# In classical logic, why is $(p\Rightarrow q)$ True if $p$ is False and $q$ is True?

Provided we have this truth table where "$p\implies q$" means "if $p$ then $q$":

$$\begin{array}{|c|c|c|} \hline p&q&p\implies q\\ \hline T&T&T\\ T&F&F\\ F&T&T\\ F&F&T\\\hline \end{array}$$

My understanding is that "$p\implies q$" means "when there is $p$, there is q". The second row in the truth table where $p$ is true and $q$ is false would then contradict "$p\implies q$" because there is no $q$ when $p$ is present.

Why then, does the third row of the truth table not contradict "$p\implies q$"? If $q$ is true when $p$ is false, then $p$ is not a condition of $q$.

I have not taken any logic class so please explain it in layman's terms.

Administrative note. You may experience being directed here even though your question was actually about line 4 of the truth table instead. In that case, see the companion question In classical logic, why is $(p\Rightarrow q)$ True if both $p$ and $q$ are False? And even if your original worry was about line 3, it might be useful to skim the other question anyway; many of the answers to either question attempt to explain both lines.

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I assume for now that when you say "p is not a condition for q", you mean p is not a necessary condition for q. And so it is not. "If p then q" does not mean p is necessary for q; it means p is sufficient for q. – Michael Hardy Oct 7 '11 at 23:00
Consider the following (totally informal) proposition: if it rains tomorrow, I'll bring my umbrella. However, the fact that I'll bring my umbrella doesn't imply it will rain tomorrow. – F M Oct 7 '11 at 23:09
@900: You're just being lazy now. – Asaf Karagila Aug 15 '14 at 0:27
@900: I see now that it was bumped before the edit; but I haven't known you the man to do half a job, just because you want to do plenty of half jobs. – Asaf Karagila Aug 15 '14 at 0:41
If pigs could fly... – ahorn Sep 13 '15 at 17:24

If you don't put any money into the soda-pop machine, and it gives you a bottle of soda anyway, do you have grounds for complaint? Has it violated the principle, "if you put money in, then a soda comes out"? I wouldn't think you have grounds for complaint. If the machine gives a soda to every passerby, then it is still obeying the principle that if one puts money in, one gets a soda out.

Similarly, the only grounds for complaint against $p\to q$ is the situation where $p$ is true, but $q$ is false. This is why the only F entry in the truth table occurs in this row.

If you imagine putting an F on the row to which you refer, the truth table becomes the same as what you would expect for $p\iff q$, but we don't expect that "if p, then q" has the same meaning as "p if and only if q".

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It does not violate the principle but we cannot be sure that the soda machine works so we cannot be sure that the principle is true. However, it seems that I have misinterpreted the purpose of the truth table. – user701510 Oct 8 '11 at 5:05
@user701510, the truth table is designed to be useful under a quantifier, so that we can define that "the soda machine works" means "every time someone walks past the machine, it is true that if they put money in, a soda comes out". We want to interpret the if-then such that we don't consider the machine to not work simply because some people walk past without buying from it. – Henning Makholm Oct 8 '11 at 17:09
Your soda example has made sure that I never forget the truth table of p->q! Thanks – Pavan Manjunath Mar 11 '15 at 0:54

$p\Rightarrow q$ is an assertion that says something about situations where $p$ is true, namely that if we find ourselves in a world where $p$ is true, then $q$ will be true (or otherwise $p\Rightarrow q$ lied to us).

However, if we find ourselves in a world where $p$ is false, then it turns out that $p\Rightarrow q$ did not actually promise us anything. Therefore it can't possibly have lied to us -- you could complain about it being irrelevant in that situation, but that doesn't make it false. It has delivered everything it promised, because it turned out that it actually promised nothing.

As an everyday example, it is true that "If John jumps into a lake, then John will get wet". The truth of this is not affected by the fact that there are other ways to get wet. If, on investigating, we discover that John didn't jump in to the lake, but merely stood in the rain and now is wet, that doesn't mean that it is no longer true that people who jump into lakes get wet.

However, one should note that these arguments are ultimately not the reason why $\Rightarrow$ has the truth table it has. The real reason is because that truth table is the definition of $\Rightarrow$. Expressing $p\Rightarrow q$ as "If $p$, then $q$" is not a definition of $\Rightarrow$, but an explanation of how the words "if" and "then" are used by mathematicians, given that one already knows how $\Rightarrow$ works. The intuitive explanations are supposed to convince you (or not) that it is reasonable to use those two English words to speak about logical implication, not that logical implication ought to work that way in the first place.

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+1 Very nice! Exactly the problem I had for years. People try to define it with "If p, then q" but that is NOT the definition :-). – TFuto Mar 28 '14 at 17:55

@user701510 Conditional ($\Rightarrow$) also known as "material implication", "material consequence", or simply "implication" follows the condition 'if...then'

| p | q | p -> q |
| T | T |   T    |
| T | F |   F    |
| F | T |   T    |
| F | F |   T    |


$p \Rightarrow q$ in the best and most simple way I understand it is by giving a situation. For example, in checking a test paper.

First row implies that "If the given statement or question is right, and you gave the right answer, then you are correct."

Second row: "If the given statement or question is right, but you gave the wrong answer, then you are definitely incorrect."

Third row: "If the given statement or question is wrong (e.g. 'partially and grammatically incorrect by its sense'), but you gave the right answer (e.g. 'you get the point', 'you comprehend on the thought of what is being asked'), then you are correct."

Fourth row: "If the given statement or question is wrong (completely wrong), whatever your answer might fail, then it may be a bonus point.

I asked my professor in Discrete Structures (Mathematics), I just applied on the given condition.

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Welcome to math.se. Tip: you can write click on equations you see in other posts, choose "show math as", "tex commands", to see how to input equations. Enjoy~ – DanielV Jul 9 '14 at 6:43

To understand why this table is the way it is, consider the following example:

"If you get an A, then I'll give you a dollar."

The statement will be true if I keep my promise and false if I don't.

Suppose it's true that you get an A and it's true that I give you a dollar. Since I kept my promise, the implication is true. This corresponds to the first line in the table.

Suppose it's true that you get an A but it's false that I give you a dollar. Since I didn't keep my promise, the implication is false. This corresponds to the second line in the table.

What if it's false that you get an A? Whether or not I give you a dollar, I haven't broken my promise. Thus, the implication can't be false, so (since this is a two-valued logic) it must be true. This explains the last two lines of the table.

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its the the table for logical implication... To understand why this table is the way it is, consider the following example:

P-"If you get an A", Q-"then I'll give you a dollar."

The statement will be true if I keep my promise and false if I don't.

Suppose it's true that you get an A and it's true that I give you a dollar. Since I kept my promise, the implication is {\it true}. This corresponds to the first line in the table.

Suppose it's true that you get an A but it's false that I give you a dollar. Since I didn't keep my promise, the implication is false. This corresponds to the second line in the table.

What if it's false that you get an A? Whether or not I give you a dollar, I haven't broken my promise. Thus, the implication can't be false, so (since this is a two-valued logic) it must be true. This explains the last two lines of the table.

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p->q If I have chocolate, then I am happy.

T T -> T If I have chocolate, then I am happy. As initially stated.

T F -> F If I have chocolate, then I cannot be not happy, by the initial statement.
..............That's why this is false.

F T -> T If I do not have chocolate, I could still be happy
.............. (perhaps because I have a cookie).
.............. This is the one you asked about. Nobody said that p is a
.............. NECESSARY condition for q, just that it is a SUFFICIENT condition.

F F -> T If I do not have chocolate, then I could also be not happy
..............(because nothing else is making me happy).

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From the other answers, the most convincing and reasonable explanation why logic implication is defined the way it is, is the idea of sufficient (versus "necessary") condition for something to be true.

NOTE: I don't buy the answer/argument that "if-then" is not an equivalent definition of "=>"; it is just that we tend to have a different notion of "if-then" in everyday life, that of a necessary one.

In short:

Any if-then statement that would "break" only when the conclusion is false while the condition is true is a logic implication.

## Examples from everyday life

The following are examples of statements that are logical implications.

1. "if (it is raining), then (there are clouds in the sky)"

• T - T (ok checks out) T
• T - F (oops!) F
• F - T (Ok, there are clouds in the sky but it doesn't rain, but that does not "break the rule") T
• F - F (no rain, no clouds, still a valid statement as far as I can tell) T
2. "if (I find my room not the way I left it), then (someone was in my room)"

• T - T (ok checks out so far) T
• T - F ("rule" breaks) F
• F - T (does not break the logic of the "rule") T
• F - F (this does not break the logic either) T

## A more elaborate explanation with example from science

Consider an example in the field of medical diagnosis. The basic (and ideal) premise of diagnosis from symptoms is to derive valid, sufficient rules that can safely conclude a diagnosis of an illness over other illnesses based on symptom observations. Let's say some medical scientist studies illness A and proposes the following diagnostic rule:

"IF (symptom B and symptom C are observed) THEN (--for sure-- the patient is under illness A)."

He then goes through all the documented cases of the illness (or conducts a new study) and tries to see whether that rule is valid:

1. If a patient in the records had the symptoms and he was also found to have illness A (1st row of the truth table), then so far so good.

2. If a patient is found to have the symptoms but not the illness, that breaks or falsifies the rule (2nd row in the truth table), and the rule has to be reconsidered and revised because it simply does not work; the rule, as a logic implication, is false.

3. If some patient is found to have illness C but not the symptoms (3rd row in the truth table), that does not reduce the validity of the rule in any way as a way of making a safe conclusion; it only reduces its usefulness, depending on how many cases it may miss.

4. If some patient is found not to have the symptoms nor the illness (4th row in the truth table), that is irrelevant to the validity of the rule.

So, if the scientist finds only records of the 1st, 3rd, and 4th case, then he has a valid rule. Moreover, the potential of the rule to break in the 2nd case makes it a logic implication.

From the above, you can see that the way logic implication is defined (with the third and fourth rows being True) finds extensive use in math and science and, eventually, is what makes sense.

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• If a dog, then has 4 legs- true
• If a dog, then has no legs- false
• If not a dog(may be a cat), then has 4 legs- true
• If not a dog( a snake), then has no legs - true
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I thought to extract (partially) from this excellent intuitive explanation from Philosophy SE, which I rewrite marginally, but I do not use blockquotes which may aggravate readability:

• As to an intuition for 'material implication' (ie: the logical 'if-then', or P → Q), consider the truth-valuation and how upset you might be given the values of P and Q.

So suppose I claim P->Q, where:

P = "it rains tonight"

Q = "I will go to the movies with you".

For each of the following 4 cases, how upset will you be and how trustworthy do you think I am?

$\Large{1.}$ P is true and Q is true:

You won't be upset at all, it's what you'd expect. If P occurs you'd expect Q to occur.

$\Large{2.}$ P is true and Q is false:

You'd be very upset. If P happens, you expect Q to happen, and when it doesn't, you should think I lied (it's raining and I didn't go to the movies with you means I'm an untrustworthy liar).

$\Large{3.}$ P is false and Q is true:

Hmmm...weird: so we went to the movies without it raining. I didn't say what I would do if it did not rain; so going to the movies is just fine, I haven't lied about it.

$\Large{4.}$ P is false and Q is false:

Also weird, but same reasoning as 3. I didn't make any claims about what would happen if it did not rain; so not going, though not great, doesn't make me a liar.

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P implies Q means that Q is true when ever P is; it does not mean in addition that Q is false when ever P is...otherwise as a net result Q will equate to P.........No.

The sentiment here is causation and in that : P is a sufficient condition for Q and there may be other as well.

Therefore when P is false Q can be both true and false in the truth table (where such entries are accepted as true) the exact value of Q depending on other sufficient conditions.

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Q: When an conclusion statement is false?

A: Only if the conclusion contradicts with a true condition.

$T\implies T$ is true, since true implies true.

$T\implies F$ is false, since true does not imply false.

$F\implies T$ is true, since there is no true condition that is contradicted.

$F\implies F$ is true, since there is no true condition that is contradicted.

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