A cosmos is a set of planes which share similar physical laws and often a similar origin. More technically, the planes must share similar physical laws because of certain details of their origin (because one plane was created from the other, for instance, or because the two were created together), though in practice cases of different planes arriving completely independently at sufficiently similar physical laws is extremely rare, and the few cases that may exist are still disputed. Though the question of what qualifies as "similar" physical laws may seem to leave the definition very fuzzy, æalogists have rigorous criteria to decide the issue, and in most cases the matter is fairly cut and dried, though there are some exceptions.
Some writers prefer to capitalize the word "cosmos", but this usage is still not considered standard by the majority of authorities. Other words used for cosmoi by some æalogists are "omne" (pronounced /ˈɒmneɪ/; plural "omnia", /ˈɒmniə/) and "cosm" (pronounced /ˈkɒzəm/), the former from a Latin word meaning "everything" and the latter, of course, a shortened form of "cosmos".
There is no one simple means to create a cosmos; cosmoi are well beyond the scope of what a single individual can easily construct. Nevertheless, cosmoi are created, though through elaborate means that differ in each case. The means by which many cosmoi were brought into being are known, or at least suspected, through various means. The answers produceable by divination are limited, but other clues to the origins of cosmoi exist, one being so-called genesis memories. These refer to the fact that some powerful or unusual individuals claim to remember the creations of the cosmoi they inhabit, and in some cases their existence in another cosmos before that. Unfortunately, some genesis memories seem to contradict each other, so they are not considered an infallible source—memory, after all, is unreliable.
Though no single spell is known to exist on any world that is capable of creating a cosmos outright, it is possible, albeit very difficult, to do so with creative combination of magical effects. Generally such mighty magics are necessary that the involvement is required of such a potent source of power as a god or an eximium, though in rare cases cosmoi seem to have been created by unusually puissant or resourceful mortal mages without such aid. Some cosmoi have even been created technologically, with no magic, per se, involved. No standard recipe can be given for creating a cosmos, however, since no two cosmoi are known to have been created in much the same way, though it seems to be frequently the case that cosmoi are created parogonically.
If all known cosmoi were created from within another cosmos, this raises the question of which cosmos was first, the fount from which all other cosmoi ultimately sprang. Formerly, there were two main divisions in possible answers to that question. Some æalogists believed that some known cosmos did give rise, directly or indirectly, to all the others, and that all stories of its origin in another cosmos were nothing more than myths. (Usually, the æalogists who held this view championed their own cosmos as the leading candidate for the first.) Others thought that the Prime Cosmos was a cosmos yet undiscovered, and perhaps now (or always) uninhabited. The prevailing modern view, however, is that due to the principle of ontological relativity, any cosmos can be regarded with equal validity as the Prime Cosmos, rendering the question meaningless.
In any case, once a cosmos is created, it is still disputed whether it can ever be truly destroyed. Many æalogists believe that even in those cases when a cosmos seems to have been destroyed, in truth it has only been severed from other cosmoi, and continues to exist in isolation—though this doesn't seem to be readily provable. On the other hand, some æalogists believe that the cosmoi are sufficiently interconnected that if one were destroyed, they would all cease to be—though fortunately there seems to be little prospect of that ever happening. (The existence of alternate worlds muddies the waters here a bit; æalogists disagree as to how alternate cosmoi are related, and whether they are interdependent or whether there are discrete sets of alternate cosmoi such that even if one entire set is destroyed the others would remain.)
Travel between cosmoi
It tends to be much easier to travel between planes within the same cosmos than between planes that belong to different cosmoi. The main reason for this is that often objects and creatures can be transferred from one plane to another plane in the same cosmos as is; since the physical laws are (usually) unchanged, no transformation or rearrangement is necessary, or perhaps just a little adjustment if the physics exhibits some minor differences. Going to another cosmos, however, may involve drastic changes to accommodate the different physical laws; since even the chemistry and underlying nature of matter may be different, the transported object may have to be completely rebuilt along entirely different principles, superficially unchanged but totally altered on a deeper level.
There are some means that intercosmic travel is possible, however. Spells can be created that effect the necessary changes in the transition, though generally they have to be tailored to specific cosmoi rather than a single spell allowing travel to any cosmos. Some rare portals between different cosmoi exist, some of which have been created by powerful beings with standing enchantments of intercosmic travel, and some of which seem to have always existed; some æalogists think these latter occur naturally as some side effect of the creation of a cosmos, but others maintain that they must have been created artificially, but so long ago that their origins are lost to time. There are also some places that include portals to many different cosmoi. Again, some of these areas are easily explicable as the work over many years of many scientists or mages, but the origins of others, such as the Orange Corridor, remain mysterious. Likewise so far unexplained is the existence of golthoi, people near whom portals to other worlds and cosmoi spontaneously appear.
For reasons still not well understood, there are certain objects and types of objects that seem to exist in all cosmoi. Objects which exhibit this property are called "panyparic", and the objects themselves are referred to as "panypares". Panypares can be almost anything, from talismans to species of organism to individual organisms, and may even include certain intangible concepts and systems—the English language, for instance, for some reason seems to be panyparic.
Obviously, panypares are not fully identical in different cosmoi. The human species, for instance, is panyparic, but while its gross anatomy is the same in different cosmoi, at the cellular and particulate levels they are necessarily very different, based on the different physics and chemistry involved. In Xi, for instance, humans are made of trillions of cells, each of them made of atoms which in turn are composed of protons, neutrons, and electrons; they pass down their genetic information through the double-helix molecules of DNA. In Dverelei, on the other hand, the particles that humans, as well as other matter, are made of, are the bubble-like xuax, each made of some combination of the fundamental humors; their genetic material is encompassed not in large molecules like DNA, but in the relative positions of sets of smaller molecules called genosystems. Nevertheless, a human from Xi and one from Dverelei will look outwardly identical. The same is true of panyparic talismans, which may operate under very different magic systems in different cosmoi, but have essentially the same effects. At least one particular panypar, the magic doorknob, even allows travel between cosmoi, opening doors that can lead to different worlds or even to different cosmoi entirely.
Despite æalogists' attempts to arrive at a rigorous definition of a cosmos, there are some proposed cosmoi that remain controversial. One of these is Hepheli, also known as the "Digest". Hepheli, strangely, seems to be a mixture of people, objects, and places from the past, present, and possibly the future of worlds of other cosmoi, all combined into one continuous whole. What makes its status as a cosmos questionable is that its physical laws likewise vary; one area may have the physics of Usm, but an area a few meters away may have the physics of Ses, such that merely moving those few meters may bring about all the largely unseen changes involved in intercosmic travel—though these changes are somehow effected automatically through the very nature of the plane. The wide variance in physical laws makes Hepheli's qualification as a cosmos under the currently accepted definition contestable, but the fact that these areas of physical laws all occur apparently in the same plane makes it hard to classify as anything else. Another dubious case is Dhaya, which seems to consist of different planes of often widely varying physical laws, but connected together by certain constants that, again, make it problematic to consider them separate cosmoi.
Likewise dubious is the concept of a "supercosmos", a collection of similar cosmoi in a somewhat analogous manner to the way in which a cosmos is a collection of planes. Several such supercosmoi have been proposed, based on rationales as diverse as common origin, certain similarities in physical laws and/or systems of magic, and which panypares are most prominent. So far, though, no adequately rigorous criteria or consistent rationales have been advanced for defining a supercosmos, so it remains a concept ignored by most mainstream æalogists.