Belief
Belief in the cosmos of Inji is an important force in directly shaping reality; what people believe is reflected in the world (and indeed the cosmos). If enough people believe something, then it tends to become true. It may not be off the mark, in fact, to say not only that the fabric of reality responds to belief, but that the fabric of reality is belief, that everything in Inji is really made of belief made manifest—and some scholars say just that. While most people of Inji are unaware of the rôle played by belief in forming and changing them and their surroundings, belief wields its works well without the witting will of the believer.
Parameters
While belief has power in Inji, not all beliefs are created equal. To say that anything that anyone believes is true is oversimplistic and antilogous. Two people may, for instance, have mutually contradictory beliefs, and therefore at most one of these beliefs could be fully realized. There is a theory that sufficiently widely held contradictory beliefs lead to the generation of alternate worlds where each is true, but no hard evidence has yet been found for this... certainly there are alternate worlds in Inji that correspond to contradictory beliefs (sometimes called dissident worlds in Injian æalogy), but it's not clear that such worlds exist for every such set of beliefs, or how or whether they arise in response to them.
Dissident worlds notwithstanding, there are some parameters that cause some beliefs to predominate over others. The best known is the number of believers. While elsewhere it is a fallacy to base the truth of a proposition on consensus gentium, the mere fact that most people believe it, in Inji this fact is, all else being equal, enough to establish it as true. If everyone believes something, then that thing is ipso facto true.
All else may, however, not be equal; the number of believers is a major factor behind the realization of a belief, but not the only one. Also significant is the proximity effect: beliefs are more effectual the nearer the believer is to the subject of the belief. In particular, this means that a person's beliefs about himself will outweigh any other person's beliefs about him—though they can still be overcome if enough other people believe differently.
There may be other factors also influencing the power of a belief, but they are not so easily quantifiable. Some studies have suggested that the beliefs of some people may be stronger than others, in the sense that they have more effect on the world around them. It seems that there is some correlation between the power of a person's belief and their intelligence. Since intelligence is itself a poorly defined and not readily quantifiable concept, however, a precise relationship cannot be determined. There is also reason to believe that the strength of a conviction may relate to its efficacy, that the more fervently something is believed the more prone it is to becoming true. Again, however, the strength of a conviction is not something easily measured or quantified, so for the moment this remains a qualitative fact of which precise use cannot be made.
An important subtlety to keep in mind is that simply not believing something is not the same thing as actively believing that it isn't true or doesn't exist. If one simply doesn't think about something, or has no opinion on it, this lack of belief will have no direct effect one way or the other, but explicitly disbelieving is another matter. This explains, among other things, how secrets can exist in a cosmos predicated on belief. Only one or a handful of people may truly believe a given secret, which may seem to imply that the greater numbers of people who don't believe it should ensure its falsity—but those others simply have never even considered the possibility of the fact in question, then they don't actively disbelieve it, and the few who do believe it are enough to maintain its existence.
Consequences
Although the causative röle of belief in Inji is not widely known, and everyday life in many parts of Inji is not too different from that in many other cosmoi, there are nevertheless a number of consequences of this principium. There are certain patterns that the human mind—or the minds of many, if not all, other ellogous species—tends to see whether they're there or not, and therefore, in Inji, they are there. The Law of Similarity, the Law of Contagion, and the Law of Uniformity, for instance, seem to be concepts independently arrived at by many people and cultures—in the first two cases as famously (if perhaps not entirely accurately) documented on True Earth by James Frazer in his celebrated anthropological opus The Golden Bough, in which those two terms were coined. Of course, many people believe in those broad principles only subconsciously, and could not express them if asked, and of course in general, on True Earth and most other worlds, they are mere superstition and do not hold true. The very fact that they are widely believed, however, makes them true in Inji, and they are recognized and studied by some scholars even in ignorance of the broader base of belief that builds the bed of them both. The Law of Uniformity is a slightly different case in that it does have more general validity behind it; in pretty much any plane or cosmos there are some principles or phenomena that do apply or exist universally, and scientists on other cosmoi can and do speak of the Law of Uniformity without delving into mysticism. In Inji, however, it's even more far-reaching, and, because people tend to imagine other places as having contents similar to their own, so they do. This is believed to account in Inji for the presence of panasters and polyypares, and perhaps, if one grants the potential for Inji to influence other cosmoi, even panypares. Aside from these general laws, there are other concepts that are also widely believed, and that also therefore exist in Inji; it may be largely due to people's belief in them, for instance, that Inji has undead, and gods.
It is not only this kind of folk belief that is instantiated in Inji. The characters and places of fiction, too, can take on a reality there, either leading to the appearance of those characters and places somewhere in an existing world, or to the creation of whole fictive worlds, or both. This may seem somewhat counterintuitive, since, after all, fiction may touch the heart or stir the soul but isn't generally actually believed. There are some stories that began as fiction but are believed, of course: urban legends, myths, in a sense even conspiracy theories. But it is not only these stories that actualize in Inji; popular fiction never intended to masquerade as anything else does too. There are those who deep down do believe in the reality of the stories they hear, after all, and those are apparently enough—and this belief is likely more common in Inji where, after all, the things believed in are real. There's also some debate as to whether belief in the power of a story and its thematic importance, even without initially any literal belief in the events it depicts, somehow counts as or metastasizes into belief in the story itself sufficient to manifest its contents. In any case, as with other beliefs in Inji, the number of believers matters. A popular novel or widely recounted tale will become real; a story invented one afternoon, told once, and forgotten will not.
There are many puzzling aspects of the way that belief shapes reality in Inji, not a few of which are still not well understood. One of the most prominent, however, is the part played by beliefs about the past. People often come to beliefs about how things came to be or about other events that predated the beliefs about them—and yet these beliefs, if sufficiently widespread, seem as prone to realization as beliefs about the present. If the past can be changed—and that certainly seems to imply that this is the case—then this introduces the similar sorts of logical pitfalls to those that arise in time travel. While no definite resolution has been proven for this palætiological paradox, possibilities have been proposed. Some have claimed that belief doesn't affect the past, but that rather the past influences belief; that people's beliefs conform even to past events they are unaware of, such that it appears that the beliefs have changed the past when in fact it was the other way around. The evidence for this is wanting, however, and a detailed analysis introduces new paradoxes anyway. Others claim that beliefs parogonically split off divergent worlds while leaving intact the worlds where the beliefs originated, but this too raises difficulties of its own. If beliefs do alter past events—and it seems that they do—then they apparently somehow do so in a way that avoids any real logical inconsistencies, and some æalogists see this as yet another facet of the Law of Ontological Relativity.
The Ultimate Paradox
There is another potential paradox concerning the effect of belief on reality, one which has attracted enough attention to be frequently referred to as the "Ultimate Paradox". This "paradox" is the question of what would happen if enough people began explicitly believing that belief has no power. Some people believe that because this would result in a paradox, it is ipso facto impossible, per the Law of Necessity. A few hold that it is hypothetically possible, but hope that it never occurs, because it would (they claim) cause the end of the cosmos. Still others say that such a widespread belief would only result in the creation of an alternate world—or rather a whole new cosmos—where belief did not have the power it does in Inji. In fact, some think it may have already done just that, and that this is precisely the reason that other cosmoi exist, disbelief in belief being a driver of cosmogenesis.
Study
While in most areas of Inji the power of belief is a secret not known to the general public, there are those epopts who are aware of it. Some of these are scholars, and some of these scholars choose to specialize in the study of belief and how it affects their surroundings. This field of study is known as nomisology, and those who pursue it are called nomisologists. The prevalance of nomisology varies between lands and worlds; in some places it is entirely unknown, whereas in others it is a common academic discipline.
Uses
Given the widespread ignorance of the power of belief, there are few who fully consciously try to take advantage of it. There are some, however, who learn of its potential power and who do try to manipulate it. At best, this meets with mixed success. It's not necessarily a simple matter to consciously force oneself to believe in something, and even if one does manage it, the beliefs of one person in general have negligible effect. And influencing the beliefs of others on a large scale may be achievable by the most charismatic of demagogues, but is certainly beyond the abilities of the average ou.
There are, however, places on Inji where the formativeness of belief is more widely known, and groups that have grown to exploit it. Some of the largest and best organized such groups, called heresies, have spread over multiple worlds and planes of Inji. In some places, different heresies have encountered each other and come into contention, and there's grown up to be almost a major conflict sometimes called the Credal War.
Magic
Magic on Inji is itself an application of belief, albeit not generally a conscious one. Magic—at least most arcana—functions because people believe that it does. Some few mages work their wonders by having attained some measure of control over their own beliefs, on a subconscious if not a fully conscious level; they can bring things about directly by believing in them. This form of magic effectively composes an arcanum in itself, known as doxurgy.
Doxurgy is, however, in most places a rare and unreliable form of magic. Most magic is produced not by people consciously manipulating their own beliefs, but in accordance to systems that people believe work. Some mages come to their own convictions of how magic should work and how their powers should function, and developed their own separate celata called idiarcana. More often, though, mages work according to arcana that have already been established by the beliefs of others. Exactly why and how belief in one particular methodology spreads rather than another seems to be a result of chance and positive feedback, much like most any meme, but in any case there are several arcana that are common throughout Inji—as well as a large number practiced (for now) only in smaller regions.