Need a logic course, fast?

I wasn’t going to put this online until it was done and cleaned up, but given the situation, maybe this can be of help.

I just developed and tried out a course on formal logic for a 13-week semester. It has:

  • a free online textbook: forall x: Calgary
  • beamer slides for lectures (or screencasts)
  • problem sets, which are mostly completed online on Carnap and graded automatically (see here if you want to use Carnap with a different textbook)
  • practice problems for Carnap (accessible on carnap.io as well)
  • 3 tests (only one converted to online/carnap so far)

Here are the beamer slides. If you’re an instructor and want the sources, drop me an email at rzach@ucalgary.ca. (Of course you’ll also get the sources to the problem sets etc.)

lectures

If you can bear it, here are screencasts of my talking through the stuff on identity in these lecture slides, and doing some proofs on Carnap.

https://ucalgary.yuja.com/V/PlayList?node=261149&a=1258679219&autoplay=1

Whiteboard Screencap

Chalk-and-talk online: whiteboard screencasting (on Linux)

Well, all my logic lectures moved online as of last week. It’s been a bit of a scramble, as I’m sure it’s been for you as well. I needed to rapidly produce videos of lectures (on logic in my case) I can give with students to watch. I thought I’d quickly share what I’m doing in case you’re in a similar situation.

My laptop runs Linux (Ubuntu 19.10 to be specific). So there are few options. If you’re on a Mac or Windows machine, there’s lots and you probably don’t need any help. Maybe your University even has a preferred solution for screencasting that integrates with your LMS.

For screencast recording on Linux I find Kazam works fine. It’s super-simple, all it does is record the microphone (or computer speaker output) together with whatever happens on your screen (or in a window). So if you want to show your students how to work Overleaf or Carnap or whatever, or if you want to show them a beamer presentation and talk over it, that’s all you need. (Well, you might want to invest in a decent microphone.)

But what if your lecture is chalk-and-talk? You need a way to let yourself “write on the board” while you talk through your proof or whatever. For that you need a handwriting/sketching app and a way to write comfortably (touchscreen/tablet and stylus). I did get a stylus and an Android tablet and tried out a few handwriting apps, but I couldn’t get the palm rejection to work on any of them. (If you rest your palm on the screen, the tablet won’t recognize what your stylus is doing, so you need an app or a stylus that can isolate the stylus from your hand. I’m told iPads are better for that and/or there are active styluses that have palm rejection built in. Not going to buy an iPad just to try that out though.)

I also have a Wacom Intuos writing tablet I got last week in paniced anticipation ($70 US/CAD 90). It works with Linux (plug-and-play USB), just takes a little getting used to. For a handwriting app, I discovered StylusLabs Write. It works really nice. I just fire it up, hit record on Kazam, start writing. It can easily add a new page/whiteboard area, you can scroll back to a previous one easily, and in the end you can save the whiteboard as a PDF. Here’s an example of me talking through the truth lemma in the completeness proof.

What is your solution? I made a Google spreadsheet where you can record your solution; maybe it’ll help other instructors who are struggling right now to adapt in the great COVID-19 rush online.

I would prefer to use my ReMarkable for all of this: it has a desktop app for Mac & Windows that shows what you’re drawing on it. So if you have one, try that out! I was hoping to make it work in Linux using srvfb, but have to wait until ReMarkable fixes a bug that turned off ssh access to the tablet. Will let you know what I find out.

Adding online exercises with automated grading to any logic course with Carnap

A couple of years ago I posted a roundup of interactive logic courseware with an automatic grading component. The favorite commercial solution is Barwise & Etchemendy’s Language, Proof, and Logic textbook that comes with software for doing truth tables, natural deduction proofs, and semantics for propositional and first-order logic, which also automatically grades student’s solutions. The problem is, that (a) it costs money and (b) will only grade problems from that textbook. I already pointed to the open-source, and free-to-use alternative Carnap by Graham Leach-Krouse and Jake Ehrlich. Graham wrote a guest post on Daily Nous about it a while ago. I’ve now used this myself with great success and thought I’d write up my experience.

Carnap is an online tool that allows you to do the following. You can upload webpages (written in a variant of Markdown) which may include logic problems of various sorts. These are, right now:

  • Translation exercises (i.e., you provide a sentence in English and the student’s task is to symbolize it in propositional or first-order logic).
  • Truth tables (you give sentence(s) of propositional logic, the student must fill in a truth table, and use it to determine, say, if an argument is valid, a sentence is a tautology, or if two sentences are equivalent, etc.).
  • Derivations (you provide a sentence or argument and the student has to construct a formal proof for it).
  • Model construction (you provide a sentence or argument, the student has to give a domain and extensions of predicates to make the sentence true, false, or show that the argument is invalid, etc.).
  • Basic multiple choice and short-answer questions.

Carnap comes with its own textbook and a collection of pre-made problem sets. But you can make up your own problem sets. That’s of course a bit of work, but you have complete control over the problems you want to assign. Here are some sample exercises that go with the Calgary version of forall x:

These are pages I give to my students to get them to become familiar with Carnap before they have to actually do problems for credit. The main difference is that for a real problem set, each exercise has a “submit” button that the student can click once they’ve found a correct solution.

To get an idea of how these problem sets are written, have a look at the documentation.

As you see, the problems are interactive: the student enters the solution, and Carnap will tell them if the solution is correct. In the case of derivations, it will also provide some feedback, e.g., tell the student why a particular application of a rule is incorrect.

You can assign a point value to each problem. Carnap also allows you to set up a course, let students sign up for the course, and lets you assign the pages you’ve created as problem sets. It will allow students to submit problems they have correctly solved, and Carnap will tally the point score earned. You can then download a spreadsheet of student scores per problem set and assign marks on the basis of that.

As you see, Carnap is incredibly flexible. Moreover, it supports the syntax and proof rules of a number of popular textbooks. I’ll highlight the free/open ones:

(Of course, the last is my favorite.)

Commercial texts supported by Carnap, which you would be evil to make your students buy of course, are:

  • Bergman, Moore, Nelson, The Logic Book (McGraw-Hill, $130)
  • Goldfarb, Deductive Logic (Hackett, $39)
  • Hausman, Kahane, Tidman, Logic and Philosophy (Wadsworth, $120)
  • Howard-Snyder, Howard-Snyder, Wasserman, The Power of Logic (McGraw-Hill, $130)
  • Kalish and Montague, Logic: Techniques of Formal Reasoning (Oxford, $90)
  • Tomassi, Logic (Routledge, $54)

All of these textbooks use a linear version of natural deduction (Fitch, Lemmon, or Montague), but Carnap now also has proof editors for Gentzen-style sequent calculus and natural deduction proofs and checks them for correctness.

How does it support different textbooks? Basically, the document you upload just tells Carnap, say, what sentence you want the student to produce as a translation, or what sentence you want them to give a proof of. You can change the “system” in which they do that, and based on that Carnap will show them the symbols differently (e.g., will ask them to do a truth table for \((\lnot P \land Q) \to R\) or for \((\mathord\sim P \mathbin\& Q) \supset R\)), and and will accept and display proofs in different formats and allow different rules. Even if your favorite text doesn’t show up above it’s likely that it is already partially supported. Graham is also incredibly helpful and responsive; last term he introduced new proof system systems and other features based on my request, often within days. (Bug reports and feature requests are handled via GitHub.)

Carnap is already pretty smart. It will accept not only solutions to translation questions that are syntactically identical to the model solution, but any equivalent solution (the equivalence check is not perfect for the first-order case, but will generally accept any reasonable equivalent translation). Graham has recently introduced a few new features, e.g., you can randomize problems for different students, or require that some conditions are met for translation problems (e.g., that the translation only uses certain connectives or is in a normal form).

To get set up, just email Graham. Once you have an instructor account and are logged in, you’ll be able to see the actual problem sets I assign in my class. You’re welcome to copy and use them of course! (If you happen to use a different textbook, you’ll just have to adjust the terminology and change the “system” Carnap is supposed to use in each problem.) Check here for more of the course, like lecture slides.

BibTeX-friendly PDF management with Zotero

For years I’ve been jealous of colleagues with Macs who apparently all use BibDesk for managing their article PDF collections and BibTeX citations in one nice program. I think I’ve finally figured out how to do both things on Linux: Zotero, with the Better BibTeX and ZotFile add-ons.

Zotero is first of all a citation management system. It’s multi-platform, open-source, not tied to a commercial publisher, widely used and well-supported. Your article database lives on your computer, but is synced with a central server. So any changes you make to the citation database gets automatically mirrored to your other computers (even if they run different OSs), and you can access it online as well. The browser extension Zotero Connector lets you import & download references and PDFs from publishers’ websites, JSTOR, etc., with a single click. It does everything a reference manager does, e.g., give you bibliographies and citations in Word or LibreOffice.

Zotero manages PDFs in one of two ways: you can store a PDF in Zotero, or you can add links to PDFs on your local drive. The former option manages the PDFs for you, syncs them across computers, etc. But you only get 300MB of online storage for free, and that’s gone quickly. But if you keep your PDF directory synced across computers (e.g., if it lives in your Dropbox), linking the PDFs is just as good. If you add a PDF, Zotero will look up the metadata for you and add a reference to your database. It keeps an index of the content of PDFs, so search will pick up hits in the PDFs and not just in the metadata. If you have a reference already, Zotero can look it up online and help you find the PDF (or library call number). The ZotFile add-on makes this even easier. For instance, with one click you can add the most recently downloaded file to a reference item, move it into your PDF directory, and rename it according to some standard schema, say “Author – Year – Title.pdf”.

All of this has worked to some extent for a while, and also works with other reference managers. What has kept me from using them is that I want my reference manager to play nice with BibTeX. That means it should export BibTeX files with (a) proper capitalization, (b) LaTeX code where necessary (e.g., mathematical formulas in titles), (c) keep track of BibTeX fields like the crossref and note fields, which may contain LaTeX code itself (e.g., “Reprinted in \cite{...}“), and (d) not change cite keys on you. On the other hand, the database itself should look as normal as possible and avoid LaTeX code whenever possible (e.g., I want Gödel’s papers to be indexed under “Gödel”, not under “G{\”o}del”). When I tried Zotero the last time (and other reference managers), it would deal with (a) by enclosing the entire title field in braces. That meant BibTeX would not lowercase anything; and sometimes the style does require lowercasing things. I don’t remember if (b) or (c) ever worked.

Anyway, Zotero’s Better BibTeX extension does an excellent job.  You can put in “Gödel” as the author name, and it will export to “G{\”o}del”. It assigns cite keys according to a configurable pattern, but it keeps cite keys the same when importing BibTeX files. You can change them manually, and it will remember them. Additional BibTeX fields not already supported by Zotero are saved on import and included on export.  It will convert HTML tags (which Zotero understands as well) to LaTeX code on export (e.g., <i>...</i> to \emph{...}). If you need LaTeX code, just enclose it in <pre>...</pre> tags. It does a very good job with capitalization and is even smart enough to do its transformations only to English language titles. Especially nice: Better BibTeX will keep your exported BibTeX files up to date. So, e.g., you can put all the references for a paper you’re working on in a Zotero “collection”, tell Better BibTeX to export it, and the .bib file will stay up to date if you change or add something to the collection in Zotero.

Want to try it?

  • Install Zotero Standalone and Connector
  • Install ZotFile
  • Install Better BibTeX
  • Check your preferences, e.g., whether you want Zotero to rename PDFs or look up metadata when saving them.
  • If you want your PDFs to be linked and collected in, say, your Dropbox, set the attachment base directory in Preferences: Advanced: Files and Folders.
  • Tell ZotFile where your downloaded files and your PDF direcories are, so it knows where to look for the most recent PDF to attach (in Tools > ZotFile preferences) and where to move them to. Make sure you set the ZotFile PDF naming pattern there to something you like.
  • Set up an account on zotero.org and link your library to it in Preferences: Sync (but uncheck “Sync attachment files” if you don’t want your PDFs on zotero.org)
  • Tweak your Better BibTeX settings, esp. the cite key pattern to make sure imported cite keys are the way you want them.

Indian Conference on Logic and its Applications 2019

The Association for Logic in India (ALI) announces the eighth edition of its biennial International Conference on Logic and its Applications (ICLA), to be held at the Indian Institute of Technology Delhi from March 3 to 5, 2019.

ICLA is a forum for bringing together researchers from a wide variety of fields in which formal logic plays a significant role, along with mathematicians, computer scientists, philosophers and logicians studying foundations of formal logic in itself. A special feature of this conference is the inclusion of studies in systems of logic in the Indian tradition and historical research on logic.

As in the earlier events in this series, we shall have eminent scholars as invited speakers. Details of the last ICLA 2017 may be found at https://icla.cse.iitk.ac.in. See http://ali.cmi.ac.in for information on past events as well as updates on this conference.

The call for papers is here: https://easychair.org/cfp/icla2019

PhD, Postdoc with Rosalie Iemhoff

Postdoc position in Logic at Utrecht University, the Netherlands.

The postdoc is embedded in the research project “Optimal Proofs” funded by the Netherlands Organization for Scientific Research led by Dr. Rosalie Iemhoff, Department of Philosophy and Religious Studies, Utrecht University. The project in mathematical and philosophical logic is concerned with formalization in general and proof systems as a form of formalization in particular. Its mathematical aim is to develop methods to describe the possible proof systems of a given logic and establish, given various criteria of optimality, what the optimal proof systems of the logic are. Its philosophical aim is to develop general criteria for faithful formalization in logic and to thereby distinguish good formalizations from bad ones. The mathematical part of the project focusses on, but is not necessarily restricted to, the (non)classical logics that occur in computer science, mathematics, and philosophy, while the philosophical part of the project also takes into account domains where formalization in logic is less common. The postdoc is expected to contribute primarily to the mathematical part of the project. Whether the research of the postdoc also extends to the philosophical part of the project depends on his or her interests.

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Raymond Smullyan

Proof by legerdemain

Peli Grietzer shared a blog post by David Auerbach on Twitter yesterday containing the following lovely quote about Smullyan and Carnap:

I particularly delighted in playing tricks on the philosopher Rudolf Carnap; he was the perfect audience! (Most scientists and mathematicians are; they are so honest themselves ‘that they have great difficulty in seeing through the deceptions of others.) After one particular trick, Carnap said, “Nohhhh! I didn’t think that could happen in any possible world, let alone this one!”

 

In item # 249 of my book of logic puzzles titled What Is the Name of This Book?, I describe an infallible method of proving anything whatsoever. Only a magician is capable of employing the method, however. I once used it on Rudolf Carnap to prove the existence of God.

 

“Here you see a red card,” I said to Professor Carnap as I removed a card from the deck. “I place it face down in your palm. Now, you know that a false proposition implies any proposition. Therefore, if this card were black, then God would exist. Do you agree?”

 

“Oh, certainly,” replied Carnap, “if the card were black, then God would exist.”

 

“Very good,” I said as I turned over the card. “As you see, the card is black. Therefore, God exists!”

 

“Ah, yes!” replied Carnap in a philosophical tone. “Proof by legerdemain! Same as the theologians use!”

 

Raymond Smullyan, 5000 BC and Other Philosophical Fantasies. New York: St. Martin’s Press, 1983, p. 24.

See Auerbach’s post for more Carnap and Smullyan anecdotes.

Why φ?

Yesterday, @gravbeast asked on Twitter,

Does anyone know why we traditionally use Greek phi and psi for metasyntactic variables representing arbitrary logic formulas? Is it just because ‘formula’ begins with an ‘f’ sound? And chi was being used for other things?

Although Whitehead and Russell already used φ and ψ for propositional functions, the convention of using them specifically as meta-variables for formulas seems to go back to Quine’s 1940 Mathematical Logic. Quine used μ, ν as metavariables for arbitrary expressions, and reserved α, β, γ for variables, ξ, η, σ for terms, and φ, χ, ψ for statement. (ε, ι, λ had special roles.) Why φ for statements? Who knows. Perhaps simply because Whitehead and Russell used it for propositional functions in Principia? Or because “p” for “proposition” was entrenched, and in classic Greek, φ was a p sound, not f?

The most common alternative in use at the time was the use of Fraktur letters, e.g., \(\mathfrak{A}\) as a metavariable for formulas, and A as a formula variable; x as a bound variable and \(\mathfrak{x}\) as a metavariable for bound variables. This was the convention in the Hilbert school, also followed by Carnap. Kleene later used script letters for metavariables and upright roman type for the corresponding symbols of the object language. But indicating the difference by different fonts is perhaps not ideal, and Fraktur may not have been the most appealing choice anyway, both because it was the 1940s and because the type was probably not available in American print shops.