For a given mathematical object, a mathematician may have:
- A mental representation of the object. This can be a metaphor, a mental image, or a kinesthetic understanding of the object.
- A physical representation of the object. This may be a (physical) picture or drawing or three-dimensional model of the object.
- A mathematical definition and one or more mathematical representations for the object. Such a representation is itself a mathematical object.
The boldface things in this list are related to each other in lots of ways, and they are fuzzy and overlap and don’t include every phenomenon connected with a math object.
I have written about these things (, , , ). So have lots of other people. In this post I summarize these ideas. I expect to write about particular examples later on and will use this as a reference.
The following examples point out a few of the relationships between the ideas in boldface above. There is much more to understand.
Function as black box
The idea that a function is a black box or machine with input and output is a metaphor for a function.
A is a metaphor for B means that A and B are cognitively pasted together in such a way that the behavior of A is in many ways like the behavior of B. Such a thing is both useful and dangerous, dangerous because there will be ways in which A behaves that suggest inappropriate ideas about B.
The function as machine is a good metaphor: for example functional composition involves connecting the output of one machine to the input of another, and the inverse function is like running the machine backward.
The function as machine is a bad metaphor: For example, it is wrong to think you could build a machine to calculate any given function exactly. But you can still imagine such a machine, given by a specification (it outputs the value of the function at a given input) and then, in your imagination, connecting the input of one to the output of another must perforce calculate the composite of the corresponding functions.
Like any metaphor, this is a mental representation. That means the metaphor has a physical instantiation in your brain. So a metaphor has a physical representation.
Different people won’t have quite the same concept of a particular metaphor. So a metaphor will have lots of slightly different physical representations, but mathematicians form a community, and communication between mathematicians fine-tunes the different physical instantiations so that they correspond more closely to each other. This is the sense in which mathematical objects have a shared existence in a community as Reuben Hersh has suggested.
A function is a mathematical object, which can be rigorously specified as a set of ordered pairs together with a domain and a codomain. There is a cognitive relationship between the concepts of function as math object and function as black box with input and output.
The physical and the mental images are not the same thing, but they are definitely related. The relationship is mediated by the neuronal circuitry behind your retinas, which performs a highly sophisticated transformation of the pixels on your retina into an organized physical structure in your brain, connected to various other neurons.
This circuitry exists because it helps us get a useful understanding of the world through our eyes. So a picture of a triangle takes advantage of pre-existing neuron structure to generate a useful mental representation that helps us understand and prove things about triangles.
This mental representation also lives in a community of mathematician. Like any community, it has subgroups with “dialects” — varying understanding of representation.
This is “sees” in the sense of what their brain reports after all that processing. The mathematician’s brain connects the “triangle I am seeing” module (in their brain) to the “looks like a right triangle” module, but does not connect it to the “is a right triangle” module because they don’t see any statement in the surrounding text that it is a right triangle. The student, on the other hand, fallaciously makes the connection to “is a right triangle” directly.
In some sense, a student who does not make that connection directly is already a mathematician.
A triangle also exists as a mathematical object in your and my brain. It is described by a formal mathematical definition. The pictures of triangles you see above do not fit this definition. For one thing, the line segments in the pictures have thickness. But the pictures trigger a reaction in your neurons that causes your brain to cognitively paste together the line segments in the drawing to the segments required by the formal definition. This is a kind of metaphor of concrete-to-abstract that connects drawings to math objects that mathematicians use all the time.
Note that this “concrete-to-abstract metaphor” itself has a physical existence in your brain. It drops, for example, the property of thickness that the line segments in the drawing have when matching them (in the metaphor) with the line segments in the corresponding abstract triangle. On the other hand, it preserves the sense the all three angles in the triangle are acute. The abstract mathematical concept of triangle (the generic triangle) has no requirement on the angles except that they add up to pi.
The discussions above describe a few of the complex and subtle relationships that exist between
- Mental representations of math objects
- Physical representations of math objects
- Formally defined math objects and their formally defined representations.
I have purported to discuss how mathematics is understood (especially in connection with language) in several articles and a book but only a few of the relationships I just described are mentioned in any of those articles. Perhaps one or two things I said caused you to react: “Actually, that’s obviously true but I never thought of it before”. (Much the way I had mathematicians in the ’60′s tell me, “I see what you mean that addition is a function of two variables, but I never thought of it that way before”.) (I was a brash category theorist wannabe then.)
A lot of research has been done on understanding math, and some research has been done on mathematical discourse. But what has been done has merely exposed the fin of the shark.
 Images and metaphors (in abstractmath).
 Representations and Models (in abstractmath).
 Mathematical Concepts (previous blog).
 Mental Representations in Math (previous blog).