There are three sorts of spren that are talked about. There's the Spren that Hesina (Kaladin's Mom) talks about, there's the spren that appear with certain cognitive/physical triggers, and then there's the Nahel Bond Spren that "bond" to receive intelligence from a human, and the human receives the ability to process Stormlight.
“Mother, look at this,” Tien said. Late afternoon sunlight streamed through the leeside window, bathing the table.“From this side, the rock sparkles red, but from the other side, it’s green.”
“Perhaps it’s magical,” Hesina said. Chunk after chunk of longroot plunked into thewater, each with a slightly different note to the splash.
“I think it must be,” Tien said. “Or it has a spren. Do spren live in rocks?”
“Spren live in everything,” Hesina replied.
“They can’t live in everything,” Kal said, dropping a peel into the pail athis feet. He glanced out the window, watching the road that led from the town to the citylord’s mansion.
“They do,” Hesina said. “Spren appear when something changes--when fear appears, orwhen it begins to rain. They are the heart of change, and therefore the heart of all things.”
“This longroot,” Kal said, holding it up skeptically.
“Has a spren.”
“And if you slice it up?”
“Each bit has a spren. Only smaller.”
Kal frowned, looking over the long tuber. They grew in cracks in the stone wherewater collected. They tasted faintly of minerals, but were easy to grow. His family needed food that didn’t cost much, these days.
“So we eat spren,” Kal said flatly.
“No,”she said, “we eat the roots.”
“When we have to,” Tien added with a grimace.
“And the spren?” Kal pressed.
“They are freed. To return to wherever it is that spren live.”
“Do I have a spren?” Tien said, looking down at his chest.
“You have a soul, dear. You’re a person. But the pieces of your body very well may have spren living in them. Very small ones.”
Tien pinched at his skin, as if trying to pry the tiny spren out.
These Spren that Hesina describes (probably in layman's terms), seem to be similar to what Shallan sees when she travels to Shadesmar, the Cognitive Realm.
The bed, the nightstand, her sketchpad, the walls, the ceiling--everything seemed to pop, forming into tiny, dark glass spheres. She found herself in aplace with a black sky and a strange, small white sun that hung on the horizon,too far away.
Shallan screamed as she found herself in midair, falling backward in a shower of beads. Flames hovered nearby, dozens of them, perhaps hundreds. Like the tips of candles floating inthe air and moving in the wind.
She hit something. An endless darksea, except it wasn’t wet. It was made of the small beads, an entire ocean oftiny glass spheres. They surged around her, moving in an undulating swell. She gasped, flailing, trying to stay afloat.
You want me to change? a warm voice said in her mind, distinct and different from the cold whisper she hadheard earlier. It was deep and hollow and conveyed a sense of great age. Itseemed to come from her hand, and she realized she was grasping something there. One of the beads.
The movement of the ocean of glass threatened to tow her down; she kicked frantically, somehow managing to stay afloat.
I’ve been as I am for a great long time, the warm voice said. I sleep so much. I will change. Give me what you have.
“I don’t know what you mean! Please, help me!”
I will change.
She felt suddenly cold, as if the warmth were being drawn from her. She screamed as the bead in her fingers flared to sudden warmth. She dropped it just as a shift in the ocean swelltowed her under, beads rolling over one another with a soft clatter.
She fell back and hit her bed, back in her room. Beside her, the goblet on her nightstand melted, the glass becoming red liquid, dropping the three spheres inside of it to the top of thenightstand. The red liquid poured over the sides of the nightstand, splashingto the floor. Shallan pulled back, horrified.
The goblet had been changed into blood.
The spheres that Shallan sees are the spren that Hesina describes- the cognitive aspect of all things. In fact, Brandon has talked about these aspects before.
What is the X in Aon Mea? Is it one of the Shard-pools?
Afraid not. Aon Mea references the expanded region within which the "Elantris Effect" will create Elantrians. The X is fertile valley with a high density of life, a place with a lot of cognitive activity. (Cognitive as defined by Realmatic Theory includes the 'thoughts' of all things that exist, not just human beings. The more complex the life form, the stronger its presence on the Cognitive Realm.)
The flames that Shallan sees are the objects (human and spren) that have a presence in the Cognitive Realm, or, in other words, are intelligent.
Spren,however, could be very elusive. Sometimes, even the most commontypes--flamespren, for instance--would refuse to appear. That made it particularly frustrating for a man who had made it his life’s work to observe, catalogue, and study every single type of spren in Roshar.
I believe (and after digging through the text and not really finding anything), that Everyday Spren are things that appear when certain cognitive or physical triggers occur. This can vary, from purely physical things, such as flamespren, which are thus more common and predictable, to more cognitive things, such as confidence spren. We see that Syl (probably) believes this about every day spren (as well as herself).
"Bindspren,” Syl said, walking upbeside his head; she was still standing in the air.
“They’re holding the rock in place.”
“Maybe. Or maybe they’re attracted to what you’ve done in affixing the stone there.”
“That’s not how it works. Is it?”
“Do rotspren cause sickness,” Syl said idly, “or are they attracted to it?”
“Everyone knows they cause it.”
“And do windspren cause the wind? Rainspren cause the rain? Flamespren cause fires?”
These Everyday Spren can be captured and utilized, if Navani's notes are right, if they are put in the proper gemstones and fed Stormlight, creating what are called fabrials.
The cut and type of the gem determines what kind of spren are attracted to it and can be imprisoned in it. There must be thousands of possible combinations. Once a spren is captured and the gem infused with Stormlight the fabrial can be used in machines.
Another thing- these Spren, require Stormlight. For example, in Shinovar, where there are no Highstorms (and thus no Stormlight), no Spren are noticed.
Nahel Spren are a subcategory of Everyday Spren. They have purely cognitive triggers. These include Honorspren, like Syl, who was attracted to Kaladin because of he "Protected those who couldn't protect themselves" and thus acted honorably.
See this quote, from when Kaladin almost committed suicide:
“Why?” he pled. “Why do you care?”
“Because I do,” she said, cocking her head. “I watchedyou, you know. Back in that army. You’d always find the young, untrained menand protect them, even though it put you into danger. I can remember. Justbarely, but I do.”
“I failed them. They’re dead now.”
“They would have died more quickly without you. You madeit so they had a family in the army. I remember their gratitude. It’s what drewme in the first place. You helped them.” A note here- it's not the gratitude that drew her. It's the fact that he helped them. The young, untrained who couldn't protect themselves. In other words, Syl was attracted to Kaladin because he was already following the Second Ideal of the Knight Radiants.
They also include Symbolspren/Truthspren (I personally prefer the second, because it describes when it appears, rather than a description of the spren), who give Shallan an experience to interact with Shadesmar, the Cognitive Realm whenever she says a truth about herself.
Creatures, she said in herhead. Can you hear me?
Yes, always, a whisper came in response. Though she’d hoped to hear it, shestill jumped.
Can you return me to that place? she asked.
You need to tell me something true, it replied. The more true, the stronger our bond.
Jasnah is using a fake soulcaster, Shallan thought. I’m sure that’s a truth.
That’s not enough, the voice whispered. I must know something true about you.Tell me. The stronger the truth, the more hidden it is, the more powerful thebond. Tell me. Tell me. What are you?
“What am I?” Shallan whispered. “Truthfully?” It was a day for confrontation. Shefelt strangely strong, steady. Time to speak it. “I’m a murderer. I killed my father.”
Ah,the voice whispered. A powerful truth indeed. . . .
A subnote- The Nahel Bond. This bond is two part. In exchange for helping the Spren gain intelligence, the Human gains the ability to process stormlight- in being able to manipulate the Surges of Pressure and Gravity for Kaladin, and the Surge of Transformation (and something else?) for Shallan.
We see the give and take of the bond in this quote from Syl:
“I’m behind what is happening to you,” she said, voice soft. “I’m doing it.”
Kaladin frowned, stepping forward.
“It’s both of us,” she said. “But without me, nothing would be changing in you. I’m . . . taking something from you. And giving something in return. It’s the way it used towork, though I can’t remember how or when. I just know that it was.”
“Hush,”she said. “I’m talking.”
“I’m willing to stop it, if you want,” she said. “But I would go back to being as I was before. That scares me. Floating on the wind, never remembering anything for longer than a few minutes. It’s because of this tie between us that I can think again, that I can remember what and who I am. If we end it, I lose that.” And I believe that it's clear, throughout the course of the book, that Syl is the one that is giving Kaladin his abilities.
Another note- the Trigger of a spren, and the ability it allows its Human to have aren't unrelated. When the Truthspren talk to Shallan the first two times, they ask not for a truth, but for her identity. They ask "What are you?" when she does Soulcast the goblet, my belief is that she helps the cognitive self of the goblet to become something else than what it is.
So this still leaves a bunch of questions left unanswered. Why do the spren only appear sometimes? What about individual spren, like the one Axial the Collector collects? And why is Shallan able to feel the Truthsprenfor a brief moment?
If there's something I missed, or something you think doesn't make sense, let me know.
Ah! I knew I forgot a quote.
“Our own natures destroy us,” the regal man said, voice soft, though his face was angry. “Alakavish was a Surgebinder. He should have known better. And yet, the Nahel bond gave him no more wisdom than a regular man. Alas, not all spren are as discerning as Honorspren.”
This quote is very important.
First off, it says that Alakavish was a Surgebinders, where Surgebinders end up being Knight Radiants.
Second, we can assume from this that at least some Surgebinders had Nahel Bond Spren.
From this quote from Jasnah:
. Suffice it to say that every Radiant’s abilities were tied to the spren.”We can amend that to all Surgebinders having Spren (presumably Nahel Bond Spren).
Third: Not all Nahel Bond Spren are Honorspren. There's a difference, otherwise Nohaden wouldn't make the distinction.
From this, I think we can say that since not all Surgebinder spren are Honorspren, and thus the Symbolspren that Shallan sees are (probably) not Honorspren.