Maverick Philosopher

Footnotes to Plato from the foothills of the Superstition Mountains

Logical Form, Instantiation, and Pattern-Matching

David Brightly comments:

We can't say that an argument is invalid because it instantiates an invalid form. The argument Socrates is a man; all men are mortal; ergo Socrates is mortal instantiates the invalid form a is F; all Hs are G; ergo a is G, but modulo equivocation, it is truth-preserving. Instantiation of form is just pattern-matching, and the argument does match the pattern of the invalid form.

I reject this of course.The sample argument is an example of correct reasoning.  But anyone who argues in accordance with the schema argues incorrectly.  Why?  Because the schema is not truth-preserving. Therefore the sample argument does not instantiate the invalid form. 

I don't think Brightly understands 'truth-preserving.'  This is a predicate of argument forms, primarily, and the same goes for 'valid' and 'invalid.'  Here are some  definitions:

D1. An argument form is truth-preserving =df no argument of that form has true premises and a false conclusion.

D2. An argument form F is valid =df F is truth-preserving.

D3. A particular argument A is valid =df A instantiates a valid form.  (This allows for the few cases in which an argument has two forms, one valid and one invalid.)

D4. A particular argument A is invalid =df there is no valid form that it instantiates. 

Now what is it for an argument to instantiate an argument form?  To answer this question we need to know what an argument is.  Since deductive arguments alone are under consideration,  I define:

D5. A deductive argument is a sequence of propositions together with the claim that one of them, the conclusion, follows from the others, the premises, taken together.

If the claim holds, the argument is valid; if not, invalid.

Now the main point for present purposes is that an argument is composed of propositions.  A proposition is not a complex physical object such as a string of marks on paper. Thus what you literally SEE when you see this: 

7 + 5 = 12

is not a proposition, but a spatiotemporal particular, a physical item subject to change: it can be deleted.  But the proposition it expresses cannot be deleted by deleting what you just literally SAW.  That suffices to show that the proposition expressed by what you saw is not identical to what you saw. Whatever propositions are (and there are different theories), they are not physical items.

What's more,  you did not SEE (with your eyes) the proposition, or that it is true, but you UNDERSTOOD the proposition and that it is true. (A proposition and its being true are not the same even if the proposition is true.)  So this is a second reason why a proposition is not identical to its physical expression.

Now what holds for propositions also holds for arguments: you cannot delete an argument by deleting physical marks, and you cannot understand an argument merely by seeing a sequence of strings of physical marks.

An argument is not a pattern of physical marks.  So there is no question of matching this physical pattern with some other physical pattern.   Instantiation of logical form is not just pattern-matching.

If a sentence contains a sign like 'bank' susceptible of two or more readings, then no one definite proposition is expressed by the sentence.  Until that ambiguity is resolved one does not have a definite proposition, and without definite propositions no definite argument.  But once one has a definite argument then one can assess its validity.  If it instantiates a valid form, then it is valid; if it instantiates an invalid form, then it is invalid.

It is as simple as that.  But one has to avoid the nominalist mistake of thinking that arguments are just collections of physical items.

Related articles
Abbreviations, Place-Holders, and Logical Form
The Stromboli Puzzle Revisited
Types, Tokens, and Logical Form
Validity and Semantics: Will the Real Frodo Baggins Please Stand Up?
On the Enforcing and Permitting of Coreferentiality by Argument-Forms

Posted

in

by

Bill Vallicella

Tags:

Comments

34 responses to “Logical Form, Instantiation, and Pattern-Matching”

  1. Ed Avatar
    Ed

    —
    D3. A particular argument A is valid =df A instantiates a valid form. (This allows for the few cases in which an argument has two forms, one valid and one invalid.)
    D4. A particular argument A is invalid =df there is no valid form that it instantiates. Equivalently: there is an invalid form that it instantiates.
    —
    David B might already have pointed out that
    (i) ‘there is no valid form that it instantiates’ is not the same as ‘there is an invalid form that it instantiates’. For it can be true both that there is some valid form it instantiates, and some non-valid form that it instantiates.
    (ii) You have already pointed this out when you say “This allows for the few cases in which an argument has two forms, one valid and one invalid“.

    Reply
  2. BV Avatar
    BV

    I have a separate post on this: http://maverickphilosopher.typepad.com/maverick_philosopher/2009/11/validity-invalidity-and-logical-form.html

    Reply
  3. Ed Avatar
    Ed

    >>I have a separate post on this
    Right, and I remember that you did. But you are wrong to say in this post that “Equivalently: there is an invalid form that it instantiates”. “There is no valid form that it instantiates” is not the same as “there is an invalid form that it instantiates”, and this is elementary.
    You are also criticising David for saying something which is perfectly correct. David says
    (a) The argument “Socrates is a man; all men are mortal; ergo Socrates is mortal” is valid. Correct.
    (b) It also instantiates the invalid form “a is F; all Hs are G; ergo a is G”. Correct.
    You object that it does not instantiate the invalid form. But it does. I have a textbook that states that “Socrates is wise implies Socrates is wise, Socrates is wise, therefore Socrates is wise” instantiates the invalid form “A implies B, B, therefore A”. Again, this is elementary.

    Reply
  4. David Brightly Avatar
    David Brightly

    Thanks, Ed. I too remembered the old post but couldn’t locate it in reasonable time.
    Bill, in the 2009 post you say, correctly I think, that ‘every valid syllogism has the invalid form p, q, therefore r‘. In pattern talk we would say that every syllogism matches the pattern p, q, r or just a pattern of three propositions. I don’t think pattern talk commits us to physical patterns, ie, patterns in physical stuff. We might equally say that the first ten natural numbers bear a pattern of alternating evens and odds. Is there another way of explaining what we mean by ‘instantiating an argument form’?
    We need to agree on the issue of validity of form, I think, before we can discuss arguments involving equivocation.

    Reply
  5. BV Avatar
    BV

    OK, now I get it. You are objecting to the equivalence. Only one half of the equivalence is true, namely,
    a. If an argument has no valid form, then it has an invalid form;
    but the following is false
    b. If an argument has an invalid form, then it has no valid form.
    But the repair is easy: I simply delete the second sentence in (D4, which I will now do.

    Reply
  6. BV Avatar
    BV

    >>I have a textbook that states that “Socrates is wise implies Socrates is wise, Socrates is wise, therefore Socrates is wise” instantiates the invalid form “A implies B, B, therefore A”. Again, this is elementary.<< If you have the textbook, then presumably you can tell me its name, author, etc. If S. is wise, then S. is wise S is wise Ergo S. is wise. I am sorry, but what you are saying strikes me as crazy. Obviously, the argument involves exactly one proposition. Therefore, you cannot use two different placeholders to symbolize it. The form is If A, then A A Ergo A and that is obviously valid. Will you also say that the argument has the form: If A, then B C Therefore D? (I fear you will answer in the affirmative.)

    Reply
  7. Ed Avatar
    Ed

    See Logic: The Laws of Truth By Nicholas Jeremy Josef Smith. “It is not true in general that every instance of an invalid argument form is an invalid argument”. They give the example
    B
    A implies B
    :. A
    which is obviously invalid. “But this does not mean that every set of three propositions of the forms B, A implies B, and A respectively, are such that the first two can be true while the third is false”. And they give the example of any argument which has the form “P, P implies P, therefore P”. For example, the Socrates example I gave earlier.
    Or try A Concise Introduction to Logic, by Patrick Hurley (who I have corresponded with in the past).
    “Now, since the form is invalid, can we say that any argument that has this form is invalid? Unfortunately, the situation with invalid forms is not quite as simple as it is with valid forms. Every substitution instance of a valid form is a valid argument, but it is not the case the every substitution instance of an invalid form is an invalid argument. The reason is that some substitution instances of invalid forms are also substitution instances of valid forms. However, we can say that any substitution instance of an invalid form is an invalid argument provided that it is not a substitution instance of any valid form.”
    >> what you are saying strikes me as crazy. Obviously, the argument involves exactly one proposition. Therefore, you cannot use two different placeholders to symbolize it.
    << Obviously if the placeholders are the same, one cannot use different concrete terms to instantiate them. But it is not the same the other way round, otherwise all mathematics would be impossible. Often in mathematics we have two different variables x and y, and we may want to prove that x = y, for example 6 = 6, or whatever. The same token can instantiate different variables or placeholders.

    Reply
  8. Ed Avatar
    Ed

    What is more puzzling, Bill, is that you have alluded to this very fact in the 2009 post.
    “Similarly, every valid syllogism has the invalid form p, q, therefore r. ”
    Correct. Every valid syllogism instantiates an invalid form.
    DB “We need to agree on the issue of validity of form, I think, before we can discuss arguments involving equivocation.”
    Agree.

    Reply
  9. Peter Lupu Avatar
    Peter Lupu

    Consider the following two rules:
    (R1) Each token sentence shall be represented by the SAME sentence-variable (regardless of the number of times it is represented in the complex sentence or its position).
    (R2) Each token sentence shall be represented by a distinct sentence-variable.
    The Socrates sentence is mapped into Bill’s valid argument above if R1 is applied. It will be mapped into the invalid second argument, if R2 is applied. The trouble is that R2 cannot be the basis of a clear definition of validity and, therefore, cannot capture the distinction between valid vs. invalid arguments. I cannot see a logic based on R2.

    Reply
  10. london ed Avatar
    london ed

    Bill, your objection is discussed in this excellent presentation here.
    “ The appeal to Logical Form also attempts to block valid arguments from being instances of an invalid form by placing a restriction on what counts as a legitimate substitution. The only allowable substitutions are into argument forms which exemplify the (true, unique?) logical form of the resulting argument. For example, the argument: “If cats like cream then cats like cream, Cats like cream, Therefore, cats like cream” can arise from substitution into both A ⊃ B, B ∴ A and A ⊃ A, A ∴ A but only the latter, it is claimed, exemplifies the argument’s logical form.”
    It’s an instance of a general problem known as the ‘asymmetry problem’. We can test for a valid argument if it has a valid form. But we can’t test for an invalid argument in the same way. It may have an invalid form, but may still be valid.

    Reply
  11. BV Avatar
    BV

    Ed says that
    B
    A implies B
    :. A
    is obviously invalid. I agree. It is the formal fallacy called Affirming the Consequent.
    But why can’t we play the same game with argument forms that Ed plays with arguments?
    Why can’t we say that the above form instantiates the valid form
    A
    A implies A
    Ergo
    A?
    If
    Socrates is wise
    ergo
    Socrates is wise
    has the form
    A
    ergo
    B
    then the latter has the form
    A
    ergo
    A?

    Reply
  12. BV Avatar
    BV

    Ed,
    I used to teach out of Hurley. Small world. He’s a Copi clone.
    In any case the quotation from him doesn’t tell against me. I agree with what he says based on the following example:
    If God created something, then God created everything
    God created everything
    ergo
    God created something.
    That instantiates Affirming the Consequent. But it also instantiates a more specific form that is valid.
    This is quite unlike the chicanery of replacing ‘Cats like cream’ with different placeholders.

    Reply
  13. BV Avatar
    BV

    Ed sez: >>Obviously if the placeholders are the same, one cannot use different concrete terms to instantiate them. But it is not the same the other way round, otherwise all mathematics would be impossible. Often in mathematics we have two different variables x and y, and we may want to prove that x = y, for example 6 = 6, or whatever. The same token can instantiate different variables or placeholders.<< It is clear that in mathematics different variables can have the same value. But a placeholder is not a variable. A placeholder is an arbitrary individual constant.

    Reply
  14. london ed Avatar
    london ed

    Naturally I am completely confused. The post began by quoting Brightly, who said “We can’t say that an argument is invalid because it instantiates an invalid form. ” This is correct. Since then I looked at the literature and this seems agreed on all sides. We can say that an argument is invalid if there is no valid form that it instantiates. But the mere fact it instantiates an invalid form does not mean there is no valid form it instantiates. Next, Brightly claimed that “Socrates is a man; all men are mortal; ergo Socrates is mortal instantiates the invalid form a is F; all Hs are G; ergo a is G”. Is this wrong or not? The textbook examples I gave, and Cheyne’s presentation, all seem agreed that David’s example is correct. Moreover Bill, in a 2009 post said “”Similarly, every valid syllogism has the invalid form p, q, therefore r. “”
    Peter: “I cannot see a logic based on R2. ” Peter, are the textbook examples correct or not? Cheyne says that the argument: “If cats like cream then cats like cream, Cats like cream, Therefore, cats like cream” can arise from substitution into both A ⊃ B, B ∴ A and A ⊃ A, A ∴ A. Is Cheyne right or wrong? This is pretty binary.

    Reply
  15. BV Avatar
    BV

    We all agree on what I said in 2009, namely, “every valid syllogism has the invalid form p, q, therefore r.” The Socrates argument illustrates this phenomenon.
    The question is whether the Socrates syllogism has or instantiates the invalid form a is F; all Hs are Gs; ergo a is G”. This is what I have been denying.
    As I see it the two cases are different. Both argument forms are of course invalid. The difference, I claim, is that the Socrates syllogism has the first form but does not have the second. Why not? Well, the first example does not violate (something like) Lupu’s R1. But the second does.
    If a sentence or a term occurs more than once in an argument, then that sentence or term must have the same placeholder in the form schema.
    Why? Well, you are given an argument and you want to know whether the reasoning it embodies is correct. That is, you want to know whether the argument is valid. To determine this, you must first determine the form of the argument since validity is a question of form. To get the form, you replace the non-logical expressions with placeholders. Obviously, you must use the same placeholder for the same non-logical expression! Does this need explaining? I don’t think so. An example:
    Maximus is a cat
    Maximus is a gladiator
    Therefore
    Some cat is a gladiator
    Here is where we start, with an actual, particular argument, not with a form diagram. Why? Because logic is concerned with the evaluation of actual arguments. Now we replace the non-logical expressions with placeholders and we get:
    a is an F
    a is a G
    Therefore
    Some F is a G.
    This is obviously the most specific form of the original argument. Now we ask whether the form is valid. It is if it impossible to substitute terms for the placeholders — the same term for the same placeholder obviously — in such a way that the premises are both true and the conclusion false. And that is impossible.
    Therefore the form is valid.
    Therefore the original argument is valid.
    The fact that the Maximus argument also has the invalid form p, q, therefore r is irrelevant since this propositional form does not capture the specific reasoning in the original argument.

    Reply
  16. Ed Avatar
    Ed

    Ah right so we are in fact arguing about one case rather than two. I now agree you are not being inconsistent in your post here and in your post in 2009.
    So the question is whether “If a sentence or a term occurs more than once in an argument, then that sentence or term must have the same placeholder in the form schema.”
    The textbook and the paper (by Cheyne) I quoted suggest that this does not have to be the case. Cheyne says
    “ The appeal to Logical Form also attempts to block valid arguments from being instances of an invalid form by placing a restriction on what counts as a legitimate substitution. The only allowable substitutions are into argument forms which exemplify the (true, unique?) logical form of the resulting argument. For example, the argument: “If cats like cream then cats like cream, Cats like cream, Therefore, cats like cream” can arise from substitution into both A ⊃ B, B ∴ A and A ⊃ A, A ∴ A but only the latter, it is claimed, exemplifies the argument’s logical form.”
    This seems to be exactly what you and Peter are arguing here, Bill. The question is whether this is simply wrong on Logic 101 (as you seem to be implying) or whether it is wrong but not for elementary reasons, or whether he is right. His page is here. “Colin studied mathematics at Otago and then taught high school mathematics for some years. He returned to Otago and completed a series of degrees in philosophy before joining the department in 1992. His research is mostly in epistemology and the philosophy of mathematics. He is the author of Knowledge, Cause, and Abstract Objects: Causal Objections to Platonism (Kluwer, 2001).”
    Cheyne has written an awful lot about the ‘Asymmetry Problem’.

    Reply
  17. David Brightly Avatar
    David Brightly

    Bill,
    I think the account you give at 11:19 is circular. You say that to show an argument is valid we have first to derive its form and then show that no substitution instance of that form is such that its premises could be true and its conclusion false. But the original argument is a substitution instance of the form and showing that its premises can’t be true and conclusion false is just to show it’s valid. But that is what we set out to do in the first place. The detour through forms has got us no further forward.

    Reply
  18. Peter Lupu Avatar
    Peter Lupu

    There are several cases to consider.
    (a) Cases restricted to propositional logic. Here Logical Form is a structure where (i) the set of logical constants are fixed either by enumeration or defined from prior logical constants that are given; (ii) The set of propositional variables are given; (iii) Logical form is defined over the set of logical constants such that all logical constants that occur in sentences are clearly displayed (i.e., none are absorbed within propositional variables); (iv) All token sentences of the same sentence type within a given argument receive the same propositional variable.
    It follows that sentences of the form “If it is raining, then it is wet” cannot be represented as a logically simple formula; it must be represented instead as a complex having the structure “A -> B”. by (iii). Similarly by (iv) arguments such as “If Socrates is wise, then Socrates is wise; Socrates is wise; therefore, Socrates is wise” cannot be represented any other way but “A -> A; A; therefore, A”. Any other proposal violates restriction (iv).
    Examples that violate (iii) and (iv) that were proposed above were completely unmotivated and result in not having any cogent system of logic. I do not see what is the point of considering them seriously.
    (b) Quantification cases: too complicated to outline here. What is clear is that David’s example will be ruled out by a similar clause regarding predicate variables (The predicate ‘man’ and ‘men’ will be both represented by a single predicate variable. David’s example capitalizes on grammatical differences that do not reflect logical distinctions. The sentences can be rephrased to eliminate all such problems.
    Now consider the following argument:
    1. John is tall
    Therefore,
    2. Someone is tall.
    This argument can be represented in propositional logic as the invalid:
    1*. P
    Therefore,
    2*. Q
    and in quantification theory as the valid:
    1**. Fa
    Therefore,
    2**. (Ex) Fx.
    Which one is it? It depends. If the intention is to display quantification structure, then the argument must be represented in the second form and therefore it is valid. If the intention is not to represent quantification structure, then the argument is not valid, since it is represented in the first form. Thus, there is a sense in which validity must be relativized to a given system. However, since the system is typically understood, there is no need to state it explicitly.

    Reply
  19. BV Avatar
    BV

    David,
    I don’t see the circularity. Let’s avoid the modal words you use. They only muddy the waters.
    An argument form is valid iff no argument having that form has true premises and a false conclusion.
    We are given an argument whose validity/invalidity is in question. We extract the form — which is a one over many — and test it for validity using a Venn diagram or by trying to find a counterexample (an argument of that form with true premises and a false conclusion). If the form is valid, then the original argument is valid.
    It is true that the original argument is an instance of the form, but we are not arguing that the form is valid because it is the form of the original argument which is valid. That would be circular. We shift from the argument, which is a particular, to its form, which is a universal, and then test that universal form for validity.

    Reply
  20. BV Avatar
    BV

    1. John is tall
    Therefore,
    2. Someone is tall.
    It makes some sense to say that this argument has two forms, an invalid one from the propositional calculus, and a valid one from the predicate calculus.
    But I think it makes more sense to say that the argument does not have the form:
    1*. P
    Therefore,
    2*. Q
    My reason is that this invalid form does not capture the reasoning embodied in the argument. Obviously, when one concludes that someone is tall from the fact that John is tall, one is not moving from one proposition to a wholly different one.
    My talk of reasoning gives no aid and comfort to psychologism. I am not saying that logic studies reasoning as a psychological process. What it studies is abstract patterns of reasoning that are what they are whether or not there are any reasoning beings, any discursive intellects.

    Reply
  21. BV Avatar
    BV

    Cats like cream
    Therefore
    It is not the case that cats do not like cream.
    Now wouldn’t it be absurd to say that this has the form
    p
    Therefore
    ~~q
    ??
    But why is it any less absurd when Cheyne says that
    If cats like cream, then cats like cream
    Cats like cream
    Therefore
    Cats like cream
    has the form
    If p, then q
    q
    Therefore
    p
    ??
    What is to stop him from saying that it has the form
    If p, then q
    r,
    therefore
    s
    ??

    Reply
  22. london ed Avatar
    london ed

    Bill, I emailed you some extracts from apparently authorative texts which contradict what you say.
    I am quoting second hand, but apparently in Thomas Aquinas: God and Explanations, pp. 161-2) Geach claims that we represent any valid argument as instantiating at least one invalid form. For there is nothing to stop us linking the premisses of any argument together with “ands” or other connectives, and representing the long sentence thus formed by the letter “p”. Representing the conclusion of the argument by “q”, we are thus able to represent any argument as a whole [my emph] as instantiating the form “p, therefore q”, which is about as invalid an argument form as you could get. I.e.
    (1) Geach’s claim is about any valid argument
    (2) The argument ‘Socrates is wise therefore Socrates is wise’ is a valid argument
    (3) Therefore Geach’s claim, if true, is true of the ‘Socrates’ argument above.
    (4) Geach’s claim is that we can represent this argument by the form ‘p, therefore q’, which is about as invalid a form as you can get.
    Can we accept the authority of Geach or not?
    >>My reason is that this invalid form does not capture the reasoning embodied in the argument.
    It may not capture it, but the question is whether the argument is an instance of the invalid form.

    Reply
  23. london ed Avatar
    london ed

    @Peter – so are you saying that
    P → (P → P)
    is not an instance of
    A → (B → A) ?

    Reply
  24. london ed Avatar
    london ed

    @Peter – so are you saying that
    P → (P → P)
    is not an instance of
    A → (B → A) ?
    And generally, can we say that A → (B → A) is true of any proposition substituted for A and any proposition substituted for B? Or shouldn’t we rather say any proposition substituted for A except the one substituted for B, and conversely?
    In which case
    P, P → P, therefore P
    is not an instance of modus ponens?

    Reply
  25. David Brightly Avatar
    David Brightly

    Bill,
    Regarding circularity, let me explain what I think is going on. The appeal to argument form is an appeal to an axiom schema. As such it needs no further justification or proof. Consider this form/schema:

    P

    Reply
  26. BV Avatar
    BV

    Gentlemen:
    There are a number of criss-crossing issues here. One issue concerns the very nature of logic. I should make a position statement about that.
    For now, I’ll refer you to Gerald J. Massey, “The Fallacy behind Fallacies,” *Midwest Studies in Philosophy* VI (1981), 489-500. I just remembered this old article which I studied carefully (as my annotations attest) back in ’81. And now I realize that one of the examples I give of an argument that has an invalid and a valid form is from Massey!
    I’ll also toot my own horn once again by reminding you of a 2010 discussion of the asymmetry thesis.
    http://maverickphilosopher.typepad.com/maverick_philosopher/2010/12/validity-invalidity-and-contravalidity.html

    Reply
  27. Ed Avatar
    Ed

    >>There are a number of criss-crossing issues here.
    Notwithstanding, the main issue is your (and Peter’s) central claim, against Brightly that the argument Socrates is a man; all men are mortal; ergo Socrates is mortal does not instantiate the invalid form a is F; all Hs are G; ergo a is G.
    David and I agree it does instantiate that form. We need to agree on the particulars, before we move to the universals, no?

    Reply
  28. BV Avatar
    BV

    If Brightly is right, then
    Tom is tall
    ergo
    Tom is tall
    instantiates the invalid form
    p
    ergo
    q.
    I say that this is a reductio ad absurdum of his view.
    As I have said many times, a sentence is not a proposition, and arguments are composed of propositions. The fact that there are two sentence tokens above does not justify using two different placeholders. Only a difference in propositions justifies a difference in placeholders.
    You guys are also not noticing the difference between the above example and this one:
    Tom is tall
    ergo
    Someone is tall.
    This has two form, one valid the other invalid. The first example has only one form, a valid one.
    You are confusing these two claims:
    A. Some valid arguments have an invalid form. (True)
    B. All valid arguments have an invalid form. (False)

    Reply
  29. BV Avatar
    BV

    > >>My reason is that this invalid form does not capture the reasoning embodied in the argument.
    It may not capture it, but the question is whether the argument is an instance of the invalid form.< You can't divorce an argument form from a pattern of reasoning. An argument form just is a pattern of reasoning. Now no one who reasons from 'Tom is tall' to 'Someone is tall' reasons according to the pattern *p ergo q.* So a case can be made that the argument does not instantiate this invalid form. For it is not the pattern of reason anyone would follow who inferred the conclusion from the premise.

    Reply
  30. David Brightly Avatar
    David Brightly

    Bill,
    Just to clarify, Ed and I are operating with the following notion of form instantiation. We say that argument A instantiates form F iff A results from substituting tokens for the placeholders in F, subject to the requirement that like placeholders are replaced by like tokens. Thus, starting with form ‘P, ergo Q’ and replacing P with ‘Tom is tall’ and replacing Q with ‘Tom is tall’ we arrive at the argument ‘Tom is tall, ergo Tom is tall’. Hence we say that the latter instantiates the former. This is the only notion of ‘instantiating a form’ that we need to define validity and invalidity of form and argument. We are careful not to use the locution ‘argument A has the form F’. Where we do, we mean just ‘argument A instantiates form F’, as I have defined it above.
    Although like placeholders must be replaced with like tokens, there is no requirement that distinct placeholders be replaced with distinct tokens. More asymmetry!

    Reply
  31. london ed Avatar
    london ed

    >>If Brightly is right, then “Tom is tall, ergo, Tom is tall” instantiates the invalid form “p ergo q”
    Precisely. Geach apparently also says the same, and numerous textbooks agree. The placeholder ‘p’ means ‘any sentence whatever’, not ‘any sentence except the one substituted into any other placeholder such as q’.
    >>I say that this is a reductio ad absurdum of his view.
    Clearly not. I see what you want to say, namely that there is a stronger sense of ‘instantiate’ which would rule out the examples you dislike. But it’s not clear how much logic you would have to rule out in order to do this. For example, if you want to prove “P → (P → P)”, you show that it is an instance of “A → (B → A)”. Under your improved rule, you would not be allowed to substitute the same sentence for a different placeholder. Or are you suggesting there are exceptions to your improved rule, where sometimes you are allowed to substitute the same sentence, sometimes not? But then you need a rule do deal with the exceptions.
    >>B. All valid arguments have an invalid form. (False)
    (True)
    I have located a copy of the Geach paper and will consult when I have time.

    Reply
  32. london ed Avatar
    london ed

    >>no one who reasons from ‘Tom is tall’ to ‘Someone is tall’ reasons according to the pattern *p ergo q.* So a case can be made that the argument does not instantiate this invalid form. For it is not the pattern of reason anyone would follow who inferred the conclusion from the premise.
    If it were the pattern of reasoning anyone would follow who inferred the conclusion from the premise, then it wouldn’t be an invalid form, would it?
    What are you trying to establish here?

    Reply
  33. David Brightly Avatar
    David Brightly

    Bill,
    Regarding Validity, Invalidity, and Contravalidity, Sanford says, via Google Books,

    Jumping ahead a couple of centuries to the early sixteenth century. we find an instructive tripartite classification in Johannes de Celaya. Instead of the more usual two-way division of consequentiae into valid and invalid, Celaya considered a symmetrical three-way division: valid, contravalid, and neither. This subdivides invalid argument forms into contravalid argument forms and those that are neither valid nor contravalid.

    Celaya’s division is thus,

    Valid: None of its instances have true premises and a false conclusion.
    Neither invalid nor contravalid: Some of its instances have true premises and a false conclusion, and some do not.
    Contravalid: All of its instances have true premises and a false conclusion.

    This is consistent with what Sanford goes on to say. The summary you give wrongly labels the middle division as the invalid forms.

    Reply
  34. David Brightly Avatar
    David Brightly

    I can’t get the label right either! Neither valid nor contravalid.

    Reply

Leave a Reply

Your email address will not be published. Required fields are marked *