[Math] Confusion with Implication

logicpropositional-calculus

Forgive me if its a dumb question, I just started reading Mathematical Logic

Question:-

Let we have an Implication,

A ⟹ B

And its Truth Table is

enter image description here

This Implication is true for All False values of “A” irrespective the value of B.

By this we can conclude that "If not A then B is false/true" both are true. So we can say "If not A then B" because B only has two value True or False and here B can be any thing.

By this reasoning "If A then B" should be equal to If not A then B

But when we write a mathematical formula and calculate their truth table then both are different

  • "If A then B"

    Mathematical Formula :-

    A ⟹ B

    Truth Table:-

enter image description here

Here I am taking only one case(i.e. B is True)

  • "If not A then B"

    Mathematical Formula :-

    not A ⟹ B

    A    B             If not A then B
    
    T    T                    T
    T    F                    T
    F    T                    T
    F    F                    F
    

I am missing something or have some conceptual flaws but unable to find, Please help me

Thank You


P.S. :- Sorry in advance because my English is not upto that mark. Edits are welcome 🙂

Best Answer

First of all, I am confused why you would say that $\neg A \rightarrow B$ should be equivalent to $A \rightarrow B$, given that you just argued that the value of $B$ should not matter, rather than the value of $A$! In fact, when $A$ is true it is no longer the case that the value of $B$ doesn;t matter, and so you immediately get that $\neg A \rightarrow B$ is not the same as $A \rightarrow B$.

What would have made a little more sense is if you would have focused on $A \rightarrow \neg B$ instead, because (as you correctly observed) if $A$ is false, then $A \rightarrow B$ has the same truth-value as $A \rightarrow \neg B$ (namely True). However, that still does not mean that they are equivalent, because equivalence means that they should have the same truth-value under any conditions (and again, you have only shown them to have the same truth-value under the condition that $A$ is False). And so $A \rightarrow \neg B$ is also not equivalent to $A \rightarrow B$

Finally, if you are trying to change the $A$ into a $\neg A$, because $A \rightarrow B$ is true when $A$ is false ... well, that makes even less logical sense. Here is an example to demonstrate your faulty logic. Take statement $\neg A$. This statement is true when $A$ is false ... ok, so by your logic we should be able to change the $A$ with a $\neg A$ and get the same statement? No, because changing the $A$ with a $\neg A$ in $\neg A$ gives us $\neg \neg A$, which is equivalent to just $A$ ... which is not at all equivalent to the original $\neg A$.

Don't confuse statements with their truth-values!!

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