What are numbers?

Question

The title is a bit of clickbait, but I think it's justified.

How did I came to ask this question

In programming, many programming languages have concepts of a hierarchy of numerical types. Often times there would be a numerical type to serve as a supertype of integers, reals, rationals and whatever other kind of numbers that particular language happens to have. This supertype is often called just the number, the properties it has are typically entirely at the mercy of the language designers (i.e. they seem to be arbitrary or motivated by language implementation).

At first, I tried to imagine what would be the next possible step in the hierarchy of familiar numerical types: complex <: reals <: rationals <: integers <: natural numbers, but nothing comes to mind. More so, in almost no programming language it is the case that, for example, an integer is a kind of rational or a rational is a kind of integer. But they often share some properties of the number type I mentioned earlier.

The question proper

Searching for different kinds of numbers in mathematics, I came to realize that there are a great many of mathematical objects eventually called "numbers" (of some kind). For example, p-ary numbers, half-integers, infinitesimals, and really lots, lots more. The common properties I could extract from the description are that these objects are typically equipped with two binary operations (from field axioms) and order relation, although sums and products wildly vary in their behaviour. Other properties may eventually appear or disappear, depending on the particularities of the numeric type in question. But this cannot be enough to select only numbers! There are other fields which have all the same properties. Then how do I distinguish numbers from non-numbers?

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The answer I came up with so far

So far, the only "definition" I could think of is that numbers are such sets, where properties of set elements are immaterial (as in urelements). This will, however, classify some groups, rings and fields, which aren't usually called "numbers" as numbers, but will definitely exclude fields of matrices, computable functions, polynomials etc. Is this a problematic answer? Is it outright wrong?

Answer 1

There's a very famous discussion in Wittgenstein's Philosophical Investigations where he talks of what have become known as "family resemblance concepts".

In §66, he takes the example of games, and writes

consider for example the proceedings that we call "games"... look and see whether there is anything common to all.

He considers various examples and points out their similarities and differences, and concludes

The result of this examination is: we see a complicated network of similarities overlapping and criss-crossing: sometimes overall similarities. ... I can think of no better expression to characterize these similarities than "family resemblances"; for the various resemblances between members of a family: build, features, colour of eyes, gait, temperament, etc. etc. overlap and criss-cross in the same way. – And I shall say: "games" form a family.

Wittgenstein goes on to suggest that many concepts are like this: for many concepts X it is a mistake to look for something in common to all and only the Xs. Rather the Xs exhibit a family resemblance (but need share no one distinctive trait in common).

He goes on to give another example -- and this is why the discussion is relevant to the question! -- namely the concept of a number.

... the kinds of number form a family in the same way. Why do we call something a "number"? Well, perhaps because it has a direct relationship with several things that have hitherto been called number; and this can be said to give it an indirect relationship to other things we call the same name. And we extend our concept of number as in spinning a thread we twist fibre on fibre. And the strength of the thread does not reside in the fact that some one fibre runs through its whole length, but in the overlapping of many fibres.

Now, if I want, for this or that purpose,

I can give the concept 'number' rigid limits ... that is, use the word "number" for a rigidly limited concept,

But Wittgenstein goes on to note that I can (and very often do) use a concept so

the extension of the concept is not closed by a frontier. And this is how we do use the word "game". For how is the concept of a game bounded? What still counts as a game and what no longer does? Can you give the boundary? No. You can draw one; for none has so far been drawn. (But that never troubled you before when you used the word "game".)

And similarly for "number": informally, we can and do stretch the concept in various ways in new applications. And there need be no one thread in common to all these applications: it is good enough that our usage has enough family resemblance to prior uses.

Moral: we can come up with restrictive technical definitions of "number" for this or that technical purpose -- but it is misguided to look for a one-off neat definition of "number" in all its mathematical uses. In its informal general use, it is a somewhat messy family resemblance concept.