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Absane
09-10-03, 08:04 PM
Quick question...

A -> ~B

This is true: B -> ~A, right?

I think so, because if B -> A -> ~B.

I just answered my own question I believe. Sorry.
lethe
09-10-03, 08:07 PM
Originally posted by 4DHyperCubix
Quick question...

A -> ~B

This is true: B -> ~A, right?
yes. this is the contrapositive of the original statement.


I think so, because if B -> A -> ~B.


so B implies ~B?!?! that s a contradiction!
Absane
09-10-03, 08:19 PM
Yes, I know it is a contradition. I forgot to include that fact. Sorry,
ele
09-13-03, 07:56 AM
No i think the logic is wrong.
Absane
09-13-03, 08:07 AM
How is that?

Basically the problem is saying this:

If A, then C.
Not C, then Not A

Because if "Not C" implies A, then A implies C... a contradicatiopn.
hotsexyangelprincess
09-13-03, 04:29 PM
'A' may imply 'C', but '~A' does not imply 'not ~C'
i think :m:
ele
09-13-03, 05:40 PM
If i love you(A) then I'll kiss you.(C)

If I'm not kissing you (~C) then that doesnt mean I don't love you(~A) does it?
Hemlock
09-16-03, 09:06 AM
I don't understand 4DHyperCubix!
A -> ~B Are you saying that A is approximately equal to B??
HallsofIvy
09-16-03, 09:38 AM
If i love you(A) then I'll kiss you.(C)

If I'm not kissing you (~C) then that doesnt mean I don't love you(~A) does it?

Yes, it does. You assert "If i love you(A) then I'll kiss you.(C)"

If you are not kissing, then according to what you just said, you must not love- if you DID, you would be kissing.

If you are going to quibble about "not kissing RIGHT NOW", then all I can say is: state you premises carefully.
Dapthar
09-16-03, 01:49 PM
Originally posted by Hemlock
I don't understand 4DHyperCubix! Are you saying that A is approximately equal to B??
He doesn't mean "approximately" by ~. He is using it as a logical not, sometimes written as NOT. The (extended) truth table for this expression is as follows.

A<font color="#ffffff">....</font>B<font color="#ffffff">..</font>~B<font color="#ffffff">....</font>A &rArr; ~B
T<font color="#ffffff">....</font>T<font color="#ffffff">....</font>F<font color="#ffffff">..........</font>F
T<font color="#ffffff">....</font>F<font color="#ffffff">....</font>T<font color="#ffffff">..........</font>T
F<font color="#ffffff">....</font>T<font color="#ffffff">....</font>F<font color="#ffffff">..........</font>T
F<font color="#ffffff">....</font>F<font color="#ffffff">....</font>T<font color="#ffffff">..........</font>T

Also note that &not; is sometimes used for NOT as well.
ele
09-16-03, 05:12 PM
The nature of kissing is that it isnt constant. The naturre of love is that it is. That the logic doesnt work is because the logic doesnt have a qualifier for this. Lots of traditional "logic" falls down in this kind of area.
Hemlock
09-17-03, 08:25 AM
He doesn't mean "approximately" by ~. He is using it as a logical not, sometimes written as NOT. The (extended) truth table for this expression is as follows.

Also note that ¬ is sometimes used for NOT as well. Oh okay Dapthar. Thanks! :)

What is all that about a truth table for it?? Could you explain it please, or will it take to long??
Dapthar
09-17-03, 03:36 PM
Originally posted by Hemlock
Oh okay Dapthar. Thanks! :)

What is all that about a truth table for it?? Could you explain it please, or will it take to long??

A truth table is essentially a large list of all the possible inputs and outputs for a logical statement. The number of outputs is related to the number of variables by the following formula o = 2<sup>v</sup>, where o is the number of possible outputs and v is the number of input variables. This is because there are only two possibilities for a logical variable, T (true) or F (false).

A page with more information on truth tables, with plenty of examples, is located here: http://www.wikipedia.org/wiki/Truth_table.
Electric Jaguar
09-21-03, 07:20 AM
Originally posted by 4DHyperCubix
I think so, because if B -> A -> ~B.
4DHyperCubix is merely using Indirect Derivation here, right? No confusion.


Originally posted by HallsofIvy
state you premises carefully.
Are you sure the wouldn't mind? ;)
Pete
10-07-03, 02:18 AM
The nature of kissing is that it isnt constant. The naturre of love is that it is. That the logic doesnt work is because the logic doesnt have a qualifier for this. Lots of traditional "logic" falls down in this kind of area.
No, it means that the premises have to be stated more carefully.

If I love you, then I kiss you at times.

Therefore, if I never kiss you, then I do not love you.
Absane
10-08-03, 11:49 PM
Originally posted by Pete
If I love you, then I kiss you at times.

Therefore, if I never kiss you, then I do not love you.

Your deductive logic is correct. However, your conclusion is false. Therefore, at least one premise is false. You only have one premise: "If I love you, then I kiss you at times." Some people do not kiss to show their love.



:rolleyes:
malkiri
10-09-03, 08:42 AM
Edit: Whoops, missed the post with this table already. Ah well.

The truth table for 'implies':

A B A -> B
T T T
T F F
F T T
F F T

If A is not true, then this statement can make no claim on the truthfulness of B. The only way this statement can be false is if A is true but B is not.

A = John is crying.
B = John is sad.

If John is crying, we can infer that John is sad. If John is not crying, we can't say whether he's sad or not - maybe he just doesn't have any tear ducts.

http://www.wikipedia.org/wiki/Logical_conditional
Absane
10-10-03, 05:31 PM
Why did you bring this up?
malkiri
10-11-03, 11:12 AM
In response to this:

'A' may imply 'C', but '~A' does not imply 'not ~C'
Absane
10-11-03, 04:37 PM
Ah. Makes sense. There is no disputing logic! :)