# FOM: Arbitrary Objects

Thu Feb 7 15:49:48 EST 2002

```It was said a lot on "arbitrary objects", often very interesting.
Of course, we could exercise in this topic a long time, playing
with epsilon symbol or with some game theoretical explanations,

First, why not to give the simplest and most straightforward
explanation of what every working mathematician seems
understand very well? Fortunately, Andrzej Derdzinski
already gave that explanation which I would like to support.

He wrote:

> Thus, "arbitrary" is a concept really appearing only in proofs: suppose that a
> mathematician/logician is proving a statement of the form  Ax P, where  P  is
> a fixed formula (in cases of interest, containing  x  as a free variable). In
> the course of the proof, the mathematician/logician tells himself/herself "Let
> x  be an arbitrary object" and proceeds to use that assumption to derive  P.
> What `arbitrary' means here is entirely subjective, and the intuition behind
> it may very from one person to another.

Let me only illustrate this: Saying "let x be an arbitrary object",
say, a triangle, or a natural number, a mathematician usually just
draws some CONCRETE triangle (say, with the sides 3,5,7) and gives
it a name, say, ABC. ("Press any key" ang give it a name k.) Then
he/she presents a proof ON THIS CONCRETE TRIANGLE, but such a proof
that actually DOES NOT DEPEND on the concrete form of this ABC.
If we consider an arbitrary natural number, we could do the same
thing: take some concrete one, say, 5377540941, denote it by n and
forget which exactly was this number. Work only with its name n!

> Note that this last property of  x  may be viewed as an "objective meaning" of
> an arbitrary object: something about which we have made no assumptions.

Exactly!

What I would like to add, is a comment to Charles Silver:

>     It seems that mathematicians do not to want to scrutinize exactly what
> they're doing when they say "let z be...", where, even if it's not
> explicitly mentioned, z is intended to be "arbitrary".   They just reason
> with arbitrary objects as a matter of course.

First, I strongly believe, mathematics is a formal science always
having, nevertheless, an informal meaning (contrary to what
many thinkers say on formalists view to mathematics; who are
those formalists personally? Hilbert? or some imaginary
mathematicians?). As it is a formal science, it is SOMETIMES
useless to ask some questions on its formalisms. They work
well, accelerating and mechanizing our thought. Just vice versa,
it is very useful for working mathematician to not ask what does
it mean "let x  be an arbitrary object". It is a part of a formal
rule (preliminary well understood, at least implicitly, by a lot
of training). Just use it like any other mechanism. Say, when
riding bicycle it is even dangerous to think simultaneously which
way are you doing that. You can fall down instead of riding
quickly (feeling intuitively how to use this wonderful
mechanism, exactly as in mathematics)!

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