[Philosophy] Logic

Philosophy is the devils tool.

 

Its not equivalent

 

False, the sentences are not equivalent.

If it rains, then the sidewalk is wet.

However, the sidewalk may be wet from another reason, and the sidewalk being wet is not causing the rain to fall.

It could have rained, even if the sidewalk is dry.

 

I neither understand philosophy nor am I a philosophy major, but I would interpret 'P is only true if Q is true' as 'P = Q'. Its either that or 'If Q is false, then P is false.' Either way, not the same as 'If P, then Q'.

 

I took Logic 1 and somehow BSed my way through it for a B. Logic 2 came around and I was like man **** this.

 

THIS I understand:

This, however, doesn't make sense:

First sentence is OK, but the second sentence doesn't make sense. Are you sure you didn't copy the Q statement incorrectly?

Could be:
- If P=[it rains], then Q=[the sidewalk is wet]
- P=[It rains] only if Q=[the sidewalk is wet]

Maybe?
If so, then NO. da_juice is right.

 

brohan, this is just the basic distinction between "if, then" statements and "only if" statements.

I can't explain this to you over the internet b/c you'll probably still be confused.

Just read this:

http://en.wikipedia.org/wiki/If_and_only_if

under where it says distinctions from "if" and "only if" because that explains "if" and "only if"
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Also, read your textbook man. This is like chapter 1 in any logic course.

 

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If you go to UH (i'm guessing you do) they have logic textbooks in the library on reserve and this is literally in the very first chapter.

 

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You actually have to look at the website for a logic class. I know you're only taking intro to phil, but since you are covering a chapter of logic..

Here, I google'd it for you

http://www.class.uh.edu/phil/garson/doc.pdf

 

P = it rains, Q = the sidewalk is wet

If it rains, the sidewalk is wet. This is always a true statement, so P -> Q (P implies Q)

If the sidewalk is wet, it does not necessarily mean that it rained (maybe someone washed their car and the water ran off onto the sidewalk). Q does not always imply P.

P iff Q means that Q -> P (Q implies P), so P & Q is always true if Q is true.

 

It's pretty much impossible to explain this over the internet, but I think the difference between the two is pretty much in the wording itself. "If X happens, then Y happens" is innately different than "Y can ONLY happen if X happens." The first one implies that Y happens if X happens, however, there are other ways for Y to happen as well. The second statement implies that there are NO other ways for Y to happen outside of X happening.

 

Just dont go to school

 

So, does this:

P is only true if Q is true..

mean

(A) P is true only if Q is true

or

(B) P is true if and only if Q is true.

I initially interpreted it as (B) (which means P = Q), but maybe (A) is more accurate. In which case, as I said earlier, this statement is equivalent to "if Q is false, then P is false."

Shayan, the best way to understand it is to just write down the truth table.

Code:

P  Q  STMT
0   0   ?
0   1   ?
1   0   ?
1   1   ?

If the truth tables are the same, then the two statements are logically equivalent.

 

It means answer choice "A" durvasa
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I only linked him the if and only if wikipedia article b/c it had a section on "if then" statements.
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Urgh, here i'll just do it for you:

Here's your basic if P->Q truth table..

P|Q|P -> Q|
t t t
t f f
f t t
f f t
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P only if Q is the logical equivalent of..

if ~Q then ~P

They are the same thing. It's just written in a different way.

Make the truth table, it'll show you better than I can

 

Someone want to take a shot at my Philosophy final?

The previous statement is true, the preceding statement is false

 

Edit: The chair has legs. The previous statement is true, the preceding statement is false. Justify.

My answer: What chair?

 

I think a logical statement is "true" so long as it doesn't contradict an existing proposition. So, P -> Q would be T so long as P is false or Q is true (i.e. equivalent to ~P V Q).