http://www.imperial-library.info/content/morrowind-arkay-enemy is clear:

"Arkay's Blessing prevents the souls of men, beastmen, and elves from being used without consent. Arkay's Law prevents those buried with the proper rituals from being raised to serve my children's will."

makes me want to ask: so how were black soul gems and all that in TES 4 justified by Lore if the above clearly, hm, pretty much "prevents" it?
even if taking into consideration the next two lines, i stil don't see how or why it is no longer "prevents" the soul trapping of man and mer :

"As you know, my children, Arkay's Blessing is flexible to those with daring, but Arkay's Law is unwavering"

explain, clear up, someone, please?

you can't re-animate a body that has undergone Arkay's rituals, but you can trap their souls if you manage to figure out a way to break the magic of Arkay (a fairly weak Et'Ada)

It has nothing to do with Arkay being a "weak" et'ada.

A blessing can be circumvented, a law can't (without fundamentally altering the theomythical nature of Aurbis). Black soul gems are one way of circumventing Arkay's blessing.

like a loophole. I mentioned Arkay's status closer to Mortals than most 'greater' Et'ada because it means it is easier to find – or make – loopholes. If he was a stronger Et'ada he wouldn't be eclipsed every seven days by a finite soul.

Even Akatosh - whose "perch from eternity" allowed the other et'ada to form - can be circumvented.

Every Aedroth (or at least that we've encountered, Zenithar or Stendarr hasn't really been seen even indirectly) has exceptions in their rule of their specific realm. Kynareth has Werewolves and Vampires, Akatosh has Dragon Breaks, Arkay has the GoW to circumvent his rule.

Err Arkay has vampires, necromancers, undead, liches and Mannimarco, and Kynnareth with werewolves.

I don't think that you actually trap the soul just the energy the body uses, or something like that. I mean you can "soul trap" reanimated corpses which is weird, but then again im not much of a lorebuff.

This begs the question of "What in blinkin' blazes is this silly soul thing you people keep rambling about?"

Goofy wording aside, I'm serious. What is a "soul" in TES?

N'Gasta was able to put up a soul snare around the island of Stros M'kai, where the only safe place to die was in the temple of Arkay, or having a priest to say the rites. That's sort of a huge subversion.

Lattice, a soul is an idea, and a mortal soul is a fragment of an idea, and thus it is a form of magic (magic is pure potential energy, and ideas have a lot of potential or something like that.) This is evidenced by Meridia – the Et'Ada of souls – being a subgradient of Magnus – the Et'ada of magic.

eh, so a loophole? OK... guess we got to have them. sure clears that up quick

A soul is a http://en.wikipedia.org/wiki/Platonic_form. It represents a primordial, elemental, that is to say non-simplifiable idea. It represents the baseness of that person. I guess you could say that everything would have its own Form with the more complex things having more animate Forms. A man or mer would have a greatly large, complex Form.

Daedric souls, on the other hand, are more malleable Form with more plasticity, since they are created from more original energy, closer to creation. Therefore, their animus can be dashed from their corporeal form without their soul being completely lost.