does not mean it cannot be proposition!

But it does follow that, since S is true, the statement "unicorns live" is also true. And now we need one more fact: there are NO living unicorns! For that reason, the statement S cannot be true and since it also cannot be false, it cannot be a proposition at all.
HallsofIvy, what I understood is how he is going about doing it breaking each preposition up and setting it up as you set as a tautology. Thanks for your help also.