Philosophical Sophistication as Mathematical Sophistication

21 06 2007

How much philosophical sophistication does someone need to count as being “mathematically sophisticated” enough to follow a graduate course in algebra?

When reading through a draft paper by Colin McLarty (addressing different issues entirely), I came upon the following passage from Serge Lang’s canonical text:

I think it illustrates a lot of issues that often arise in understanding mathematical writing.

In actuality, mathematicians almost never write the statement that Lang wrote, except in the same sort of definitional statement. In particular, in place of “f(x)” they would write some expression in terms of “x” that one might use, such as “x3+2x-1″ or the like. Because this expression is just a placeholder, we might expect some neutral term, like “t“. But instead he uses a term that gives the reader the idea of what the overall expression is supposed to mean, at the expense of some abuse of notation.

Another issue of use and mention at work here is what the term “x” to the left of the arrow is doing. He doesn’t say whether “x” is a placeholder for a term denoting a specific element of A, or whether it is a sort of meta-placeholder, representing a variable that itself takes values in A.

In practice, I believe that both options are allowed. By a minor abuse of notation, one can write either “Under function f, 3 \mapsto 9″, or “Under function f, x \mapsto x2“. (I’m using “\mapsto” to stand for the arrow used in Lang’s statement.) In particular, the latter type of statement derives from the former by the standard practice of ignoring certain types of use-mention distinction, and allowing variables to stand either for elements of A or the names of elements of A. This abuse is allowed just about everywhere except in some parts of model theory, where it’s important to distinguish objects and their names.

So getting back to my original point, I think that an ability to know when a term is being used or mentioned, and whether it’s standing for itself, an expression that is partly composed in the way that it’s composed (this might relate to Lang’s famous statement that “notation should be functorial over the language”), or something totally different is important. I suspect that a non-sophisticated math student (or a sophisticated philosopher) would read the statement Lang wrote and suspect that the arrow would never be useful, because we’d always have to specify in some other place what f(x) was (that is, what expression “f(x)” refers to).

One aspect of mathematical sophistication seems to rely being aware of these different levels on some subconscious level, so that you can always jump to the right one, even through multiple abuses of notation.

5 responses

22 06 2007

One can, following Dijkstra, call this a deficiency of notation rather than sophistication. Another example of that is right-to-left notation for function application f(g(h(x))) (and right-to-left notation for numerals) somewhat awkward for predominantly left-to-right writing culture.

The problem here is that knowledge, required to jumps through those levels is nowhere formalized, which means that the only way to acquire it, is through osmosis, making mathematical education more akin to medieval guild training process rather than to modern discipline.

24 06 2007

I think that wordpress is automatically set up to allow tex. So you could just do $3 \mapsto 9$ if you want. For details on the syntax see Ben’s explanation.

24 06 2007

Wonderful!

12 07 2007

Huh. I’m not good at philosophy, but let me try to understand this. I always interpreted “Under function f, $x \mapsto x^2$” to be part of a grammatical rule that lets us elide universal quantifiers when they are clear enough. So the full meaning of this sentence is “Under function f, for every $x \in A$, we have $x \mapsto x^2$.” I don’t see the use-mention issue that you are referring to.

12 07 2007

The use-mention issue here is in Lang’s actual statement – he’s using $f(x)$ to stand for some other expression, while he’s using $x$ and $\mapsto$ to stand for themselves. In some sense he’s describing schematically a bunch of statements that individually make sense, but he’s making some use-mention confusions in the description of the schema.