What you're after is a more robust relationship between premises and conclusions, something more like grasping why it being the case that P, in the real world, brings about Q being the case, in the real world, and then just representing that as 'P ⇒ Q' or whatever. Not just a matter of truth-values, but of an intimate connection between the conditions that 'P' and 'Q' are used to represent. Yes? — Srap Tasmaner
Now the question arises: is it invalid? I don't claim that. — Leontiskos
Well, what do we say here ― leaving aside whether color exclusion is a tenable example? What you're after is a more robust relationship between premises and conclusions, something more like grasping why it being the case that P, in the real world, brings about Q being the case, in the real world, and then just representing that as 'P ⇒ Q' or whatever. Not just a matter of truth-values, but of an intimate connection between the conditions that 'P' and 'Q' are used to represent. Yes? — Srap Tasmaner
Validity is a relationship between premises and conclusion. This is what I say is the common interpretation of your sources on validity:
1. Assume all the premises are true
2. See if it is inferentially possible to make the conclusion false, given the true premises
3. If it is not possible, then the argument is valid
...
...validity is an inferential relationship between premises and conclusion. — Leontiskos
TL;DR. If you think of the material conditional as a containment relation, its behavior makes sense. — Srap Tasmaner
Material implication is the way it is for much the same reason that humans are the way they are given Epimetheus' mistake. When the logic gods got around to fashioning material implication they basically said, "Well if the antecedent is true and the consequent is true then obviously the implication is true, and if the antecedent is true and the consequent is false then obviously the implication is false, but what happens in the other cases?" "Shit! We only have 'true' and 'false' to work with! I guess we just call it 'true'...?" "Yeah, we certainly can't call it 'false'."
I haven't thought about this problem in some time, but last time I did I decided that calling the vacuous cases of the material conditional 'true' is like dross. In a tertiary logic perhaps they would be neither true nor false, but in a binary logic they must be either true or false, and given the nature of modus ponens and modus tollens 'true' works much better. It's a bit of a convenient fiction. This is not to say that there aren't inherent problems with trying to cast implication as truth-functional, but it seems to me that an additional problem is the bivalence of the paradigm. — Leontiskos
...Soon after this, Frege expresses frustration that 28 years after he introduced the material conditional mathematicians and logicians continue to resist it as something bizarre! — Leontiskos
To be sure, one might use disjunctive syllogism to prove that B is A from the contradiction, but that doesn't make the form of the above valid. — Count Timothy von Icarus
But surely we don't want to claim that the fallacy of exclusive premises is true just in cases it is possible for its premises to be true. — Count Timothy von Icarus
"There are a number[11] of people voting for me for President on Tuesday — Srap Tasmaner
I affirm that it is valid by any of these considerations:
(1) Apply the definition of 'valid argument'. — TonesInDeepFreeze
Your interpretation is mistaken because validity is an inferential relationship between premises and conclusion. You would establish an inferential relationship without examining the inferential structure and relations. To say, "The premises are contradictory, therefore an inferential relationship between premises and conclusion holds," is to establish an inferential relationship without recourse to inferential relations. — Leontiskos
In natural deduction systems, if you assume A and then eventually derive B, you may discharge the assumption by writing 'A → B'; this is just the introduction rule for →, and it is exactly the same as the '→' that might appear in a premise. — Srap Tasmaner
You're giving a different reason for why it's valid versus Tones. — frank
Lots of people are not paying attention to the differentiation of arguments for why the OP might be valid. Three options have been given: modus ponens, explosion, and the definition of validity. TonesInDeepFreeze's is the latter... — Leontiskos
Another one:
"a major topic in the study of deductive logic is validity. This is a
relationship between a set of sentences and another sentence; this relationship holds whenever it
is logically impossible for there to be a situation in which all the sentences in the first set are true
and the other sentence false." [bold added]
https://logiclx.humnet.ucla.edu/Logic/Documents/CORE/LogicText%20Chap%200%20Aug%202013.pdf — TonesInDeepFreeze
The idea that it is a relationship already excludes your reading. If a relationship between A and B must be established, then one must know something about both A and B. Yet you think that merely knowing something about A—that it is inconsistent—proves validity. If an isolated fact about A proved validity then validity would not be a relationship between A (premises) and B (conclusion). This is another source that excludes your view. The other (single-sentence) sources you presented favor my view but do not exclude your tendentious view. — Leontiskos
. . .The validity relation is a relation in the ordinary formal sense of a set of ordered pairs. That is distinct from any of the ordered pairs themself. — TonesInDeepFreeze
I mentioned it several posts back, but it seems possible to have an invalid argument with necessarily false premises. — Count Timothy von Icarus
Edit:
This is a matter of different modal levels, so to speak, or different domains or levels of impossibility. Tones is committing a metabasis eis allo genos. He is committing a category error where the genus of discourse is not being respected. Contingent falsity, necessary falsity, and contradictoriness are three different forms of denial or impossibility. The definition of validity that Tones favors is dealing in the first category, not the second or third. The domain of discourse for such a definition assumes that the premises are consistent. It does not envision itself as including the degenerate case where an argument is made valid by an absurd combination of premises. An "argument" is not made valid by being nonsense. — Leontiskos
The OP's question was not about ordinary English at all. — Srap Tasmaner
And, yes, the equivalence is per the material conditional. — TonesInDeepFreeze
English as a meta-language regarding formal logic. In that meta-language, 'if then' is taken in the sense of the material conditional. — TonesInDeepFreeze
Any argument with inconsistent premises is valid, according to Tones. Weird indeed. It requires a strained reading of the fine print of portions of definitions of validity, taken out of context. Earlier posters usefully leveraged the word "sophistry."
(Note that this is different from the modus ponens reading of the OP and it is different from the explosion reading of the OP. The effect of explosion requires explicit argumentation. The OP, for example, is susceptible to explosion, but it is not wielding explosion. Tones is just doing a weird, tendentious, definitional thing.) — Leontiskos
I think, though, we can allow a somewhat negative connotation because reliance in argumentation on degenerate cases is often inadvertent or deceptive. "There are a number of people voting for me for President on Tuesday [and that number happens to be 0]." — Srap Tasmaner
"Therefore, an argument with contradictory/inconsistent premises cannot have a false conclusion while the premises are true" [Paraphrase of Tones] — Leontiskos
"a major topic in the study of deductive logic is validity. This is a relationship..." — TonesInDeepFreeze
That is the second time you put quotes around words I didn't say. — TonesInDeepFreeze
"A sentence Phi is a consequence of a set of sentences Gamma if and only if threre are no interpretations in which all the sentences in Gamma are true and Phi is false." (Elementary Logic - Mates)
"An argument is deductively valid if and only if it is not possible for the premises to be true and the conclusion false." (The Logic Book - Bergmann, Moor and Nelson). — TonesInDeepFreeze
Oh. So then any argument that has no true premises is valid. That's weird. — frank
There is no question. He does not presuppose it. — TonesInDeepFreeze
I did not claim that validity requires that there is no interpretation in which the premises are all true. — TonesInDeepFreeze
We’re just saying that the conclusion follows from the premises – that if the premises were all true, then the conclusion also would have to be true. — Gensler, Introduction to Logic, Second Edition, p. 3
And the argument is valid by Gensler, Enderton, SEP and Wikipedia. — TonesInDeepFreeze
Gensler:
"An argument is valid if it would be contradictory (impossible) to have the premises all true and conclusion false."
It is impossible to have both A -> ~A and A true. Perforce, it is impossible to have the premises all true and the conclusion false. — TonesInDeepFreeze
An argument is valid if it would be contradictory (impossible) to have the premises all true and conclusion false. In calling an argument valid, we aren’t saying whether the premises are true. We’re just saying that the conclusion follows from the premises – that if the premises were all true, then the conclusion also would have to be true. — Gensler, Introduction to Logic, Second Edition, p. 3
And with the argument mentioned in the original post, it is the case that there is no interpretation in which all the premises are true. — TonesInDeepFreeze
An argument is valid if and only if there are no interpretations in which all the premises are true and the conclusion is false.
In this case there are no interpretations in which all the premises are true. Perforce, there are no interpretations in which all the premises are true and the conclusion is false. So the argument is valid. — TonesInDeepFreeze
An argument is valid if and only if there are no interpretations in which all the premises are true and the conclusion is false.
In this case there are no interpretations in which all the premises are true. Perforce, there are no interpretations in which all the premises are true and the conclusion is false. So the argument is valid. — TonesInDeepFreeze
What you've done is imported the artificial truth-functionality of the material conditional into the consequence relation itself. You have contradicted ↪Hanover's "flows from." You are effectively saying, <Any "argument" with nonsense premises is "valid."> — Leontiskos
As I said, in this particular regard, I'm merely applying the definitions of ordinary formal logic. — TonesInDeepFreeze
Ordinary formal logic does not define the consequence relation as identical to the material conditional. — Leontiskos
An argument is valid if it would be contradictory (impossible) to have the premises all true and conclusion false. In calling an argument valid, we aren’t saying whether the premises are true. We’re just saying that the conclusion follows from the premises – that if the premises were all true, then the conclusion also would have to be true. — Gensler, Introduction to Logic, Second Edition, p. 3
What is surprising is that the concept of validity turns out to be equivalent to another concept (deducibility)... — Enderton, A Mathematical Introduction to Logic, p. 89
A good argument is one whose conclusions follow from its premises; its conclusions are consequences of its premises.
...
...the argument is valid [when] the conclusion follows deductively from the premises... — Logical Consequence | SEP
In logic, specifically in deductive reasoning, an argument is valid if and only if it takes a form that makes it impossible for the premises to be true and the conclusion nevertheless to be false. It is not required for a valid argument to have premises that are actually true, but to have premises that, if they were true, would guarantee the truth of the argument's conclusion. — Validity | Wikipedia
analogous predication — Count Timothy von Icarus
I don't know what you mean. — TonesInDeepFreeze
Well, while I think Srap has a good point about our being able to live without A→~A in most situations, I think it is important that statements like "nothing is true," are able to entail their own negation—that logic captures how these claims refute themselves. — Count Timothy von Icarus
It may seem bizarre that a valid argument could have at least one premise that is necessarily false at first glance, but I think it is fairly intuitive if one thinks in terms of truth-preservation. If the premises were true, it would preserve truth. But the "truth" of a false premise cannot be preserved.
And it's a good thing that it is valid because we often can reason from necessarily false conclusions in valid arguments to identifying false premises. — Count Timothy von Icarus
It may seem bizarre that a valid argument could have at least one premise that is necessarily false at first glance, but I think it is fairly intuitive if one thinks in terms of truth-preservation. — Count Timothy von Icarus
...prioritizing truth-functional process over logical telos. — Leontiskos
Yes. I edited that post. It's just weird that any argument that can't have all true premises is going to be valid. — frank
As I said, in this particular regard, I'm merely applying the definitions of ordinary formal logic. — TonesInDeepFreeze
An argument is valid if and only if there are no interpretations in which all the premises are true and the conclusion is false.
In this case there are no interpretations in which all the premises are true. Perforce, there are no interpretations in which all the premises are true and the conclusion is false. So the argument is valid. — TonesInDeepFreeze
Just trying to think of real world examples of a formula like "A → ~A", likely dressed up enough to be hard to spot. Excluding reductio, where the intent is to derive this form. What I want is an example where this conditional is actually false, but is relied upon as a sneaky way of just asserting ~A.
I suppose accusations of hypocrisy are nearby. "Your anti-racism is itself a form of racism." "Your anti-capitalism materially benefits you." "Your piety is actually vanity." — Srap Tasmaner
A→B
B→~A
A
∴ B — Leontiskos
I'm sure there are more convoluted ways to go about it, but does that satisfy your objection? — Hanover
Only arguments are valid, and "A, therefore A," is not an argument. Argument, at the very least, involves rational movement. — Leontiskos
Another argument:
A -> ~A
A
therefore A
valid — TonesInDeepFreeze
The losing party, in one sense, grants that they lost, but continues in the competitive spirit, which means they have to shift ground from whether they "officially" or "technically" lost to whether that was a "real" loss, or whether there had a been a "real" competition in the first place. — Srap Tasmaner
As Banno notes, validity is determined by asking if the conclusion flows from the premises, and so he argues under mp, it does, so it is valid.
The wiki cite adds criteria, namely (1) that the negation of the conclusion cannot also flow from the premises for validity and (2) the premises under any formulation must also reach the same conclusion. — Hanover
But it's not validity we usually disagree over, but soundness, and inconsistent premises make valid inferences unsound. — Srap Tasmaner
I find this particularly unconvincing as respects "afterlife" beliefs because many ancient visions (and the dominant modern vision) of the afterlife seem significantly more unpleasant than just ceasing to exist. — Count Timothy von Icarus
I don't think it's that hard to define at all. — Count Timothy von Icarus
Their argument is roughly that the intuitive/informal notion of logical consequence is multiply-realizable (granted it is more technical in its details). — Count Timothy von Icarus
Were debating whether to call certain formulations "modus ponens." — Hanover
The basic idea is "formally correct but misleading". Akin to sophistry. Or to non-cooperative implicature, like saying "Everyone on the boat is okay" when it's only true because no one is left on the boat and all the dead and injured are in the water. — Srap Tasmaner