A different way of disagreeing would be this: examples are often what make a clear explantion clear.

http://www.wisdom.weizmann.ac.il/~oded/etc.txt

(i) a,b in F then a+b in F
(ii)…etc

Whether this is put first or second is perhaps neither here nor there; the central thing, from my perspective, is to emphasize that we care about fields because they clarify a lot of things about N, Z, Q, and C (and perhaps even more to the point, they distinguish Z_n from Z_p, both of which can be defined without a recourse to abstraction). This may seem like an obvious statement, but I think a good deal of university mathematics is unfortunately written from the perpsective that this would ‘dumb-down’ the material.

So I can’t agree with the sentiment that “This is University, playtime’s over, it’s time for some real work.” In my personal experience I have been able to learn math much more quickly (and deeply) if I view it as play, rather than ‘real work.’ In the former instance, the subject itself motivates me; in the second, it’s not the math, but just a desire to impress people or just do what’s expected. And there are easier ways to do these thing than to learn about tensor products, etc. From an even broader viewpoint, surely less sterile math will be developed if people learn in their undergraduate days to pursue abstractions because they are useful and clarifying, rather than pursue the abstractions because someone of authority has written them down.

Tim Gowers has two very interesting posts on using examples early on in a mathematical exposition of a subject. I can only second that and say that this is my favorite way of understanding mathematical concepts: Try to think through the simplest non-…

Although my answer to the second question is very often yes, it’s really my answer to the first question that I’d like to persuade people of. I may even end up writing “Examples first III” to deal with it at greater length, but a brief summary of my view is this. It depends on a distinction between “direct” memory (the sort you’d need to remember digits of , say) and “derived” memory (the sort you’d need to reconstruct a proof of a theorem from one key idea using your years of mathematical training). One of the difficulties of reading mathematics linearly is that you sometimes have to rely on direct memory because you don’t yet have anything from which to derive a memory. This would be true if a proof had steps that appeared to be arbitrary, or a complicated lemma was stated and you didn’t know how it was going to be applied, or a definition was given that turned out later to be exactly what you needed. Putting examples (and other kinds of motivation) first is a way of reducing the reliance of the reader on direct memory, since it sometimes makes it possible to derive the memory instead. (E.g., in the fields example, one could derive the list of axioms, or at least get a long way towards doing so, from the simple idea that they are the properties that hold of addition and multiplication in the rationals.) It’s this factual point that I’m mainly trying to push. (By “factual” I mean that it makes a statement about the world that could be true or false.) The normative point (this is how we ought to present mathematics) is secondary — of course, it’s no secret what my views are here too, but I don’t hold them in a rigid way and can think of situations where presenting examples first would not be helpful.

I think people should be especially skeptical about transforming their tastes into strong principles.

I second this. As one much wiser than I has said, “It is my firm belief that it is a mistake to hold firm beliefs.”

Regarding ‘fields’ I like the way Amitsur do it basing the definition of a field on trying to examine properties of the real numbers; I see no problem in mentioning Q and C as “positive” examples and N and Z as “negative” examples before giving the formal definition provided the students already know these examples. I am not sure it makes a big difference. If the main objective is to define “fields” and the students need an introduction to the complex numbers I would not suggest to give this example first but to wait after fields are defined.

Actually my own style/taste of teaching is in the direction of examples-first plenty of preliminary chat, stories, philosophy, non linear development and plenty apropos and even dubious humor. It is perhaps a good pedagogical principle that people should follow, within reason, their taste and style. But I think people should be especially skeptical about transforming their tastes into strong principles.

Apropos Amitsur, while I do not remember him giving much chat and non linear stories at class I do remember we had many chats in our faculty club (Belgium house) and we even tried once to work on a problem: Take a non Papussian projective space (3-dimensional), say a projective space
over the Quaternion. Is it possible to find there seven pairs of lines e1, e2, … e7 ; f1 f2 …f7 so that ei is disjoint from fi for every i and ei intersects fj whenever i and j are different. Such a configuration is impossible for a Papussian projective space (over a field).
As far as I know, this problem is still open.