A Cognitive Approach to Teaching Logic and Proof

Susanna S. Epp

Department of Mathematical Sciences
DePaul University
sepp@condor.depaul.edu

In the late 1970's I started teaching a course to help students make the transition from a computationally oriented calculus class to more abstract, theoretical classes in mathematics and computer science. Members of my department were concerned that students in our more advanced classes had great difficulty answering what we considered to be quite simple abstract questions involving proof and disproof, and certain foundational mathematical material was receiving short shrift because teachers in the advanced classes wanted to move quickly to more interesting topics. The rationale for the new course was that it would solve these problems by giving students an adequate amount of time in which to focus exclusively on techniques of proof and disproof and topics like sets, functions, and relations.

Unfortunately, the solution was not so simple. In teaching the course I found that student difficulties were much more profound than I had imagined. In fact, I discovered, most of my students lived in a different logical and linguistic world from the one I inhabited. Yet to be able to engage effectively in abstract mathematical thinking, students had to learn to function in my world. (For more details see Epp, 1987.) My aim in teaching, therefore, became to help my students develop new habits of mind as well as new knowledge.

Over the years of evolution of the course I've experimented quite a lot. As a general principle I've tried to remain sensitive to the kinds of difficulties students encounter and be constantly alert for more effective ways to help them. In the two examples which follow I describe how I've come to present two particular topics, validity and basic notions of proof and counterexample.

Example: Teaching about Validity

My main purpose in teaching about validity is to lay the groundwork for understanding the process of deductive reasoning. Thus I want students to develop insight into the fact that validity is conditional, namely that in a valid form of argument when all the premises are true the conclusion is necessarily true and that it's only when all the premises are true that we have any information about the truth or falsity of the conclusion. I also want at least the better students to understand that the word "necessarily" means that in any other argument of the given form, if the premises are true then the conclusion is also true.

When I first started teaching a course in mathematical reasoning, I decided against having students test the validity of forms of argument in the statement calculus by checking to see whether the corresponding conditional statement form was a tautology. This approach seemed too static to insure that students who used it correctly would develop the kind of dynamic understanding I was aiming for. In the tautology approach the conditional aspect of validity, which I wanted students to think about explicitly, might be hidden as a mechanical part of truth table construction.

Instead I elected to use the format where students have to identify explicitly the columns of the truth table that represent the premises and the conclusion, consider each case where the premises are all true, and then check that in each such case the conclusion is also true.

Over time I discovered that simply using this method was not sufficient to achieve my goal. It turned out that I needed to be much more careful in my choice of examples and exercises than I had at first realized was necessary. A small but significant number of students made the mistake of declaring an argument to be valid if they found one row of the truth table in which the premises and the conclusion were all true. Recently I've come to see that this mistake is part of a broader cognitive tendency to ignore quantification and confound universal conditional statements and conjunctions. (Other manifestations of this tendency are seen if one asks students to write what it means for a set A to be a subset of a set B. Quite a few respond "x is in A and x is in B" , or the equivalent. [endnote1] Similarly, many students define a relation R to be symmetric if "xRy and yRx".)

To counter the mistake, I began including, at the earliest stage of the discussion of validity, examples and exercises designed to make as obvious as possible that each individual row having true premises must be checked. So now the first argument form I test for validity (with much participation by members of the class) fails the test but its truth table has several rows in which all the premises and the conclusion are true. Of course, in discussing this example I make much of the fact that some (but not all!) rows have all true premises and a true conclusion so that (be careful!) every single row with true premises must be checked. I also make sure that the first homework assignment includes at least two exercises of this type (with answers in the back of the book to give students immediate feedback if they make a mistake), and I give a follow-up quiz with a similar problem. (Often I sharpen the focus and shorten the length of this quiz by supplying the truth table myself so that all the students have to do is interpret it.) Actually I give the quiz to students in pairs so that those students who have successfully caught on to the idea become my allies in passing it along to their less receptive colleagues.

I've found the combination of these measures to be quite successful in achieving good student performance. Another benefit is that it gives students concrete practice in determining the truth or falsity of a universal conditional statement (which many of them clearly need) and thus helps sharpen students' sensitivity to the meaning of quantified statements and prepare the way for our subsequent treatment of the predicate calculus.

Despite the success of these measures, however, many of my students were still not coming to understand validity in the conceptual way I was hoping for. I discovered that some rather good students were able to perform the steps to test for validity but were doing so mechanically without being able to articulate the connection between the steps and the definition. For instance, in connection with a class example one day a student stated, "It is invalid because the result in row 3 is F." "What do you mean by 'result'," I asked (nicely). It took several minutes of discussion, which involved most of the rest of the class, until one student finally said that "result" actually meant truth value of the conclusion.

The first countermeasure I took was to change the way I wrote solutions to validity problems in the book I had developed to use for the course. In the first edition I wrote the explanations for the answers as ordinary text underneath the truth tables. (Epp, 1990, pp. 37-8) In the second edition I made the explanation part of the truth table display itself. (Epp, 1995, p. 30) The reason I did this was that in a problem solution containing a display students tend to regard the text outside the display as extra explanatory material rather than as a formal part of the answer. Incorporating a brief explanation into the display sends an (admittedly subtle) message that students are to include it in their own solutions.

When I next taught the course, I went a step further in explicitness and changed the wording of my questions. Now instead of just asking students to "determine whether the following argument form is valid," I routinely add (both in class and on quizzes and exams): "Include a few words of explanation for your answer." During class, after student discussion has produced a truth table and a correct analysis of it, I matter-of-factly summarize the result and the reason for it on the board as if inclusion of these words is simply part of a standard answer. For instance, on the basis of the discussion I might draw an arrow to one row and write "This row shows that it is possible for an argument of this form to have true premises and a false conclusion; hence this argument form is invalid." Or I might write: "Thus for each possible argument of this form in which the premises are true, the conclusion is also true; hence the argument form is valid." Hence all students are participants in a process that leads to an answer which reinforces conceptual understanding, and students taking notes have the answer available as a model for future reference. Now when I ask for explanations about validity on quizzes and exams, a remarkably larger number of students give very well expressed answers, and almost all the rest express their answers reasonably well. This result encourages me in the belief that relatively small changes in instructional technique can have a major impact on student understanding.

Example: Teaching about Proof and Counterexample

In introducing students to the concepts of proof and counterexample, my primary goal is to help them develop the ability to evaluate the truth and falsity of simple mathematical statements. I want them to get a feel for the flow of deductive argument, to appreciate the role of definitions in mathematical reasoning, and to learn to express themselves with clarity and coherence.

When I first started trying to teach students how to write proofs, I was almost overwhelmed by the poor quality of their attempts. Often their efforts consisted of little more than a few disconnected calculations and imprecisely or incorrectly used words and phrases which did not even necessarily advance the substance of their cases. A few of my colleagues are so put off by such student work that they avoid asking for proof altogether, focusing on the purely computational even in advanced courses. Some argue that the really "good" students will develop an ability to reason mathematically by osmosis as they watch the teacher at the board and that the other students are simply incapable of complex mathematical abstraction. But in all likelihood those who make this argument have themselves had the experience of improving their performance of a complex skill by dint of persistent effort. We do not all have the same innate gifts; hardly any of us will become Mozarts in music or Gausses in mathematics, not by a long shot. And the level of achievement attained will certainly vary from one person to another. But with sustained effort and encouragement, I believe that everyone is capable of improvement.

It seems to me that the very existence of student difficulties with abstract thinking is reason for us to address them. The students I teach are majoring in mathematics and computer science at a selective university. In fact many are computer professionals with a decade or more of experience. If they are having problems with what we consider the most fundamental aspects of mathematical reasoning, it should be one of our prime responsibilities to help them better their performance.

A rather widely held belief among cognitive psychologists is that the human brain consists of many parts which operate more or less independently and which often do not communicate very well with each other. [For instance, see Gazzaniga, 1985 or Minsky, 1987.] The implication for education is that to learn a complex process such as proof and disproof, many component intellectual skills must be developed and effectively integrated. It is the teacher's job to devise ways to help different students achieve such development and integration.

Earlier in this century the Russian psychologist L. S. Vygotsky coined the phrase "zone of proximal development." (Vygotsky, 1935). I confess that when I first heard these words, I dismissed them as jargon. I've since come to view them as articulating a profound truth about the educational process, namely that at any given point in the learning process, the insight and intuitions the learner has previously developed provide a basis that defines and limits the amount that can be accomplished in the next stage of instruction. Thus the depth of my understanding of the background ideas for a given topic area and my skill in working with them determine how much I can absorb in the next instructional session.

When I first started teaching a course in mathematical reasoning, I covered preliminary material on logic in the briefest and most perfunctory way before launching into specific topic such as sets, functions, and relations. Over the years, however, I've come to see the benefit of helping students broaden their zone of proximal development before I introduce the concepts of proof and disproof proper. So I now spend a few weeks at the beginning of the course discussing basic notions of elementary logic and giving students formal and informal practice in working with the language of the logical connectives and the quantifiers. Also I now order the mathematical subject matter in a way that takes students' cognitive development into account. I begin the discussion of proof and counterexample by considering properties of numbers (which are familiar objects) and wait to cover set theory (whose proofs involve nested quantification) until somewhat later.

One benefit of this approach is that students like it. It provides them with a supportive framework which they can lean on while the various aspects of proof and counterexample are falling into place. It also gives them the impression that the course as a whole has been carefully thought out and thereby increases their confidence in the educational enterprise and their willingness to cooperate with me. Moreover, preceding the discussion of proof and disproof with a treatment of logic gives me a language in which to explain why we do the things we do in proving and disproving mathematical statements and a vocabulary with which to communicate with my students when they make mistakes.

To help make the transition from elementary logic to proof, I assign a number of puzzle problems similar to Raymond Smullyan's knights and knaves. (See Smullyan, 1978.) When I discuss solutions in class, quite a number of students make it clear that the kind of indirect reasoning needed to solve most of the puzzles does not come naturally to them. But almost all seem to enjoy working on the puzzles, and doing so helps them develop a sense for the flow of deductive reasoning they will use later in mathematical proofs of all types and provides a basis of intuition for proof by contradiction.

The first statements I ask students to prove or disprove concern such things as properties of even and odd integers, rational numbers, divisibility, and so forth. In explaining what is involved in establishing the truth or falsity of such statements, I rely heavily on the previous work I've asked students to do in the sections on logic. For instance, in the logic sections I try to motivate acceptance for the forms of the negations of each type of statement by discussing them interactively with the class, and I give students a lot of practice writing negations (in English) of statements of each form. My aim is to get students to incorporate into their own thinking patterns the ability to take negations according to the rules of formal logic. I reason that if students are eventually to be able to evaluate mathematical statements for their truth and falsity, they need to have a good intuition for what it means for various kinds of statements to be false. In addition, the analysis of truth and falsity in which students engage during the weeks devoted to logic gives them insight into aspects of proof and disproof even before these ideas are introduced formally. So, for example, when I introduce the concept of disproof by counterexample quite a number of students immediately understand its relation to the fact that the negation of a universal statement is existential. And when I present proof by contradiction, I can point out that such a proof begins by supposing the negation of the statement to be proved and be confident that every student will have had experience in learning how to write negations correctly.

One of the most serious problems which arises in trying to teach mathematical proof is that even rather simple proofs are built atop a normally unexpressed substructure of great complexity _ logical, linguistic, and sociolinguistic (i.e., the use of mathematical slang, in French "abuse de langage," and other mathematical conventions). A teacher can easily write a "clarifying analysis" for a proof which if students could read with understanding they wouldn't need in the first place. Figuring out a way to describe proof construction in simple terms while explaining enough to be effective is probably a teacher's greatest challenge.

In my view the most important idea to communicate about proof is that all proofs flow from something supposed to something to be shown. It then becomes natural to consider what is supposed and what is to be shown. The most common type of proof is direct and the most common type of mathematical statement has the form "For all elements x in a set D, if (hypothesis) then (conclusion)". A direct proof of such a universal conditional statement has the following outline: "Suppose that x is a particular but arbitrarily chosen (or generic) element of D for which the hypothesis is true. We must show that x makes the conclusion true also." I sometimes refer to this method, descriptively albeit ponderously, as "generalizing from the generic particular."

When I begin discussing this approach to proof, I try to dramatize its power by showing how one can use it to outline proofs couched in language one doesn't even understand. For instance, given the statement "For all toths T, if T has a rath then every wade of T is brillig," the outline of a proof would be "Suppose T is any toth which has a rath. We must show that every wade of T is brillig." This transformation may seem simple to us, but I am convinced that the student who truly understands how to do it has taken a giant step toward mathematical maturity.

Later on, after I've introduced proof by contraposition and proof by contradiction as well as direct proof, I try to help students keep the three basic proof methods separate by pointing out that while for each method there is something that we suppose and something that we show, these "somethings" are dramatically different in each case. In a direct proof we suppose we have a particular but arbitrarily chosen object that satisfies the hypothesis and show that this object satisfies the conclusion. In a proof by contraposition we suppose we have a particular but arbitrarily chosen object for which the conclusion is false and we show that for this object the hypothesis is also false. In a proof by contradiction we suppose that the entire statement to be proved is false and show that this supposition leads to a contradiction.

I must admit that while my approach works very well with a certain fraction of students (in the sense that they do excellent work at each stage of the course but also give me every impression that they consider the course challenging and worthwhile), a significant number of students have difficulty when they come actually to write proofs. Partly, I think this is because the few weeks we've spent on background material is not sufficient to overcome the casual attitudes toward language and reasoning that they've developed over their lifetimes. Some of them can't believe I'm serious about my demand for coherence, while others accept it but have difficulty putting all the pieces together correctly.

One countermeasure I've found very effective is to have students present proofs from their homework assignment for the rest of the class starting with the very first class period after proofs have been assigned. I should mention that I try to do this in a way that preserves the self-esteem of the presenters, thanking them for being good sports when they volunteer and pointing out that to the extent that they make mistakes, their mistakes and our discussion of them will help the rest of the class develop a better understanding. If the proofs are good, the other students get to see that the demands made by their teacher can actually be met by one of their own kind. If the proofs contain mistakes or sections that are not well expressed, I ask for suggestions for improvement from the rest of the class, urging them to be super critical, to imagine, for instance, that the class as a whole is a research team for a company and if they can come up with a "perfect" answer they'll get to share a large monetary prize. After the class has finished its critique and some changes have been recorded, I take my turn, explaining that I'm using the opportunity to show them the kind of things I'll be looking for when I grade their papers. I try to be "picky" but also to put my criticisms in perspective, explaining frankly that certain corrections are more important than others but that I also care about what might seem to be relatively small points. For instance, if a student's proof states that a certain number, say n, is even because it equals 2k, I would ask what was missing. Most likely, based on the emphasis I'd previously placed on definitions, one of the other students would say I should add "for some integer k." I would agree, pointing out that 1 = 2x(1/2) and yet 1 is not even and adding that it's not enough that n is 2 times something _ that "something" has to be an integer.

My main reason for engaging in this process is to give students immediate feedback on their proof writing. But I also try to use these occasions to counteract student anxiety about how their proofs will be evaluated. Since there is more than one right way to construct any given proof and since different instructors may well have different standards of "correctness," I feel obliged to try to give my students a sense of the range of proof styles I consider acceptable and to indicate which parts of a proof I consider most important. So when I critique student proofs, present my own, and write proofs at the board that have been developed collaboratively with members of the class, I try to discuss various alternative ways of expressing the steps which I would consider acceptable. I also talk about conventions of mathematical writing (such as giving only part of the reason for a certain step, enough to indicate that the writer of the proof has considered and resolved the issue but not so much as to overload the proof with verbiage) and the fact that the amount of detail included in a proof varies considerably depending on the audience. In my classes I generally suggest that students aim their proofs at a fellow student who has missed a few classes.

Other techniques I use to help students learn to write proofs are (1) having them complete a few fill-in-the-blank proofs before they start writing their own, (2) supplying complete answers for some of their homework problems so that they come to see that I really mean for their individual work to look something like the kind of thing we've been doing in class, (3) making sure that even in a large class I give each student detailed feedback about the work they've done on each type of proof I assign (one good way to do this is to have students submit one or two drafts of their solutions to a few selected problems and make suggestions for improvement on each draft), (4) discussing some of the kinds of errors often made in writing proofs and assigning homework problems of the find-the-mistake variety. In one way or another all these techniques reflect my belief that to learn as complex a skill as proof construction, most students need quite a bit of back-and-forth interaction with an instructor. To the extent that I am not able to supply this personally, I have tried to devise effective substitutes.

Final Thoughts

I don't claim that the kind of approaches I've described above will turn every student who is exposed to them into a fully competent mathematical reasoner. But I do see significant intellectual development in most of my students, and I don't think the experience has harmed those who appear to have benefited less from it. Indeed, I've sometimes been surprised when students who in my view fell far short of achieving the levels of accomplishment I strive for tell me how valuable they found the course in helping them do better work in their other courses or (I am always pleased to hear) in their jobs.

A couple of years ago I had an experience with one particular class that made a special impression on me. The class was unusually small, only twelve students, and was the second quarter of our sequence. The previous quarter had dealt with logic, an introduction to direct and indirect proof, mathematical induction, and elementary combinatorics, all interwoven with various computer science applications. This second quarter was to cover set theory, function properties, recursion, some analysis of algorithms, relations on sets, and an introduction to graph theory, also with an admixture of applications. The class met only once a week but for a three-hour session.

The small size of the class and the length of the sessions gave me a chance to work with students more intimately than I'm used to. I began each period by having students discuss in groups of three or four the homework they had prepared for that day. I went from group to group talking with each at length. Overall the class atmosphere was excellent, and there were several students especially who showed the kind of eager, enthusiastic intelligence that is a teacher's joy. What surprised me was that as the course moved from one topic to the next, almost all the students who had attained a relatively sophisticated level of achievement in dealing with a previous topic made it clear that they felt they had to struggle to succeed with the next. Yet as we worked through their questions and difficulties, they ultimately achieved an excellent level of performance with the new topic as well. Their understanding of general methodological principles clearly made it easier for them to learn the new material but it didn't make it trivial for them.

Somehow this experience brought home to me more effectively than any before that abstract mathematical thinking is not something that either one is able to do or one is not able to do. Because of the experience I've become especially conscious of the need to respect my students and never to act surprised by their questions. Even when a student asks a question whose answer I've already discussed, I try to respond to it as if it were fresh. After all, nobody can concentrate 100% of the time when new ideas are coming in fast and furiously. In all likelihood the student was not mentally prepared to absorb the answer when I previously talked about it. For the student to formulate the question at this point means that they've thought about the issue, want to know the answer, and are probably ready to understand it. That is cause to celebrate. And, very likely, clarifying the issue at this point in the course (if possible in a slightly different way from that presented earlier) will give the other students in the class greater insight also.

My main advice to those teaching courses whose goal is to develop students' mathematical reasoning powers is to play an activist role but recognize that achieving success is a long-term process. The analogy I like to draw is of a child learning to walk. It takes months of daily effort for most children to take their first steps and several more months until they actually become steady on their feet. When a child is trying to move from one stage to the next in learning to walk and has failed several times, we don't say "forget it." We remain calm, good humored, and encouraging. And when the child finally succeeds, we spare nothing in expressing our delight.

Endnotes

  1. The tendency to confuse conditionals and conjunctions is very strong. Since I know students tend to make this mistake, I discuss the definition of subset at length with them, eliciting from students various alternative formal and informal ways of expressing it and assigning homework exercises designed to force them to think about the definition in various situations, and I specifically warn them against confusing "if x e U then x e V" with "x e U and x e V". Considering all these efforts, one might expect a virtually perfect response to the following question asked on a quiz given the period after the initial discussion and when students had had a chance to do the homework: "Given sets U and V, what does it mean for U to be a subset of V?" In fact, in a class of 38 students, only 20 gave a correct answer, 1 answered essentially correctly, 6 answered ambiguously (e.g., "Every element of U is in V. x e U and x e V."), 5 students answered "x e U and x e V", 3 answered "Ax, x e U and x e V", and 3 gave miscellaneous incorrect answers. (In this note, A stands for "for all" and e for "element of".)
  2. I point out that we follow such conventions so that we don't drown the readers of our proofs in words but that in doing so we increase our risk of error. Any given class is sure to give rise to examples to illustrate this point, and I try to take advantage of them in my discussions with the students. I've actually come to believe that one of my functions as a teacher is to help students become comfortable about making mistakes. After all when human beings engage in mathematical reasoning, mistakes are inevitable. My best students occasionally made howlers and so do I. Doesn't every mathematician? Paradoxically, once students accept that mistakes in mathematical reasoning are part of the game, they become more willing to make the effort to avoid them.

References

  1. Epp, S. (1987). The logic of teaching calculus. In R. G. Douglas (Ed.), Toward a Lean and Lively Calculus (pp.41-60). Washington, D. C.: The Mathematical Association of America.
  2. Epp, S. (1990). Discrete Mathematics with Applications. Belmont, California: Wadsworth Publishing Company. (1995) Discrete Mathematics with Applications, Second Edition. Boston, Massachusetts: PWS Publishing.
  3. Gazzaniga, M. S. (1985). The Social Brain: Discovering the Networks of the Mind. New York: Basic Books.
  4. Minsky, M. (1987). The Society of Mind. London: Heinemann.
  5. Smullyan, R. (1978). What Is the Name of This Book? Englewood Cliffs, New Jersey: Prentice-Hall.
  6. Vygotsky, L. S. (1935). Mental Development of Children and the Process of Learning. Translated as part of M. Cole et al. (Ed.) (1978). Mind in Society. Cambridge, Massachusetts: Harvard University Press.