Clairvoyance

Clairvoyance is an interesting consequence of the conflict in reference frames between the physical and celestial realities. The former is bound by relativity, while the latter uses an absolute reference frame. As a result, the Celestial mind of an individual exists in a world where everything happens very briefly before it does in their physical mind (and technically, before it happens from their perspective). A clairvoyant thinks with the reference frame of their soul instead of their neural impulses, allowing them to act 'before' events have happened.
Most spirits and all demons are clairvoyant. Surprisingly, ghouls aren't clairvoyant: in fact, it's impossible for an ordinary ghoul to become clairvoyant. This has to do with the fact that all of a ghoul's internal functions work off Celestial flubbing in the first place. A ghoul's existence fundamentally relies on detachment for reality- the exact opposite of what is required for clairvoyance. If a ghoul were to have a perfectly functional physical body (if they learned about the body and practiced fleshcraft it for decades or something), it'd be possible to obtain clairvoyance without also collapsing into a pile of flesh. However, the ghoul sense of smell is almost clairvoyance. It's much weaker and only detects extreme Celestial distortions, unlike true clairvoyance, which detects essentially all Celestial phenomenon relevant to the user. Since there's no reaction time buff to accompany it, there's no functional benefit to gaining the information infinitesimally sooner.

Mechanism of perception

Clairvoyance functions because every action in the physical world leaves a slight imprint in the Celestial, a force known as Celestial Weight. Actions taken by souls are especially visible (see The Soul-Force Effect ). As a result, Chaotic phenomena (including their aftereffects, assuming no further Celestial input) are exempt from clairvoyance, as would any theoretical action taken in a Celestial vacuum (though obviously having a person in the vacuum renders it no longer one by definition).
Overall, Clairvoyance is essentially a passive version of the Titan soul effect. The latter also uses flubbing to shift things to the bearer's subconscious, while clairvoyance only creates warnings for the user to react to. As a result, all Titans can be clairvoyant quite easily with some self analysis. Thing is, many Titans exist in a state closer to dreaming than any sort of reasonable lucidity, primal titans especially.

Celestial Defense

An interesting side-effect of clairvoyance is that it translates certain Celestial forces that would ordinarily act on the Mind-Soul Interface into physical ones. Combined with the clairvoyant ability to instantly react, this essentially allows most incoming Celestial forces to be completely neutralized with the equivalent expenditure of Will. Even without taking that second step, the natural physical force conversion renders clairvoyants immune to many typical forms of Celestial attack- especially those which rely on metaphorical sharp edges to compensate for low force; the conversion process disperses this across the entire body, spreading the minimal force into complete negligibility.

In Humans

(mostly deals with archmages in particular because they're the most common case)
Clairvoyance is considered 'true' hallmark of an archmage of The Sunstone Citadels: the ability to instantaneously react. Most more powerful individuals have very low reaction times: generally in the 0.05-0.1s range. However, archmages can react instantly, or even before that. In humans, this can usually be split into two stages: focus, and true clairvoyance.

Focus

Fundamentally, this completely surrenders the neural system to the soul. This allows the individual to 'think' on a higher reality level, bypassing standard processing times that a biological brain would have to handle, allowing for functionally instantaneous reaction times. It also roughly doubles the efficiency of spellweaving, as one no longer translates from Celestial (thought) to Physical (neural impulse) to Celestial (soul manipulation) to Physical (Hard Light)- instead only going from Celestial to Physical once.
While focus allows perception, it doesn't allow one to meaningfully make use of this information. It's the act of thinking with the soul, but still treating it as if it were a physical brain.

Clairvoyance

One step further lies true clairvoyance. Souls exist in a world in an absolute reference frame, while the physical world is bound by relativity. A soul can perceive something the moment that it happens in the absolute reference frame: a tiny fraction of a second before it strictly happens in the physical world from the body's perspective. On its own, this fact wouldn't mean much: the physical body can't really do anything meaningful in 0.00000001s of extra time. However, because a prerequesite of clairvoyance is thought entirely through soul, something can be made of it. The Celestial plane functions less on linear time and more on sequential actions.
Consequently, because the event was perceived before at all, no matter how shortly before, the sequence of actions changes. This generally allows the archmage to get one simple spell in 'before' they should realistically be aware of something happening: this is both useful in combat (for obvious reasons) and experimentation. In the latter, it's useful as a way to perform much riskier tests, as one can instantly end a reaction before it technically starts to be a danger or compensate for a hole in an algorithm runtime without rewriting and recompiling the whole thing.
The limitation of this prescient cast is that it has to be executed in a single thought: no incantations, preparation, or iterative layers. It's the exact opposite of what mages learn to do leading up to this level of ability: before, they learn to break problems down into simple steps and minimize error by taking their time. Clairvoyance spells are the instant projections that apprentices learn before they understand the underlying math well enough to dissect the problem.

Becoming Clairvoyant

Obtaining the first of these traits is most typically accomplished through what is theorized to be perfect synchronization of the soul. MSI is strengthened by emotion, so maximize emotion to maximize MSI, or so the theory goes. Seems to hold up: many gain it after slaying a great beast, winning a duel, losing a loved one, or the birth of a child. Higher soul flow seems to make this easier, to the point that it seems impossible to occur without a certain baseline soul flow.
Once one has focus, clairvoyance is mostly a matter of introspection and re-organizing one's thought patterns to fully take advantage of soul-based thought. The exact difference between focus and clairvoyance is hard to perceive from the outside, but to the individual in question, the latter manifests very clearly as a sort of instinctive sense which the former lacks.

Temporary Clairvoyance

True clairvoyance is a permanent state of existence, but momentary focus can also be temporarily reached under certain circumstances. When immensely focused on a task, true masters can often achieve flashes of it, lasting a second or two. This can allow one to shift ever so slightly to achieve a better result, whether in crafting an artwork or fighting a battle.