How are Seons off of Sel?

[JustQuestin2004 he/him]

Somehow, up until this moment I never questioned how Seons, Splinters of Devotion, could leave Sel when most other Splinters like Spren cannot.

There doesn't seem to be any special method, since if there was it would probably work on Spren and yet we know that no Spren has managed to leave Roshar as of ROW.

So it must be innate so Seons, but why? What makes them so different that they can break the rule of Splinters being unable to leave the planet they came from?

[Treamayne]

43 minutes ago, JustQuestin2004 said:

Somehow, up until this moment I never questioned how Seons, Splinters of Devotion, could leave Sel when most other Splinters like Spren cannot.

There doesn't seem to be any special method, since if there was it would probably work on Spren and yet we know that no Spren has managed to leave Roshar as of ROW.

So it must be innate so Seons, but why? What makes them so different that they can break the rule of Splinters being unable to leave the planet they came from?

The WoB we have on this is not definitive, but does have some implications:

Spoiler

 

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Weiry Writer

Why are Seons stuck as floating balls of light in the Cognitive Realm, while true spren get to be people shaped? I feel that is unfair.

Brandon Sanderson

That is a little unfair isn't it. Boy, Sel has all sorts of unfair things going on. But they can leave Sel, so there's some sort of clue there. But lots of unfairness to everybody from Sel in various ways. But the thing that is not unfair is: they're able to get places.

Chaos

Just gotta escape the deadly plasma, you know. No big deal.

Brandon Sanderson

If they can escape the deadly Investiture plasma covering the Cognitive Realm... not even covering, like... suffusing the Cognitive Realm where they are, the Expanse of Densities. If they can escape that they can go places. 

I'll be honest. I couldn't decided if I should use a seon there at the end or a Tamu Kek. In one draft it was a Tamu Kek, and then I thought, "Eh, seons are way more interesting because they have volition. Tamu Kek is just a bone." It was a Tamu Kek originally, and then it was a seon, then back to a Tamu Kek, and then I released it as a seon. 

Shardcast Interview (Jan. 23, 2021)

 

Further, comparing Spren and Seons:

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Questioner

What's the relationship between spren and seons?

Brandon Sanderson

They are the same thing from different magics.

Questioner

Okay, exact same thing from different magics?

Brandon Sanderson

"Exact," may be a little too strict, but yeah.

Questioner

So a seon can be transferred to another person, right?

Brandon Sanderson

Yes.

Questioner

Can a spren?

Brandon Sanderson

Yes.

Skyward Houston signing (Nov. 19, 2018)

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Questioner

[Spren] are used to Invest a lot of things, fabrials, they turn into Shardblades. Are we ever going to see something similar with the seons, from Sel?

Brandon Sanderson

Yeah, they are basically the same thing. They're a little more formalized, a little more structured but they are basically the exact same thing so yes. Some of the things you see spren doing seons are capable of, some things that seons do spren will be capable of.

Shadows of Self release party (Oct. 5, 2015)

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DoritoJH

We know that there are spren that are partially of Honor, partially of Cultivation, and Odium. Can there be spren made of any combination of Shards?

Brandon Sanderson

Yes. Well, you would have to call them... Under that definition if you call a seon a spren, then yes. If you don't call a seon a spren, if you define a spren as, "On Roshar, related to the natural world of Roshar," then no. Theoretically yes, but it wouldn't really work. But it depends on how you define spren. If a Shard were to come and reside on Roshar like the other ones have, then you could theoretically see other new spren appearing out of them.

DoritoJH

Could there be a spren of all 16 Shards combined all at once?

Brandon Sanderson

*hands out RAFO card*

DragonCon 2019 (Aug. 31, 2019)

 

While Brandon does say they can "go places," he does not say why. Based on the way he says "escape the Expanse of Densities" implies three primary differences, with either or both possibly being the reason they can "go places." It is unclear if any of these factor, or (possibly) all three together are what allows them to move about.

1. Radiant Spren are of two shards, Honor and Cultivation, only one of which is splintered - while Seons are Devotioin only (and Skaze Dominion) which is Splintered
2. Because the base Investiture is different (Devotion) it could be the nature of their bond to "Follow their bonded mortal" - which is an aspect that Honor and Cultivation's Intent would not share
3. Seon's are wholly in the Physical Realm because of how Odium squished the remnants of Devotion and Dominion into the Cognitive - whereas Spren are mostly into the Cognitive, and the Bond pulls them slightly into the Physical (with increased Oaths pulling them more firmly into the Physical)
   • Which implies that if you can get a Seon through a Perpendicularity (protected from the Dor Plasma) and escape the Sel Shadesmar, then they can Worldhop because they are not split between two realms at once (it's closer to how a person Worldhops).
     • (this is my bet as most likely, if only one reason applies)

TLDR: We don't really know, but there are some valid lines for theorizing on which, if any, known differences allow Seons to do what they do.