Three More Putative Instances of Valid Is-Ought Inferences

I thank Tully Borland for pushing the discussion in this fascinating direction.

A

Affirming the Consequent is an invalid argument form.
Ergo
One ought not (it is obligatory that one not) give arguments having that form.

B

Modus Ponens is valid
Ergo
One may (it is permissible to) give arguments having that form.

C

Correct deductive reasoning is in every instance truth-preserving.
Ergo
One ought to reason correctly as far as possible.

An argument form is valid just in case  no (actual or possible) argument of that form has true premises and a false conclusion. An argument form is invalid just in case some (actual or possible) argument of that form has true premises and a false conclusion.  Deductive reasoning is correct just in case it proceeds in accordance with a valid argument form.  'Just in case' is but a stylistic variant of 'if and only if.'

Now given these explanations of key terms, it seems  that validity, invalidity, and correctness are purely factual, and thus purely non-normative, properties of arguments/reasonings.  If so, how the devil do we get to the conclusions of the three arguments above?

View One:  We don't.  A, B, and C are each illicit is-ought slides. 

View Two:  Each of the above arguments is valid.  Each of the key terms in the premises is normatively loaded from the proverbial 'git-go,' in addition to bearing a descriptive load..  Therefore, there is no illict slide.  The move is from the normative to the normative.  Validity, invalidity, and correctness can be defined only in terms of truth and falsity which are normative notions.

View Three:  We have no compelling reason to prefer one of the foregoing views to the other.  Each can be argued for and each can be argued against.  Thus spoke the Aporetician.

Related articles

Syntactic Versus Semantic Validity

Truth and Normativity

From the Laws of Logic to the Existence of God

The Stromboli Puzzle Revisited

Comments

[Tully Borland]

Now given these explanations of key terms, it seems that validity, invalidity, and correctness are purely factual, and thus purely non-normative, properties of arguments/reasonings.

But there is "truth" in the definition. The premises need to be true, i.e. correct, i.e. right. Or are you not buying that truth is normative?

I also wonder about "permissible"/"obligatory" in the conclusions. Is it morally obligatory not to give affirming the consequent argument forms? I should think the "ought" in the conclusion would be one of rationality or proper function but not morality. How about this:

1. Affirming the Consequent is an invalid argument form.
2. Thus it ought not be employed as if it were valid.

The "ought" would be an ought of proper function. Given the nature of affirming the consequent, it has a proper function (to crank out conclusions that don't follow from premises). If you want to use the argument form as an example in a classroom to illustrate an invalid argument form, go for it. But if you want to use it as if it were valid, you're being irrational for its nature prevents it from ever being valid.

Perhaps argument forms don't have natures but then maybe that line of thought would still work with something that does.

[Miloš]

I believe that all this examples are problematic because one premiss contain some statement which is not pure factual or it presupposes some broader ''value network'' in which such statements operate perfectly (something similar can be said for Aristotelian MacIntyre's examples).

For example in first case ''One ought not (it is obligatory that one not) give arguments having that form.'' we presume more one premise, something like ''One ought not to give arguments having invalid form''. [Sorry for messy English, I hope that text above is understandable]

[Bill Vallicella]

Tully,

Be careful with 'correct' and 'right.' My definitions of validity and invalidity are in terms of truth and falsity. Now you could say that a true belief (judgment, etc.) is a correct or right belief. But how do you know that 'correct' and 'right' are normative terms? Even if we suppose that 'true' and 'correct' can be used interchangeably across all the relevant contexts, you can't show that 'true' is normative on the ground that 'correct' is normative. For they both may be non-normative.

How? If a true judgment is one that corresponds to a truth-maker (an obtaining state of affairs, say), then isn't it a purely factual claim that this correspondence holds? And likewise if it doesn't hold?

You are right that the sense of 'ought' needs clarification. And you are right if you think that we OUGHT (!) to distinguish between theoretical (epistemic) and practical norms.

But I don't understand how a logical fallacy such as Affirming the Consequent could have a proper function. A logical fallacy is a typical error in reasoning that has some non-negligible tendency to seduce people. What would it mean for such an argument pattern to have a function, let alone a proper function? The heart has a proper function, and maybe a man does, but a fallacy?

If I say that you ought to distinguish between theoretical and practical norms, why isn't this a moral and thus a practical ought? Making distinctions is something we do. Or suppose you fail to distinguish between validity and soundness, is that not a low-level moral failure?

And then there is the whole topic of the ethics of belief which presupposes that there are moral oughts and ought nots with respect to beliefs. W. K. Clifford and the boys.

[Bill Vallicella]

Tully,

I am not maintaining that truth is not normative; I am exploring the issue of whether it is or not and what that would mean.

[Tully Borland]

Bill,

Thanks for the corrections. Helpful points all around.
"Correct" seems to me a normative notion as well as "right." I can't see my way around that, but maybe I'm just not thinking hard enough. But you're right (what you said is TRUE!) that perhaps "truth" could ultimately be understood in terms other than those (and indeed many HAVE defined "truth" in non-normative terms).

Also, I agree that the failure to distinguish between validity and soundness could be a low-level moral failure, but in trying to get an "ought" from an "is" it seems to me more promising (less controversial) to derive a different "ought" than a moral one using examples like the ones above. If modus ponens is valid, it's in some sense irrational (even if not, say, prudentially irrational) to believe it's an invalid form of inference. To me, that seems more obvious than it being morally permissible to give arguments having that form following from modus ponens being valid, but I guess I'm assuming a relevance logic in so thinking (since if your conclusion is a necessary truth--which presumably it is if true--your conclusion does indeed follow).

Hmmmm....Before I succumb to the aporia (which I'm more than happy to do) here's a thought:

1. Tullius est.
2. Thus, necessarily, one shouldn't kill babies for fun.

Did I just derive an "ought" from an "is"? According to (so-called) "classical logic" the argument is sound.
So perhaps for there to be an "is-ought" problem, we'll have to either reject classical logic or specify that the derivation in question is something other than logical entailment, which we've more or less been doing in this discussion. Anyhow, in my mind that at least puts an interesting spin on the problem, which I at least didn't notice until now.

[Bill Vallicella]

You make a good point: whether one can derive an 'ought' from an 'is' depends on what 'derive' is taken to mean.

Arguments of this form are of course technically valid:

p
ergo
q v ~q

For there is no possible circumstance in which the premise is true and the conclusion false.

Your (2) above, however, is not a tautology or a logical truth either (every tautology is a logical true but not conversely).

Here is a better example that serves your purposes:

Tullius Maximus est et Tullius Maximus non est.
ergo
One ought to be kind to kitty cats and other sentient critters.

If anything follows from a logical contradiction, then 'ought' statements follow from it.

But surely this doesn't count as deriving an "ought' from an 'is' except in an intolerably degenerate sense of 'derives.'

[Tully Borland]

But surely this doesn't count as deriving an "ought' from an 'is' except in an intolerably degenerate sense of 'derives.

Right. I'm all for intolerance but not for degeneracy!

I have to play chess with my little boys tonight so I'm signing off. Thanks, Bill, for the helpful dialogue.