Logical Methods — on modal logic

Moving on through Greg Restall and Shawn Sandefer’s Logical Methods, Part II is on propositional modal logic. So the reader gets to find out e.g. about S4 vs S5 and even hears about actuality operators etc. before ever meeting a quantifier. Not an ordering that many teachers of logic will want to be following. But then, as I have already indicated when discussing Part I on propositional logic, I’m not sure this is really working as the first introduction to logic that it is proclaimed to be (“requires no background in logic”). I won’t bang on about that again. So let’s take Part II as a more or less stand-alone treatment that could perhaps be used for a module on modal logic for philosophers, for those who have already done enough logic. What does it cover? How well does it work?

Part I, recall, takes a proof-theory-first approach; Part II sensibly reverses the order of business. So Chapter 7 on ‘Necessity and Possibility’ is a speedy tour of the Kripke semantics of S5, then S4, then intuitionistic logic. I can’t to be honest say that the initial presentation of S5 semantics is super-clearly done, and the ensuing description of what are in effect unsigned tableaux for systematically searching for counterexamples to S5 validity surely is too brisk (read Graham Priest’s wonderful text on non-classical logics instead). And jumping to the other end of the chapter, there is a significant leap in difficulty (albeit accompanied by a “warning”) when giving proofs of the soundness and completeness of initutionistic logic with respect to Kripke semantics. Rather too much is packed in here to work well, I suspect.

Chapter 8 is a shorter chapter on ‘Actuality and 2D Logic’. Interesting, though again speedy. But for me, the issue arises of whether — if I were giving a course on modal logic for philosophers — I’d want to spend any time on these topics as opposed to touching on the surely more interesting philosophical issues generated by quantified modal logics.

Chapter 9 gives Gentzen-style natural deduction systems for S4 and S5. Which is all technically fine, of course. But I do wonder about how ‘natural’ Gentzen proofs are here, compared with modal logic done Fitch-style. I certainly found the latter easier to motivate in class. So Gentzen-style modal proof systems would not be my go-to choice for a deductive system to introduce to philosophy student. Obviously Restall and Sandefer differ!

Overall, then, I don’t think the presentations will trump the current suggested introductory readings on modal logic in the Study Guide.

Logical Methods — on propositional logic

I have now had a chance to read the first part of Greg Restall and Shawn Sandefer’s Logical Methods, some 113 pages on propositional logic.

I enjoyed this well enough but I am, to be frank, a bit puzzled about the intended readership. The book’s Preface starts “Welcome to Logical Methods, an introduction to logic for philosophy students …”. And the text does indeed seem to start right from scratch. But Restall’s web-page for the book says “The text was developed through years of teaching intermediate (second-year) logic at the University of Melbourne.” While their Amazon blurb says “suitable for undergraduate courses and above.” Which suggests a rather unstable focus. And indeed, a significant amount of the material here, as we’ll see in a moment, is at what strikes me as a decidedly non-introductory level.

Certainly, things that can (and often should!) give pause to a philosophy student encountering formal logic for the first time are often skated over at speed. For example, when we do propositional logic, just what is the relation between the formal systems and our everyday inferences using the ordinary-language connectives? So, exactly what are these dratted “p”s and “q”s doing? On p.8 we are told that “declarative sentences express propositions”, and that we are going to be looking at propositional languages “where there are declarative sentences”. But then are also immediately told that our formal language is just designed “to express the forms of propositions combined with [the connectives]” (my emphasis). So do the “p”s and “q”s get interpretations as expressing propositions or not?

On p. 9 we are baldly told that “disjunctions will always be inclusive in this text” without a moment’s discussion of how things might or might not stand in ordinary language. And later, the much more vexed question of how the logician’s conditional might be related to the ordinary language conditional is relegated to a “challenge question” on p. 32. I wonder: if we don’t say rather more about the ordinary-language logical apparatus, how do we rack up a persuasive score sheet of the costs and benefits of various alternative formal choices? (Teachers using this book with real beginners might well be adding quite a bit of appropriate classroom chat on such matters as they go along — but I’m thinking here of a student reader taking the book “neat”.)

Again, the beginning reader is given just one worked example of a truth-table test for validity in action. And nothing is said e.g. about standard heuristics to speed things up (as in “you don’t need to work further on a line where the conclusion is true because that can’t give us a counterexample”) Yes, yes, of course truth-table testing complicated examples is as boring as heck. But surely(?) we do want our beginning students to be just a bit more au fait with how things can work out in practice.

So already, I’m not sure how well this is going to work with real beginners. But there are more serious worries. Restall and Sandefer advertise their book as presenting “proof construction on equal footing with model building” — but in fact that briskness over truth-tables is just one sign that their presentation is really skewed to emphasize proof-theoretic ideas. And so, long before we ever hear about the classical truth-functional interpretation of the connectives, we are tangling with why we might want detour-free proofs in a Gentzen-style natural deduction system. (By the way, much as though I like the elegance of Gentzen trees, I’m yet to be really persuaded that they trump Fitch-style proofs for introducing ND to students.)

And now, not only is the — I agree! — reasonably intuitive idea of a detour-free proof canvassed, but we actually get a full-on, ten-page, proof of normalizability for intuitionistic propositional logic (starting as early as p. 53 in the book). I honestly can’t imagine too many thinking that this is where they want their beginning philosophy students to be concentrating, so early in their logical encounters!

Now, I don’t want to carp, so let’s now recalibrate our expectations, and think of this as in fact a second-level text with some brisk reminders of the more elementary stuff. Then, on positive side, it can be said that the normalization proof and other parts of the discussion of Gentzen style ND are very accessibly done. So I can e.g. well foresee the relevant sections getting into the next edition of the Study Guide as warmly recommended reading on entry-level proof theory. But yes, for me at least, that is where this material really belongs, a step or two up from a first introductory text for philosophers. Call me old-fashioned!

I note that the text was typeset by the authors (and some of their aesthetic choices are a bit wonky!). But that does raise a question. I do wonder why, in 2023, since they have a nice PDF to hand, they have gone done the route of conventional publication when they could have got the book into so many more students’ hands by going down the free-PDF-plus-cheapo-print-on-demand route? Just saying.

Restall & Standefer, Logical Methods

A new introductory logic textbook has just arrived, Greg Restall and Shawn Standefer’s Logical Methods (MIT).

This promises to be an intriguing read. It is announced as “a rigorous but accessible introduction to philosophical logic” — though, perhaps more accurately,  it could be said to be an introduction to some aspects of formal logic that are of particular philosophical interest.

The balance of the book is unusual. The first 113 pages are on propositional logic. There follow 70 pages on (propositional) modal logic — this, no doubt, because of its philosophical interest. Then there are just 44 pages on standard predicate logic, with the book ending with a short coda on quantified modal logic. To be honest, I can’t imagine too many agreeing that this reflects the balance they want in a first logic course.

Proofs are done in Gentzen natural deduction style, and proof-theoretic notions are highlighted early: so we meet e.g. ideas about reduction steps for eliminating detours as early as p. 22, so we hear about normalizing proofs before we get to encounter valuations and truth tables. Another choice that not everyone will want to follow.

However, let’s go with the flow and work with the general approach. Then, on a first browse-and-random-dipping, it does look (as you’d predict) that this is written very attractively, philosophically alert and enviably clear. So I really look forward to reading at least parts of Logical Methods more carefully soon. I’m turning over in my mind ideas for a third edition of IFL and it is always interesting and thought-provoking to see how good authors handle their introductory texts.

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