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Math Help - how come false AND false is true?

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    how come false <--> false is true?

    For truth tables and the biconditional proposition operator, how comes false <--> false is true?

    It seems counter intuitive, if I may make a stab at sounding mathematical.

    Thanks!
    Last edited by Truthbetold; September 23rd 2010 at 09:13 PM. Reason: I messed up meaning of my question.
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    Quote Originally Posted by Truthbetold View Post
    For truth tables and the AND operator, how comes false AND false is true?

    It seems counter intuitive, if I may make a stab at sounding mathematical.

    Thanks!
    false AND false is false
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    Yeah....
    I'm getting loopy or something. That's the second time today I haven't made sense.

    I confused AND with the biconditional proposition.

    If I may rephrase my question, how come the truth value of p <--> q where p = q = false is true?
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    Quote Originally Posted by Truthbetold View Post
    Yeah....
    I'm getting loopy or something. That's the second time today I haven't made sense.

    I confused AND with the biconditional proposition.

    If I may rephrase my question, how come the truth value of p <--> q where p = q = false is true?
    Biconditional indicates logical equivalence. Note that p <--> q is the same as (p -> q) AND (q -> p). Other ways you might hear it described are "if and only if", "is a necessary and sufficient condition for", and "is equivalent to".
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    Hello, Truthbetold!

    For truth tables and the biconditional proposition operator,
    how come F \leftrightarrow F is true?

    The biconditional p \leftrightarrow q means that \,p and \,q have the same truth value.

    Hence: . \begin{Bmatrix} T\leftrightarrow T\, \text{ is true.} \\ F \leftrightarrow F\,\text{ is true.} \end{Bmatrix}
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    Quote Originally Posted by Truthbetold View Post
    Yeah....
    I'm getting loopy or something. That's the second time today I haven't made sense.

    I confused AND with the biconditional proposition.

    If I may rephrase my question, how come the truth value of p <--> q where p = q = false is true?

    I think the real problem here, and the one on which yours lays, might be: why the value of A\rightarrow B is true

    whenever the value of A is false, no matter what the value of B is?

    The short answer is that it is so because it is defined so. Period. The long answer may pass through all kinds

    of explantions including what truth really is and stuff. You may want to grab a book with some history of logic in it and read it.

    Tonio
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