Maikrosian magic is, to a degree, well understood by the late Third Age. Such understanding is within the realm of the manipulation of magic. Of course, there is still more to understand, but the knowledge on the matter gathered over the millennia by Maikrosian mages is quite extensive. There are two widely recognized forms of magic - spark magic and formulaic magic. Spark magic is the sort of magic where the mage simply casts the spell, with no tools of any kind. This is achieved via thought, and there is a specialized neural organ among the structures of the brainstem that governs this connection between brain and iss+The Propho-Salenzian name for the energy used in the casting of magic.. It is present in all known species of vertebrate, explaining those various species thereof which employ some kind of magic.+While magical ability is known to exist in various invertebrates, the structures governing it are either unknown or wildly different from the vertebrate structure. However, just because a creature has this structure in their brain, doesn't mean they can cast magic.+Why have the structure if it has no apparent function? Current theory says that it provides the connection between the brain and the spirit, even that this is its primary function, thus that casting is a secondary function that evolved later - and due to the scattered appearance of the trait amongst species, many times. In most, it is usually determined by species: some species can cast magic,+almost always very specialized, for use typically in hunting, predator avoidance, or finding a mate while others cannot. In hominins it is determined on an individual level.+yes, some few chimps can do magic, but it's heavily selected against because chimp mages are innate casters, and without the natural magical specialization had by other non-sapient species thus tend to end up blowing themselves up or something
Those individuals of Homo who are capable of magic are known to have two or three ways to cast their magic. In the two-variant system, there are knowledgeable casters and innate casters. Basically, knowledgeable casters understand what they are doing, while innate casters do not. Knowledgeable casters need training and practice, while innate casters do not. With such a lack of training and practice, there are understandably few innate casters present in the world, because they tend to end up blowing themselves up or something. In the three-variant system, knowledgeable casters are further subdivided into studied and intuitive casters. Studied casters are by far the most common today. As the name suggests, they reach their knowledge via study, whether from a book or from another mage. Intuitive casters, on the other hand, have an intuitive grasp of their magic - they just get it. These days, intuitive casters are more common in non-literate cultures, where their intuition has more time to develop on its own - while in literate cultures, those who might become intuitive mages usually end up studying their magic like all the others.
The physics of magic, on the other hand, are not well understood at all. Most of what is known about the physics of magic is related to its manipulation. It is rather uniformly distributed across every landmass and body of water. Iss saturation of Maikros is known to go well below the deepest underground any person has ever been so far.+~1km The use of magic requires the mage to pull iss out of the ground or a body of water. Of course, the more iss, the more powerful the magic. When spent, the iss is no longer present in the surface, but gradually returns. As most spells are cast aboveground, this spent iss is speculated to linger in the air, though no spent iss has ever been conclusively detected.+The gradual replenishing of iss has much evidence recorded by mages from around the world for millennia, but no mage can detect spent iss the same way they can detect ready iss, and no instrument has been invented so far that could detect spent iss directly. Its eventual return into Maikros suggests that if the spent iss does descend from the air, then iss is affected by gravity.+Newtonian physics are well known, having been discovered by Crown scientists in Zarcos in the sixth century Third Age.
Furthermore, the larger the spell, the more iss spent, the longer it takes to return. There are various competing theories as to why iss does this, when - by all rights, unaffected by the air as it is - it should always fall back to Maikros at the same rate, if it is indeed being affected by gravity. The theory currently accepted by most mages is iss interference, which states that in its natural state, iss interferes with itself constantly, creating complex, interlocking structures. These structures, being mostly random, are usually completely harmless, but iss interference is also sometimes used to explain rare phenomena of sudden magical outbursts. However, almost all recorded instances of sudden magical outbursts were made by laypeople, many in earlier ages, before the scientific method. As such, the existence of sudden magical outbursts is debated.+They could, for instance, be the work of capricious mages, or unintentional casting by innate mages. Iss interference is also used to explain the existence of iss spirits, that occasionally, the random structures result in an intelligence that subsequently stabilizes itself. However, this too is sometimes debated.+Iss spirits are notoriously reticent, so mages have yet to learn if they all arise naturally from ambient groundiss, or if other iss spirits fashion new ones, or if something else might be the case. Of course, again, this all relies on iss being affected by gravity. If it is not, then there must be some other mechanism not only driving its return to the ground, but also governing the rate at which it does so.
With the knowledge that spent iss is unusable for a while in mind, it is a reasonable conclusion that iss is a finite, though renewable, resource. The fact that this finite resource is rendered unusable when cast and refreshes over time is called the Iss Cycle. This is a very important concept in magic, because it means that if one were to cast a large enough spell, the iss in the ground for many many kilometres could be spent, creating an area where magic simply no longer functions for many years. The iss spirits, sapient beings composed entirely of iss, have charged themselves the protectors of the Iss Cycle. There are many tales across the world of hubristic mages trying to cast enormous spells to only be stopped - usually lethally - by the iss spirits - both within recorded history and without. Scant little is otherwise known about the iss spirits, since they typically only interact with biological sapients when warning them against casting too large spells or physically stopping them from doing do.
On the other hand, the Underworld provides a source of infinite iss+of course, just because there is infinite iss available, doesn't mean any one mage is capable of channeling an infinite amount of iss - people get tired, after all that many mages on Salenzis and Toletska utilize today. However, the Underworld is not without its dangers - it is the home of darkness spirits,+that is, iss spirits of the element of darkness and they are quite malevolent.
Magic is commonly grouped by mages into schools, as shown below.