Why does the bible forbid mixed cloth?

Camel hair, goat, and maybe cotton. Leather. Furs. *Maybe *hemp.

Or a third:

  1. The word “shatnez” had a meaning, something like “adulterated” or “mixture”. Thus, the first verse gave the rule in a general way, and the second clarified exactly what was meant.

Similar to how a modern law might first talk about a tax on “income”, and later on it would clarify what counts as income and what doesn’t.

It’s because some (or more likely, a lot of) people were mixing wool and linen fibers, and advertising and selling the cloth as the more expensive linen. In other words, they were lying to and cheating their customers, and it’s not Biblical to do that.

So are there rabbis that go around to textile mills and certify that their cloth is <Hebrew word for not shatnez>? Is there a special symbol that <not shatnez> cloth has?

If I remember correctly, this is done in Israel, and the certification appears on one of the various tags that the manufacturer attaches. Outside Israel, there is a network of laboratories which will take a few sample threads from various parts (linings are the most frequent problem) and then examine them with microscopes and chemical tests. These labs also keep the public informed aboutwhat types of clothing, and which manufacturers are more and less problematic. (Simple cheap stuff like underwear and shirts are no problem at all. Generally, only fancy stuff like a suit would have a reason to use linen. But a big problem in recent years has been mixtures of recycled fibers, where even the manufacturer doesn’t know what’s in it.)

I
Yes, for this thread, in two ways that have been mentioned (perhaps I am not disagreeing with you, but am simply fleshing out your correct observation):

in the meta-sense of the discussion of many of the Jewish commandments (the translation of mitzvot)–all 613 of them, one of which is the OP, another is kashrut/“keeping kosher” (and everyone knows the Big Ten, so that leaves another 601 for the religious Jew, which certainly keeps him busy if nothing else)–in two areas:

  1. within the consensus of normative Talmud thinking about the legal classification of the mitzvot, taught in yeshivas for nearly two millennia (although I’m sure the de elopement of the concept among earlier and later commentators can be traced); and

  2. within many of the commentators in this thread of the secular SDGQ community,and of anthropologists and sociologists from different angles

which is, for 1) because God said so, and for 2) because “someone said so/it is so ordained.” In neither case, at this level, is it implied that the discussion is fruitless–of course, considering the case of 1), discussion of Torah is never fruitless and is constantly encouraged. (Speaking of mitzvot, and of meta, I can’t remember if that is also one.)

II
Yes, as above, both of which are cited in this thread, for both thought communities , as above the 1) the strictly religious, “internal Talmudic” discussion, as well as the secular mode of 2) above, about the idea of separation of categories as an element of holiness. Which bears comparison with an alternate-reality Aristotle–a notion, unsurprisingly, of great interest to scholastic Jewish philosophers of the Middle Ages.

III
No, as mentioned in the definition of the mitzvot above, kashrut is a separate mitzvah.
IV
Another reason, not Leo’s but I can’t think of any cites within 1) or 2) groups, and I am sure exist at length, is to attempt to settle, once and for all, at least economically in one of the chief products (a: physical–essential to the consumer–clothing-- and to the economic health of the community and b: the “spiritual product” of the peace of the community): why can’t the farmer and rancher be friends?

Oh the farmer and the cowman should be friends!
Oh the farmer and the cowman should be friends!
One man likes to pull a plow!
One man likes to milk a cow!
But that’s no reason why they can’t be friends!

One more reason for me to never become a rabbi.

If the Architect of All The Universe wants me to do something, s/he can make the minimal effort of giving a good, rational reason.
(“Because I said so!” sounds like a bully or a Bad Gym Teacher.)
Until then, I’ll be over here, following the commandments that make a lick of sense
(so I won’t be killing anyone, but I just might be coveting my neighbor’s ox…).

Well, maybe. It could mean that God is just making up laws willy-nilly, just for the sake of forcing us to follow them. Or it could mean that God has perfectly good and rational reasons for those laws, and that we should take His word that the reasons are good ones, since we don’t know them ourselves (and possibly can’t even know them: God after all has far greater knowledge and understanding than we do, so maybe there’s a perfectly good reason which is simply beyond human comprehension).

Two good reasons for “because” as an answer:

  1. The book of rules would be huge–explaining every single guideline would take volumes. Even after that, there would still be folks trying to nitpick and game the system.
  2. Military commanders don’t explain their orders to the lowest enlisted man–there are great strategic efforts going on and it is not practical to explain everything throughout the ranks. Indeed, there are many things that could not be explained satisfactorily, but still need to be done.

Like, over 6,200 pages.