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Can a statement be true ( materially) while contradicting a true general rule of mathematics? ( An apparent problem with a conditional statement).
Logic and Principle of InductionStoll, Set Theory and Logic (pg 165): Logic for P->Q and P<->QImplications and Ordinary languageTruth table of proof by contradictionTranslate Quantified FOL Statement into EnglishDefining Material ConditionalTruth-functionality of the “If-Then” connective in EnglishHow does the negation form of a conditional proposition help to build an intuition for the vacuously true cases?Problems teaching introductory logic. Is this a statement? “If x is an integer, then…”How can we prove soundness property if it's possible for our assumption set to contain false assumptions?
$begingroup$
Suppose a student says : "if 17 is even, then 2 is not a divisor of 17".
Surely his teacher would tell him he is wrong, saying that when a number is even, this number has 2 as divisor. The teacher would correct with " if 17 were even, then 2 would be a divisor of 17". In other words, the student's claim contradicts the general rule : " For all number x, if x is even , then x has 2 as divisor." So, if 17 is even...
But, since the sentence uttered by the student is a conditional with a false antecedent, this sentence ( the whole conditional) is true ; in vitue of " ex falso sequitur quodlibet" ( from a false proposition, anything follows).
My question is : what is wrong in the student's claim?
Can this hypothetical case be clarified by saying that
(1) the student's sentence is materially true
(2) the teacher is right in saying that the sentence is false in case it is understood as asserting a consequence relation ( logical consequence) between the antecedent and the consequent?
Or , am I wrong in saying that " if 17 is even , then 2 is not a divisor of 17 " contradicts ( or is incompatible with) " For all number x, if x is even, then x is divisible by 2" ?
logic propositional-calculus
$endgroup$
add a comment |
$begingroup$
Suppose a student says : "if 17 is even, then 2 is not a divisor of 17".
Surely his teacher would tell him he is wrong, saying that when a number is even, this number has 2 as divisor. The teacher would correct with " if 17 were even, then 2 would be a divisor of 17". In other words, the student's claim contradicts the general rule : " For all number x, if x is even , then x has 2 as divisor." So, if 17 is even...
But, since the sentence uttered by the student is a conditional with a false antecedent, this sentence ( the whole conditional) is true ; in vitue of " ex falso sequitur quodlibet" ( from a false proposition, anything follows).
My question is : what is wrong in the student's claim?
Can this hypothetical case be clarified by saying that
(1) the student's sentence is materially true
(2) the teacher is right in saying that the sentence is false in case it is understood as asserting a consequence relation ( logical consequence) between the antecedent and the consequent?
Or , am I wrong in saying that " if 17 is even , then 2 is not a divisor of 17 " contradicts ( or is incompatible with) " For all number x, if x is even, then x is divisible by 2" ?
logic propositional-calculus
$endgroup$
6
$begingroup$
"Surely his teacher would tell him he is wrong"... unless the teacher knew logic, which one hopes is rather common.
$endgroup$
– Lee Mosher
7 hours ago
2
$begingroup$
I don't see any logical distinction between saying "if 17 is even" and "if 17 were different". Yes, they are grammatically different and, since 17 is NOT even, the first form would not be grammatically correct. But in logic, they say the same thing. Was this an English class or a logic class?
$endgroup$
– user247327
7 hours ago
4
$begingroup$
There's nothing to clarify - if the teacher says he's wrong the teacher is simply wrong.
$endgroup$
– David C. Ullrich
7 hours ago
add a comment |
$begingroup$
Suppose a student says : "if 17 is even, then 2 is not a divisor of 17".
Surely his teacher would tell him he is wrong, saying that when a number is even, this number has 2 as divisor. The teacher would correct with " if 17 were even, then 2 would be a divisor of 17". In other words, the student's claim contradicts the general rule : " For all number x, if x is even , then x has 2 as divisor." So, if 17 is even...
But, since the sentence uttered by the student is a conditional with a false antecedent, this sentence ( the whole conditional) is true ; in vitue of " ex falso sequitur quodlibet" ( from a false proposition, anything follows).
My question is : what is wrong in the student's claim?
Can this hypothetical case be clarified by saying that
(1) the student's sentence is materially true
(2) the teacher is right in saying that the sentence is false in case it is understood as asserting a consequence relation ( logical consequence) between the antecedent and the consequent?
Or , am I wrong in saying that " if 17 is even , then 2 is not a divisor of 17 " contradicts ( or is incompatible with) " For all number x, if x is even, then x is divisible by 2" ?
logic propositional-calculus
$endgroup$
Suppose a student says : "if 17 is even, then 2 is not a divisor of 17".
Surely his teacher would tell him he is wrong, saying that when a number is even, this number has 2 as divisor. The teacher would correct with " if 17 were even, then 2 would be a divisor of 17". In other words, the student's claim contradicts the general rule : " For all number x, if x is even , then x has 2 as divisor." So, if 17 is even...
But, since the sentence uttered by the student is a conditional with a false antecedent, this sentence ( the whole conditional) is true ; in vitue of " ex falso sequitur quodlibet" ( from a false proposition, anything follows).
My question is : what is wrong in the student's claim?
Can this hypothetical case be clarified by saying that
(1) the student's sentence is materially true
(2) the teacher is right in saying that the sentence is false in case it is understood as asserting a consequence relation ( logical consequence) between the antecedent and the consequent?
Or , am I wrong in saying that " if 17 is even , then 2 is not a divisor of 17 " contradicts ( or is incompatible with) " For all number x, if x is even, then x is divisible by 2" ?
logic propositional-calculus
logic propositional-calculus
edited 7 hours ago
Eleonore Saint James
asked 8 hours ago
Eleonore Saint JamesEleonore Saint James
1,3581 silver badge18 bronze badges
1,3581 silver badge18 bronze badges
6
$begingroup$
"Surely his teacher would tell him he is wrong"... unless the teacher knew logic, which one hopes is rather common.
$endgroup$
– Lee Mosher
7 hours ago
2
$begingroup$
I don't see any logical distinction between saying "if 17 is even" and "if 17 were different". Yes, they are grammatically different and, since 17 is NOT even, the first form would not be grammatically correct. But in logic, they say the same thing. Was this an English class or a logic class?
$endgroup$
– user247327
7 hours ago
4
$begingroup$
There's nothing to clarify - if the teacher says he's wrong the teacher is simply wrong.
$endgroup$
– David C. Ullrich
7 hours ago
add a comment |
6
$begingroup$
"Surely his teacher would tell him he is wrong"... unless the teacher knew logic, which one hopes is rather common.
$endgroup$
– Lee Mosher
7 hours ago
2
$begingroup$
I don't see any logical distinction between saying "if 17 is even" and "if 17 were different". Yes, they are grammatically different and, since 17 is NOT even, the first form would not be grammatically correct. But in logic, they say the same thing. Was this an English class or a logic class?
$endgroup$
– user247327
7 hours ago
4
$begingroup$
There's nothing to clarify - if the teacher says he's wrong the teacher is simply wrong.
$endgroup$
– David C. Ullrich
7 hours ago
6
6
$begingroup$
"Surely his teacher would tell him he is wrong"... unless the teacher knew logic, which one hopes is rather common.
$endgroup$
– Lee Mosher
7 hours ago
$begingroup$
"Surely his teacher would tell him he is wrong"... unless the teacher knew logic, which one hopes is rather common.
$endgroup$
– Lee Mosher
7 hours ago
2
2
$begingroup$
I don't see any logical distinction between saying "if 17 is even" and "if 17 were different". Yes, they are grammatically different and, since 17 is NOT even, the first form would not be grammatically correct. But in logic, they say the same thing. Was this an English class or a logic class?
$endgroup$
– user247327
7 hours ago
$begingroup$
I don't see any logical distinction between saying "if 17 is even" and "if 17 were different". Yes, they are grammatically different and, since 17 is NOT even, the first form would not be grammatically correct. But in logic, they say the same thing. Was this an English class or a logic class?
$endgroup$
– user247327
7 hours ago
4
4
$begingroup$
There's nothing to clarify - if the teacher says he's wrong the teacher is simply wrong.
$endgroup$
– David C. Ullrich
7 hours ago
$begingroup$
There's nothing to clarify - if the teacher says he's wrong the teacher is simply wrong.
$endgroup$
– David C. Ullrich
7 hours ago
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
First of all, the fact that a material implication is considered true as soon as its antecedent is false, is not the same as ex falso sequitur quodlibet, which says that any statement follows from a contradiction.
But yes, if you interpret the student's claim as a material conditional, then technically the student's claim is considered true. It would be just as true as "If I live in London, then I live in Germany" ... interpreted as a material conditional this is considered true because I don't live in London.
Still, the teacher would say the student is wrong, and offer the correction exactly as you indicated. This is because in practice, the use of mathematics is such that under normal circumstances, when the student makes a statement like this, the student is expected to have used the definition of what it means for a number to be even, rather than that the student is making some kind of smart-aleck claim trying to exploit the paradox of the material implication.
Indeed, note that the teacher does not say that the student's claim is false, but rather that the student did something wrong: the student wrongly applied the definition of even-ness. Thus, the teacher says: "No no, you did that wrong: if 17 is even, then 2 is a divisor of 17".
$endgroup$
$begingroup$
@Bram28.Thanks for the point you made by distinguishing ex falso from the paradox of material implication. As to the fact that the sentence would be considered as mathematically false , which explanation could be given? I mean : there is something wrong in the student's claim, but what precisely?
$endgroup$
– Eleonore Saint James
7 hours ago
2
$begingroup$
I had at least one math teacher who emphatically taught that an "if ... then" sentence in a mathematical setting is a material implication, and therefore the student is correct. I do not recall ever being taught otherwise in a math class. What kind of teachers did you have?
$endgroup$
– David K
7 hours ago
$begingroup$
@EleonoreSaintJames Again, while logically the student is seen as true, the use of mathematics is such that when you make a statement like that, you are expected to have applied some kind of definition. So we can say that the student has not correctly used the definition of what it means for a number to be even. The math teacher would say to the student: "No. You are not using the definition of even correctly. Using the definition of even correcty, we get that if 17 is even, then 2 is a divisor of 17".
$endgroup$
– Bram28
7 hours ago
$begingroup$
Application of a definition would be "2 is a divisor of 17, therefore 17 is even," which is clearly incorrect.
$endgroup$
– David K
7 hours ago
1
$begingroup$
Now that the answer has been edited, I agree with it more that before, but I think the teacher's reaction would depend on context, and it would likely range from "that's correct" to "that's technically correct, but what is your point" or even "OK, that's enough from you, may we please continue with the lesson", depending on how annoyingly smart-alecky he student is being.
$endgroup$
– David K
6 hours ago
|
show 2 more comments
$begingroup$
The student would be right in claiming "if $17$ is even, then $2$ does not divide $17$", as well as in claiming "if $17$ is even, then $2$ divides $17$".
The fact of the matter is that for any proposition $P$, "if $17$ is even then $P$" is correct, no matter how contradictory $P$ is : for instance, one rightfully conclude from these two statements that "if $17$ is even, then $2$ divides and does not divide $17$". One must not forget that there is a "if $17$ is even"-assumption, which of course makes all the rest irrelevant, because $17$ isn't even.
The teacher would be wrong on two accounts: saying that the student is wrong; but also transforming the mathematically relevant "if $17$ is even" into the irrelevant "if $17$ were even", which is not a mathematical statement.
By the way, if you believe that intuitionism has managed to go beyond material implication, then that isn't a property of material implication, just of implication.
$endgroup$
add a comment |
$begingroup$
You have stated a general rule, which is mathematically true:
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor.
Since the part following "for every number $x$" is true for every number $x,$
it is true in the particular case where $x=17,$ that is,
If $17$ is even, then $17$ has $2$ as divisor.
Interpreted as mathematical statements, both the statement about the number $17$ and the "if-then" clause of the general rule are material conditionals,
which are true whenever the antecedent is false.
To put it another way, if this were a false statement:
If $17$ is even, then $17$ has $2$ as divisor,
then it would be a counterexample of the general claim
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor,
and therefore the general claim would be false.
If you believe the general claim but simultaneously disbelieve a specific example of that claim (such as the example when $x=17$), you are not reasoning consistently.
By the way, the principle "ex falso sequitur quodlibet" is applied to deductions.
An implication can be the subject of a deduction, but it is not itself a deduction.
So I would not say that a material conditional with false antecedent is true because of "ex falso sequitur quodlibet."
The falseness of the antecedent in a material conditional does not cause the consequent to follow (that is, it does not cause the consequent to become true);
rather, it allows the consequent to be false without falsifying the entire conditional statement.
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add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
First of all, the fact that a material implication is considered true as soon as its antecedent is false, is not the same as ex falso sequitur quodlibet, which says that any statement follows from a contradiction.
But yes, if you interpret the student's claim as a material conditional, then technically the student's claim is considered true. It would be just as true as "If I live in London, then I live in Germany" ... interpreted as a material conditional this is considered true because I don't live in London.
Still, the teacher would say the student is wrong, and offer the correction exactly as you indicated. This is because in practice, the use of mathematics is such that under normal circumstances, when the student makes a statement like this, the student is expected to have used the definition of what it means for a number to be even, rather than that the student is making some kind of smart-aleck claim trying to exploit the paradox of the material implication.
Indeed, note that the teacher does not say that the student's claim is false, but rather that the student did something wrong: the student wrongly applied the definition of even-ness. Thus, the teacher says: "No no, you did that wrong: if 17 is even, then 2 is a divisor of 17".
$endgroup$
$begingroup$
@Bram28.Thanks for the point you made by distinguishing ex falso from the paradox of material implication. As to the fact that the sentence would be considered as mathematically false , which explanation could be given? I mean : there is something wrong in the student's claim, but what precisely?
$endgroup$
– Eleonore Saint James
7 hours ago
2
$begingroup$
I had at least one math teacher who emphatically taught that an "if ... then" sentence in a mathematical setting is a material implication, and therefore the student is correct. I do not recall ever being taught otherwise in a math class. What kind of teachers did you have?
$endgroup$
– David K
7 hours ago
$begingroup$
@EleonoreSaintJames Again, while logically the student is seen as true, the use of mathematics is such that when you make a statement like that, you are expected to have applied some kind of definition. So we can say that the student has not correctly used the definition of what it means for a number to be even. The math teacher would say to the student: "No. You are not using the definition of even correctly. Using the definition of even correcty, we get that if 17 is even, then 2 is a divisor of 17".
$endgroup$
– Bram28
7 hours ago
$begingroup$
Application of a definition would be "2 is a divisor of 17, therefore 17 is even," which is clearly incorrect.
$endgroup$
– David K
7 hours ago
1
$begingroup$
Now that the answer has been edited, I agree with it more that before, but I think the teacher's reaction would depend on context, and it would likely range from "that's correct" to "that's technically correct, but what is your point" or even "OK, that's enough from you, may we please continue with the lesson", depending on how annoyingly smart-alecky he student is being.
$endgroup$
– David K
6 hours ago
|
show 2 more comments
$begingroup$
First of all, the fact that a material implication is considered true as soon as its antecedent is false, is not the same as ex falso sequitur quodlibet, which says that any statement follows from a contradiction.
But yes, if you interpret the student's claim as a material conditional, then technically the student's claim is considered true. It would be just as true as "If I live in London, then I live in Germany" ... interpreted as a material conditional this is considered true because I don't live in London.
Still, the teacher would say the student is wrong, and offer the correction exactly as you indicated. This is because in practice, the use of mathematics is such that under normal circumstances, when the student makes a statement like this, the student is expected to have used the definition of what it means for a number to be even, rather than that the student is making some kind of smart-aleck claim trying to exploit the paradox of the material implication.
Indeed, note that the teacher does not say that the student's claim is false, but rather that the student did something wrong: the student wrongly applied the definition of even-ness. Thus, the teacher says: "No no, you did that wrong: if 17 is even, then 2 is a divisor of 17".
$endgroup$
$begingroup$
@Bram28.Thanks for the point you made by distinguishing ex falso from the paradox of material implication. As to the fact that the sentence would be considered as mathematically false , which explanation could be given? I mean : there is something wrong in the student's claim, but what precisely?
$endgroup$
– Eleonore Saint James
7 hours ago
2
$begingroup$
I had at least one math teacher who emphatically taught that an "if ... then" sentence in a mathematical setting is a material implication, and therefore the student is correct. I do not recall ever being taught otherwise in a math class. What kind of teachers did you have?
$endgroup$
– David K
7 hours ago
$begingroup$
@EleonoreSaintJames Again, while logically the student is seen as true, the use of mathematics is such that when you make a statement like that, you are expected to have applied some kind of definition. So we can say that the student has not correctly used the definition of what it means for a number to be even. The math teacher would say to the student: "No. You are not using the definition of even correctly. Using the definition of even correcty, we get that if 17 is even, then 2 is a divisor of 17".
$endgroup$
– Bram28
7 hours ago
$begingroup$
Application of a definition would be "2 is a divisor of 17, therefore 17 is even," which is clearly incorrect.
$endgroup$
– David K
7 hours ago
1
$begingroup$
Now that the answer has been edited, I agree with it more that before, but I think the teacher's reaction would depend on context, and it would likely range from "that's correct" to "that's technically correct, but what is your point" or even "OK, that's enough from you, may we please continue with the lesson", depending on how annoyingly smart-alecky he student is being.
$endgroup$
– David K
6 hours ago
|
show 2 more comments
$begingroup$
First of all, the fact that a material implication is considered true as soon as its antecedent is false, is not the same as ex falso sequitur quodlibet, which says that any statement follows from a contradiction.
But yes, if you interpret the student's claim as a material conditional, then technically the student's claim is considered true. It would be just as true as "If I live in London, then I live in Germany" ... interpreted as a material conditional this is considered true because I don't live in London.
Still, the teacher would say the student is wrong, and offer the correction exactly as you indicated. This is because in practice, the use of mathematics is such that under normal circumstances, when the student makes a statement like this, the student is expected to have used the definition of what it means for a number to be even, rather than that the student is making some kind of smart-aleck claim trying to exploit the paradox of the material implication.
Indeed, note that the teacher does not say that the student's claim is false, but rather that the student did something wrong: the student wrongly applied the definition of even-ness. Thus, the teacher says: "No no, you did that wrong: if 17 is even, then 2 is a divisor of 17".
$endgroup$
First of all, the fact that a material implication is considered true as soon as its antecedent is false, is not the same as ex falso sequitur quodlibet, which says that any statement follows from a contradiction.
But yes, if you interpret the student's claim as a material conditional, then technically the student's claim is considered true. It would be just as true as "If I live in London, then I live in Germany" ... interpreted as a material conditional this is considered true because I don't live in London.
Still, the teacher would say the student is wrong, and offer the correction exactly as you indicated. This is because in practice, the use of mathematics is such that under normal circumstances, when the student makes a statement like this, the student is expected to have used the definition of what it means for a number to be even, rather than that the student is making some kind of smart-aleck claim trying to exploit the paradox of the material implication.
Indeed, note that the teacher does not say that the student's claim is false, but rather that the student did something wrong: the student wrongly applied the definition of even-ness. Thus, the teacher says: "No no, you did that wrong: if 17 is even, then 2 is a divisor of 17".
edited 6 hours ago
answered 7 hours ago
Bram28Bram28
66.7k4 gold badges47 silver badges93 bronze badges
66.7k4 gold badges47 silver badges93 bronze badges
$begingroup$
@Bram28.Thanks for the point you made by distinguishing ex falso from the paradox of material implication. As to the fact that the sentence would be considered as mathematically false , which explanation could be given? I mean : there is something wrong in the student's claim, but what precisely?
$endgroup$
– Eleonore Saint James
7 hours ago
2
$begingroup$
I had at least one math teacher who emphatically taught that an "if ... then" sentence in a mathematical setting is a material implication, and therefore the student is correct. I do not recall ever being taught otherwise in a math class. What kind of teachers did you have?
$endgroup$
– David K
7 hours ago
$begingroup$
@EleonoreSaintJames Again, while logically the student is seen as true, the use of mathematics is such that when you make a statement like that, you are expected to have applied some kind of definition. So we can say that the student has not correctly used the definition of what it means for a number to be even. The math teacher would say to the student: "No. You are not using the definition of even correctly. Using the definition of even correcty, we get that if 17 is even, then 2 is a divisor of 17".
$endgroup$
– Bram28
7 hours ago
$begingroup$
Application of a definition would be "2 is a divisor of 17, therefore 17 is even," which is clearly incorrect.
$endgroup$
– David K
7 hours ago
1
$begingroup$
Now that the answer has been edited, I agree with it more that before, but I think the teacher's reaction would depend on context, and it would likely range from "that's correct" to "that's technically correct, but what is your point" or even "OK, that's enough from you, may we please continue with the lesson", depending on how annoyingly smart-alecky he student is being.
$endgroup$
– David K
6 hours ago
|
show 2 more comments
$begingroup$
@Bram28.Thanks for the point you made by distinguishing ex falso from the paradox of material implication. As to the fact that the sentence would be considered as mathematically false , which explanation could be given? I mean : there is something wrong in the student's claim, but what precisely?
$endgroup$
– Eleonore Saint James
7 hours ago
2
$begingroup$
I had at least one math teacher who emphatically taught that an "if ... then" sentence in a mathematical setting is a material implication, and therefore the student is correct. I do not recall ever being taught otherwise in a math class. What kind of teachers did you have?
$endgroup$
– David K
7 hours ago
$begingroup$
@EleonoreSaintJames Again, while logically the student is seen as true, the use of mathematics is such that when you make a statement like that, you are expected to have applied some kind of definition. So we can say that the student has not correctly used the definition of what it means for a number to be even. The math teacher would say to the student: "No. You are not using the definition of even correctly. Using the definition of even correcty, we get that if 17 is even, then 2 is a divisor of 17".
$endgroup$
– Bram28
7 hours ago
$begingroup$
Application of a definition would be "2 is a divisor of 17, therefore 17 is even," which is clearly incorrect.
$endgroup$
– David K
7 hours ago
1
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Now that the answer has been edited, I agree with it more that before, but I think the teacher's reaction would depend on context, and it would likely range from "that's correct" to "that's technically correct, but what is your point" or even "OK, that's enough from you, may we please continue with the lesson", depending on how annoyingly smart-alecky he student is being.
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– David K
6 hours ago
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@Bram28.Thanks for the point you made by distinguishing ex falso from the paradox of material implication. As to the fact that the sentence would be considered as mathematically false , which explanation could be given? I mean : there is something wrong in the student's claim, but what precisely?
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– Eleonore Saint James
7 hours ago
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@Bram28.Thanks for the point you made by distinguishing ex falso from the paradox of material implication. As to the fact that the sentence would be considered as mathematically false , which explanation could be given? I mean : there is something wrong in the student's claim, but what precisely?
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– Eleonore Saint James
7 hours ago
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I had at least one math teacher who emphatically taught that an "if ... then" sentence in a mathematical setting is a material implication, and therefore the student is correct. I do not recall ever being taught otherwise in a math class. What kind of teachers did you have?
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– David K
7 hours ago
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I had at least one math teacher who emphatically taught that an "if ... then" sentence in a mathematical setting is a material implication, and therefore the student is correct. I do not recall ever being taught otherwise in a math class. What kind of teachers did you have?
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– David K
7 hours ago
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@EleonoreSaintJames Again, while logically the student is seen as true, the use of mathematics is such that when you make a statement like that, you are expected to have applied some kind of definition. So we can say that the student has not correctly used the definition of what it means for a number to be even. The math teacher would say to the student: "No. You are not using the definition of even correctly. Using the definition of even correcty, we get that if 17 is even, then 2 is a divisor of 17".
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– Bram28
7 hours ago
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@EleonoreSaintJames Again, while logically the student is seen as true, the use of mathematics is such that when you make a statement like that, you are expected to have applied some kind of definition. So we can say that the student has not correctly used the definition of what it means for a number to be even. The math teacher would say to the student: "No. You are not using the definition of even correctly. Using the definition of even correcty, we get that if 17 is even, then 2 is a divisor of 17".
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– Bram28
7 hours ago
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Application of a definition would be "2 is a divisor of 17, therefore 17 is even," which is clearly incorrect.
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– David K
7 hours ago
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Application of a definition would be "2 is a divisor of 17, therefore 17 is even," which is clearly incorrect.
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– David K
7 hours ago
1
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Now that the answer has been edited, I agree with it more that before, but I think the teacher's reaction would depend on context, and it would likely range from "that's correct" to "that's technically correct, but what is your point" or even "OK, that's enough from you, may we please continue with the lesson", depending on how annoyingly smart-alecky he student is being.
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– David K
6 hours ago
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Now that the answer has been edited, I agree with it more that before, but I think the teacher's reaction would depend on context, and it would likely range from "that's correct" to "that's technically correct, but what is your point" or even "OK, that's enough from you, may we please continue with the lesson", depending on how annoyingly smart-alecky he student is being.
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– David K
6 hours ago
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show 2 more comments
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The student would be right in claiming "if $17$ is even, then $2$ does not divide $17$", as well as in claiming "if $17$ is even, then $2$ divides $17$".
The fact of the matter is that for any proposition $P$, "if $17$ is even then $P$" is correct, no matter how contradictory $P$ is : for instance, one rightfully conclude from these two statements that "if $17$ is even, then $2$ divides and does not divide $17$". One must not forget that there is a "if $17$ is even"-assumption, which of course makes all the rest irrelevant, because $17$ isn't even.
The teacher would be wrong on two accounts: saying that the student is wrong; but also transforming the mathematically relevant "if $17$ is even" into the irrelevant "if $17$ were even", which is not a mathematical statement.
By the way, if you believe that intuitionism has managed to go beyond material implication, then that isn't a property of material implication, just of implication.
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add a comment |
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The student would be right in claiming "if $17$ is even, then $2$ does not divide $17$", as well as in claiming "if $17$ is even, then $2$ divides $17$".
The fact of the matter is that for any proposition $P$, "if $17$ is even then $P$" is correct, no matter how contradictory $P$ is : for instance, one rightfully conclude from these two statements that "if $17$ is even, then $2$ divides and does not divide $17$". One must not forget that there is a "if $17$ is even"-assumption, which of course makes all the rest irrelevant, because $17$ isn't even.
The teacher would be wrong on two accounts: saying that the student is wrong; but also transforming the mathematically relevant "if $17$ is even" into the irrelevant "if $17$ were even", which is not a mathematical statement.
By the way, if you believe that intuitionism has managed to go beyond material implication, then that isn't a property of material implication, just of implication.
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add a comment |
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The student would be right in claiming "if $17$ is even, then $2$ does not divide $17$", as well as in claiming "if $17$ is even, then $2$ divides $17$".
The fact of the matter is that for any proposition $P$, "if $17$ is even then $P$" is correct, no matter how contradictory $P$ is : for instance, one rightfully conclude from these two statements that "if $17$ is even, then $2$ divides and does not divide $17$". One must not forget that there is a "if $17$ is even"-assumption, which of course makes all the rest irrelevant, because $17$ isn't even.
The teacher would be wrong on two accounts: saying that the student is wrong; but also transforming the mathematically relevant "if $17$ is even" into the irrelevant "if $17$ were even", which is not a mathematical statement.
By the way, if you believe that intuitionism has managed to go beyond material implication, then that isn't a property of material implication, just of implication.
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The student would be right in claiming "if $17$ is even, then $2$ does not divide $17$", as well as in claiming "if $17$ is even, then $2$ divides $17$".
The fact of the matter is that for any proposition $P$, "if $17$ is even then $P$" is correct, no matter how contradictory $P$ is : for instance, one rightfully conclude from these two statements that "if $17$ is even, then $2$ divides and does not divide $17$". One must not forget that there is a "if $17$ is even"-assumption, which of course makes all the rest irrelevant, because $17$ isn't even.
The teacher would be wrong on two accounts: saying that the student is wrong; but also transforming the mathematically relevant "if $17$ is even" into the irrelevant "if $17$ were even", which is not a mathematical statement.
By the way, if you believe that intuitionism has managed to go beyond material implication, then that isn't a property of material implication, just of implication.
answered 6 hours ago
MaxMax
20k1 gold badge12 silver badges46 bronze badges
20k1 gold badge12 silver badges46 bronze badges
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You have stated a general rule, which is mathematically true:
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor.
Since the part following "for every number $x$" is true for every number $x,$
it is true in the particular case where $x=17,$ that is,
If $17$ is even, then $17$ has $2$ as divisor.
Interpreted as mathematical statements, both the statement about the number $17$ and the "if-then" clause of the general rule are material conditionals,
which are true whenever the antecedent is false.
To put it another way, if this were a false statement:
If $17$ is even, then $17$ has $2$ as divisor,
then it would be a counterexample of the general claim
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor,
and therefore the general claim would be false.
If you believe the general claim but simultaneously disbelieve a specific example of that claim (such as the example when $x=17$), you are not reasoning consistently.
By the way, the principle "ex falso sequitur quodlibet" is applied to deductions.
An implication can be the subject of a deduction, but it is not itself a deduction.
So I would not say that a material conditional with false antecedent is true because of "ex falso sequitur quodlibet."
The falseness of the antecedent in a material conditional does not cause the consequent to follow (that is, it does not cause the consequent to become true);
rather, it allows the consequent to be false without falsifying the entire conditional statement.
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add a comment |
$begingroup$
You have stated a general rule, which is mathematically true:
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor.
Since the part following "for every number $x$" is true for every number $x,$
it is true in the particular case where $x=17,$ that is,
If $17$ is even, then $17$ has $2$ as divisor.
Interpreted as mathematical statements, both the statement about the number $17$ and the "if-then" clause of the general rule are material conditionals,
which are true whenever the antecedent is false.
To put it another way, if this were a false statement:
If $17$ is even, then $17$ has $2$ as divisor,
then it would be a counterexample of the general claim
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor,
and therefore the general claim would be false.
If you believe the general claim but simultaneously disbelieve a specific example of that claim (such as the example when $x=17$), you are not reasoning consistently.
By the way, the principle "ex falso sequitur quodlibet" is applied to deductions.
An implication can be the subject of a deduction, but it is not itself a deduction.
So I would not say that a material conditional with false antecedent is true because of "ex falso sequitur quodlibet."
The falseness of the antecedent in a material conditional does not cause the consequent to follow (that is, it does not cause the consequent to become true);
rather, it allows the consequent to be false without falsifying the entire conditional statement.
$endgroup$
add a comment |
$begingroup$
You have stated a general rule, which is mathematically true:
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor.
Since the part following "for every number $x$" is true for every number $x,$
it is true in the particular case where $x=17,$ that is,
If $17$ is even, then $17$ has $2$ as divisor.
Interpreted as mathematical statements, both the statement about the number $17$ and the "if-then" clause of the general rule are material conditionals,
which are true whenever the antecedent is false.
To put it another way, if this were a false statement:
If $17$ is even, then $17$ has $2$ as divisor,
then it would be a counterexample of the general claim
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor,
and therefore the general claim would be false.
If you believe the general claim but simultaneously disbelieve a specific example of that claim (such as the example when $x=17$), you are not reasoning consistently.
By the way, the principle "ex falso sequitur quodlibet" is applied to deductions.
An implication can be the subject of a deduction, but it is not itself a deduction.
So I would not say that a material conditional with false antecedent is true because of "ex falso sequitur quodlibet."
The falseness of the antecedent in a material conditional does not cause the consequent to follow (that is, it does not cause the consequent to become true);
rather, it allows the consequent to be false without falsifying the entire conditional statement.
$endgroup$
You have stated a general rule, which is mathematically true:
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor.
Since the part following "for every number $x$" is true for every number $x,$
it is true in the particular case where $x=17,$ that is,
If $17$ is even, then $17$ has $2$ as divisor.
Interpreted as mathematical statements, both the statement about the number $17$ and the "if-then" clause of the general rule are material conditionals,
which are true whenever the antecedent is false.
To put it another way, if this were a false statement:
If $17$ is even, then $17$ has $2$ as divisor,
then it would be a counterexample of the general claim
For every number $x,$ if $x$ is even, then $x$ has $2$ as divisor,
and therefore the general claim would be false.
If you believe the general claim but simultaneously disbelieve a specific example of that claim (such as the example when $x=17$), you are not reasoning consistently.
By the way, the principle "ex falso sequitur quodlibet" is applied to deductions.
An implication can be the subject of a deduction, but it is not itself a deduction.
So I would not say that a material conditional with false antecedent is true because of "ex falso sequitur quodlibet."
The falseness of the antecedent in a material conditional does not cause the consequent to follow (that is, it does not cause the consequent to become true);
rather, it allows the consequent to be false without falsifying the entire conditional statement.
answered 6 hours ago
David KDavid K
57.5k3 gold badges46 silver badges128 bronze badges
57.5k3 gold badges46 silver badges128 bronze badges
add a comment |
add a comment |
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"Surely his teacher would tell him he is wrong"... unless the teacher knew logic, which one hopes is rather common.
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– Lee Mosher
7 hours ago
2
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I don't see any logical distinction between saying "if 17 is even" and "if 17 were different". Yes, they are grammatically different and, since 17 is NOT even, the first form would not be grammatically correct. But in logic, they say the same thing. Was this an English class or a logic class?
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– user247327
7 hours ago
4
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There's nothing to clarify - if the teacher says he's wrong the teacher is simply wrong.
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– David C. Ullrich
7 hours ago