What's the most harmful heuristic (towards proper mathematics education), you've seen taught/accidentally taught/were taught? When did handwaving inhibit proper learning?

8$\begingroup$ In view of many of the answers to this question, it might help to have in the statement a definition of heuristic as it is applied to mathematics. $\endgroup$ – Pete L. Clark Apr 26 '10 at 3:56

10$\begingroup$ In fact, the harmful entity in most answers is not a heuristic at all! $\endgroup$ – Victor Protsak May 22 '10 at 15:07
"Mathematical knowledge is contained and communicated primarily by documents."
I'm not sure if this is a heuristic, but in terms of beliefs that inhibit learning, this is definitely the one that hurt my mathematical development the most.
I would say the correct statement is "Mathematical knowledge is contained primarily in the minds of mathematicians and communicated primarily by informal oral communication."
This problematic belief grew out of the way that I (and pretty much everyone else) was taught mathematics at the undergraduate and beginning graduate level. In this setting texts are a central authority and a complete, wellwritten resource for the knowledge needed to solve any mathematical problem encountered.
In the world of mathematical research, this is no longer the case. I finally figured this out by reading Thurston's essay "On proof and progress in mathematics", which I would strongly recommend for any beginning mathematician.
Maybe it is possible to do research mathematics using papers as a primary resource, but I believe this is highly inefficient. I spent several years trying to learn the noncommutative standard model by reading the available papers on the subject and made no real progress. Looking back, I don't think I ever had a chance of succeeding with this approach.
I would guess that to be successful in mathematics, it is absolutely vital to become regularly involved in conversations with working mathematicians, as awkward and intimidating as that might be.

4$\begingroup$ This has nothing to do with heuristics, whatsoever! $\endgroup$ – Mariano SuárezÁlvarez Apr 17 '13 at 20:52

1$\begingroup$ Sorry but I disagree. There is so much buried knowledge in the unread works of the past that I wouldn't be surprised if it surpasses the knowledge of currently living mathematicians. Take into account that many authors forget their own papers after a couple of decades... $\endgroup$ – darij grinberg Apr 17 '13 at 22:56

2$\begingroup$ I disagree with the disagreeers. Sure, this answer is a little more "meta" than the question likely intended, but not overwhelmingly so: if we take "a heuristic in math" to mean "a rule of thumb for how to prove things in math", then this answer is arguably on target, even though it is more methodological and less domainspecific. Besides, I think it's an important message to have out there, a realization that every mathematician will have to come to in order to be successful. Even though it's not the sort of issue you'll find discussed in papers :). $\endgroup$ – Tim Campion Nov 14 '13 at 21:52

1$\begingroup$ Especially a lot of the intuition is communicated orally and informally. And it is impossible to do mathematics without having an intuition about what you're doing. $\endgroup$ – Manuel Bärenz Sep 10 '14 at 15:13
Talking about "functions" when we are actually talking about equivalent classes of functions almost everywhere equal
An element of a $L^p(X)$ space is usually called a "function", and is usually denoted by letters that are used typically for functions ($f$, $g$, $h$, etc.).
It seems to be a harmful heuristic to act "as if" $L^p(X)$ is made of functions, as a function is really something that should give you a value for each point $x$ in $X$. I am aware that it is now common practice, but I am sure it would help to actually introduce an actual name besides "function" to call "equivalent classes of functions modulo equality almosteverywhere". This concept is fundamental, and should be given a proper name. I don't have a proposition for such name, but I kind of wish someone in the past did.

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1$\begingroup$ @GerryMyerson It can lead to errors in the theory of Hilbert spaces. Many people incorrectly think of the Hilbert space $L^2(\mathbb{R})$ as the space $\mathcal{L}^2(\mathbb{R})$ of squareintegrable functions. But $\mathcal{L}^2(\mathbb{R})$ isn't an inner product space at all, because the naive "inner product" isn't positive definite. Moreover, the "squareintegrable function" intuition often leads people to believe that orthonormal bases of $L^2(\mathbb{R})$ have the cardinality of the continuum, which would imply that the Hilbert space is nonseparable, when in fact it is separable. $\endgroup$ – tparker Oct 20 '18 at 3:35

4$\begingroup$ @GerryMyerson It can lead people to ask for "values" of elements of $L^p(X)$ at points of $X$. For an individual element of $L^p(X)$ this may actually be made sense of (due to some kind of regularity), but it is surely misleading to think that there is a linear functional like "evaluation at a point". $\endgroup$ – Kapil Oct 27 '18 at 5:19

$\begingroup$ Why not build the term "ekafunction" from sanskrit eka meaning "one" ? The sound "ek" evocates "Equivalence Class" while the meaning of the prefix suggests that one single equivalence class corresponds to several different genuine functions. $\endgroup$ – Sylvain JULIEN Oct 27 '18 at 8:26
The following refers to the school system in Germany, it may be different in other countries:
In my opinion, one really bad heuristic happens in elementary school, when children learn arithmetics with natural numbers. They learn that addition and subtraction are two entirely different things, because they are taught $a+b=b+a$ but $ab\neq ba$. Thus addition is commutative, and subtraction is not. At that level, numbers are solely understood as enumerations of objects.
Then they learn about numbers with units, such as lengths or prices or weights. Also they learn that numbers might have geometric meaning, e.g. as lengths of line segments. But still no concept of negative numbers.
Years later, when they finally get to know negative numbers as well, they have so much incorporated that subtraction is something different from addition that they have difficulties to grasp that $ab=a+(b)=(b)+a$, i.e. that subtraction is nothing else than addition of a negative number.
I think that postponing negative numbers so long is a mistake, and that children in elementary school would very well be capable to understand them.

1$\begingroup$ Not a heuristic, but my leastfavourite mannerism in additive groups is the habit of reading '$x$' as 'negative $x$'. This is at best meaningless (for example, when $x$ is a nonreal complex number) and at worst false (students with this habit cannot understand how $x = x$ can be true, since the lefthand side is positive (not necessarily, but that's not the real issue) and the righthand side is negative …). $\endgroup$ – LSpice Nov 28 '17 at 20:08
Division by Zero is Infinity. This was taught in my seventh grade while the teacher was explaining the concept of infinity and it's definition(s).
False: division by zero is Undefined.

3$\begingroup$ Well, as a heuristic that works sometimes, as long as one remembers it's just a heuristic and takes limits to doublecheck the conclusions. $\endgroup$ – Michael Nov 16 '16 at 16:59

2$\begingroup$ 1. Only $0/0$ should be thought of as being defined. Otherwise, it is usually a good idea to consider $1/0=\infty$ as a heuristic. After all, the mapping $x\mapsto 1/x$ becomes an automorphism of the Riemann sphere if and only if one sets $1/0=\infty,1/\infty=0.$ $\endgroup$ – Joseph Van Name Jan 10 '17 at 23:39

2$\begingroup$ $0/0$ should be thought of as defined? $\endgroup$ – naveen dankal Jan 11 '17 at 10:42

$\begingroup$ I meant to say 0/0 should be undefined. $\endgroup$ – Joseph Van Name Jan 24 '17 at 22:35
"Basic (and useful) mathematics is about calculations and higher (pure) mathematics is about proof."
One reason I think this is harmful is that there is no sharp line between calculations and proofs. Very often a certain calculation is essentially the proof except for a few logical connectives. Conversely, in formal logic, one can create a "calculus" that makes proofs appear to be calculations.
Another reason is that it leads students (and more importantly teachers!) to think that a drastic change of mindset is required to learn higher mathematics.
It is indeed true that analysis is quite different from calculus even though there is strong linkage. However, the former also leads to better techniques to calculate things. Putting too much emphasis on the "proof" aspect of analysis tends to put off a lot students who enjoyed playing with polynomials, trigonometry and calculus. Conversely, many students who like to work with proofs are encouraged to believe that what they are doing is somehow "superior" (higher) to "mere" calculus; they then do not do enough computational drills which illserves them if they actually take up mathematics!

$\begingroup$ I think a drastic change of mindset is required to learn higher mathematics. That doesn't mean throwing away computation, but embracing proof  partly recognizing the importance of proof, and partly coming to grips with proofs having structure beyond that of just algebraic manipulation (e.g. the first time a student learns about proof by induction). I do agree that devaluing computation is bad, but I think you're underestimating the conceptual change needed to move into higher mathematics. $\endgroup$ – Noah Schweber Oct 27 '18 at 15:24
The excluded middle ( A Law or an Heuristic) .
On a more general level given any closed question: Is it A or B ? , the heuristic says it is one or the other disregarding the option : the question is wrong or stupid or irrelevant or incomplete.
The principle of excluded middle disregards intuitionist logic. And has been harmful in not providing direct (constructive) proofs which are often more clear  yet can be harder to find.
Intuitionism is is also rather natural : being against anticommunists does not means you are a communist.

6$\begingroup$ Perhaps more proponents of intuitionism should have readily available examples for the glaring question: what are some natural settings where classical logic is faulty next to an (intuitionistic) alternative. Compelling answers to this question are much scarcer than suggestions to consider intuitionism. $\endgroup$ – AndrewLMarshall Aug 10 '11 at 23:51

6$\begingroup$ A topology is an example of a Heyting algebra, not a Boolean algebra. How's that? $\endgroup$ – Todd Trimble♦ Aug 25 '12 at 20:02