# Modus Ponens as Substitute for Syllogism

This is a pretty basic question, but I haven't been able to find a clear answer in the various sources I've looked at, including the Kneales' chapter on conditionals in 'The Development of Logic'.

In propositional logic, when modus ponens stands in for a syllogistic/categorical argument, with the antecedent being the conjunction of the original syllogism's premises, and the consequent the conclusion, is it the standard interpretation to regard the affirmation of the antecedent as thereby entailing the conclusion only on the assumption that the original argument was valid, i.e the validity is external to the conditional itself, as opposed to that validity somehow being transferred to the conditional so long as the antecedent-as-premises is affirmed?

And more broadly, is propositional logic just a logic of "this is what follows, assuming not only that the statements p, q, etc, are true but also that the rules of inference do actually represent valid arguments" (insofar as validity is the aim, rather than, say, causal arguments)?

In answer to Graham, I'm adding this to the original question because of the text limit for comments.

In propositional logic, syllogistic, i.e. categorical, arguments are regularly expressed using modus ponens, with the conjunction of the two premises (e.g. "all men are mortal & Socrates is a man") serving as the antecedent of the conditional, if p then q, and the consequent ("Socrates is mortal") as the conclusion. Stating this in the form of a conditional statement is recognized to not be sufficient for the conclusion to be entailed by the premises per se, but when the conjunction of the premises, 'p', is then affirmed to be true, which turns the conditional into a modus ponens argument, then the logic textbooks describe the conclusion, 'q', as then being 'inferred'. But surely this can't be the case, and my question was "is it generally recognised that this is not the case".

As an example of why this can't be so, take the enthymeme "Socrates is a man, therefore Socrates is mortal", which is an invalid argument as it stands, but if you add the missing premise "all men are mortal" then it becomes a valid syllogism, yet modus ponens doesn't differentiate between the two - so long as the premises are affirmed to be true, then the conclusion that "Socrates is mortal" can in both cases be 'inferred'. Surely this means that whatever validity there was in the original argument has not been carried over to the modus ponens formulation.

Yes, a valid syllogism doesn't ever become invalid when expressed using modus ponens, but only because the validity provided by the original logical form has disappeared (if this were not the case, then we should not be able to 'infer' a consequent that stands for a syllogism's conclusion, from the affirmation of an antecedent that stands for an enthymeme's premise, yet we can). It seems to me that the conclusion is only 'inferred' if the syllogism has already been proved outside the system and then assumed within it. And even then, what kind of inference is "this is a valid argument and its premises are true, so it's a sound argument too, which means its conclusion is also true"? This merely declares that a valid argument, made elsewhere, is sound. You don't infer something from an affirmation that something is the case, but rather, if an inference is valid to the effect that something must follow from something else, then an affirmation of that first thing entitles you to affirm, not infer, the second thing.

In other words, in a syllogism, a conclusion is true on the condition that the premises are true, only because such an argument is already valid, whereas with modus ponens, when used to express such an argument, the conclusion is only true on the condition that the premises are true AND that it's a valid argument to begin with. And affirming the antecedent merely affirms the former. If that is right, then propositional logic, which is heavily reliant on material implication, is not 'truth-preserving' in the sense that actually valid arguments are truth-preserving, i.e. where you can only go from true premises to true conclusions because there's a valid form that is prior to any claim of soundness, but instead, if we grant from the outset that certain statements are true and that certain material implications are also true, i.e. truly stand for valid arguments, then from this we can say (not 'infer' or 'conclude', except indirectly and implicitly) that other things have to be true. Again, for all I know all of this all might be commonplace, but the purpose of my original question was just to check if that was so.

• I made an edit mainly to break the text into smaller pieces. You may roll this back or continue editing. Welcome! Aug 14, 2018 at 19:53
• There are two different notions of consequence, semantic and syntactic. For semantic inference it makes no difference what the rules of inference are, it is defined in terms of truth in models, for syntactic inference whatever rules of inference are adopted are automatically "valid" since following them is how validity is defined, see Implies vs. Entails vs. Provable on Math SE. But in either case it makes no difference whether premises are true for a conditional to be valid, so I am not sure what the "original argument" is doing here. Aug 14, 2018 at 21:26
• @ScottB. Please divide this question into even shorter statements. Aug 14, 2018 at 23:16
• It's not clear what you are asking. Perhaps provide an example of an "original sylogism" and how "modus ponens stands in for" it. Aug 15, 2018 at 2:20
• Hi Graham, I've replied to you by editing the question. Aug 15, 2018 at 21:40

## 1 Answer

In propositional logic, syllogistic, i.e. categorical, arguments are regularly expressed using modus ponens, with the conjunction of the two premises (e.g. "all men are mortal & Socrates is a man") serving as the antecedent of the conditional, if p then q, and the consequent ("Socrates is mortal") as the conclusion. Stating this in the form of a conditional statement is recognized to not be sufficient for the conclusion to be entailed by the premises per se,

No, "All men are mortal" is the conditional statement (a universal one to be precise). "Socrates is a man" is another predicate. They conjointly entail the consequent "Socrates is mortal."

Ɐx (Man(x)→Mortal(x)), Man(Socrates) Ⱶ Mortal(Socrates)

but when the conjunction of the premises, 'p', is then affirmed to be true, which turns the conditional into a modus ponens argument, then the logic textbooks describe the conclusion, 'q', as then being 'inferred'. But surely this can't be the case, and my question was "is it generally recognised that this is not the case".

It is not. It is the case that q will be infered from p and p→q using the rule of 'modus ponens'.

p→q, p Ⱶ q

PS:

You seem to be confusing the rule of modus ponens with the the tautology: ((p → q) ˄ p) → q , which can be proven by using that rule of inference.

``````0. |___
1. |  |_ (p → q) ˄ p      Assumption
2. |  |  p → q            ˄ Elimination (1)
3. |  |  p                ˄ Elimination (1)
4. |  |  q                → Elimination (2,3)  aka Modus Ponens
5. |  ((p → q) ˄ p) → q   → Introduction (1-4)
``````
• Hi Graham, thanks for the response, as to the first point, I don't dispute the validity of the way predicate logic represents syllogistic argument, only the way propositional logic does. Your line of predicate logic isn't a proof and still requires UI and then MP. And my point was precisely that MP adds nothing unless there's a prior categorical demonstration such as you gave: Ɐx (Man(x)→Mortal(x)), Man(Socrates) Ⱶ Mortal(Socrates). I.e. to prove that argument, Man(Socrates) Ⱶ Mortal(Socrates) must be instantiated then Man(Socrates) affirmed, from which we then conclude Mortal(Socrates) by MP. Aug 16, 2018 at 21:11
• As to the second point, I know that MP says you can infer q from (p>q and p), but in my question I was arguing that i) unless there's a prior demonstration of the kind that is explicit in predicate logic, or (I believe) assumed in propositional logic, then you cannot meaningfully 'infer' anything, and ii) even if you do have either an explicit or assumed categorical argument prior to your 'proof' by MP, such a proof merely amounts to the assertion that the argument presented using modus ponens is sound. Aug 16, 2018 at 21:51
• Also, I don't see that there's any difference in modus ponens as a purported argument form and modus ponens expressed as a truth-functionally tautologous conditional statement. The very fact that it can be represented as a tautology is an indication to me of its ultimate vacuity. Aug 16, 2018 at 22:03
• @ScottB Another way to say that is that it is "self-evidently true", or axiomatic, that Q is inferable from P and P implies Q. Modus ponens is a fundamental rule of inference essentially justified by "that's what implication means". The Socratic sylogism can be seen to be composed of two such rules ... modus ponens and universal instantiation ... and so is not fundamental. Aug 16, 2018 at 22:43
• IMO, logical deduction shouldn't itself be expressed axiomatically, but rather, self-evident axioms such as the laws of identity and non-contradiction provide the fundamental basis for logically-valid form that says: if certain things involving identity are the case, these other things must follow. Although such axioms express how things metaphysically must be (and so are tautologous), the point of a logical argument is to show specific consequences of necessity, not to be a vacuous imitation of it, as if a valid argument itself rather than its conclusion were what is necessarily true. Aug 20, 2018 at 20:30