The Walkabout world is similar to our own, but it has two drastic changes (and these changes lead to a cascade of lesser changes). The first change is the tilt of the planet, this has been discussed fairly extensively in the first posts of this series. The second change is the manifestation of the spirit world in the physical plane, this has been touched on but hasn't really been detailed.
In some of those early posts it was even said that the nature of spirits either remains unknown, or isn't really important to the objectives of the characters within the setting. This was an oversimplification to allow us to progress with the other description that help set the scene for the stories within the Walkabout world.
At other times, statements were made about spirits being in a state of quantum flux, their exact parameters defined by the observer. There were also comments made about the ways were somehow linked to nuclear energy, with the effects of electromagnetic radiation playing a mysterious part.
The idea was to make spirits an integral part of the setting, certainly supernatural, but only in a way that they exist beyond the boundaries we have put on nature. The spirits force people in the setting to expand their concept of the natural, or reveal themselves for narrow minded.
Religions are forced to change, shattered technologies are harder to rebuild (because they often lure curious spirits), the news ways prove the be a dangerous deviation from the natural order (as environmentalists and radicals had been saying for decades) and the old ways prove their virtue.
In this setting, some might claim that the spirits are travellers from outer space, carried aboard an asteroid crashed into the earth with such force that the spin was altered. Running through the physics, an asteroid large enough to divert the spin of the earth this much, and an impact that big...it just isn't right. Not only are there no major craters on the planet, the planet would have probably shattered into a plethora of new asteroids.
No, the spirits have always been here. The most popular theory states that it was probably their intervention that tilted the earth. The most powerful among them, once known as gods, celestines, archangels or demon lords, were fed up with the abuse of humanity but knew it took conscious thought to anchor their forms. They couldn't get rid of all humanity, but sought to resume their power within the world. Different religions filter this theory through their assorted beliefs and dogma, but the pattern of powerful beings re-asserting their power in the world remains a common theme.
Scholars of pre-Christian Norse faith (and neo-vikings) might make claims about Ragnarok, justifying their words with the manifestation of nightmarish creatures, fiery earthquakes and the freezing of the world. The assorted Christian denominations whisper about the time of revelations and apocalypse. No religion is more correct than any other, and just like any other period of history they cherry-pick the facts that back up their dogma while ignoring or disputing any evidence that might run contrary to their beliefs.
Let's not get caught up in too much dogma though, we'll look at what the inhabitants of the world know about the spirits, as well as what they think they know, what is rumoured, and what is just downright wrong. No-one in the Walkabout world has a complete understanding of the spirits; almost like an electron, where you can know either its position or its velocity (but never be 100% certain of both); there are those who understand the spirits from a clinical and scientific perspective and there are those who understand them from a social and anthropological perspective, but even most spiritually aware individuals are unable to grasp the alien-ness of the spirits compared to themselves while upholding a meaningful discourse with them.
(This idea could end up playing a part as a significant mechanism within the final game, especially among those characters who regularly deal with spirits; those who have little spirit lore tend to believe more of the fallacies and some of the rumours, while those who learn more about the spirits gradually develop the accepted beliefs and the known...still working on this idea.)
Spirits are not human. It is clear that spirits are otherworldly beings, existing in a state somewhere between energy and an uncollapsed quantum wave-form. It is unknown whether they evolved along a different developmental path to matter based life-forms, if they were engineered by a progenitor race (or divine being), or if they are simply universal constants somehow linked to living consciousness.
The range of spirits is nearly endless. For every person that has ever existed, there seems to be at least one spirit. The means tens of billions, if not more. Most spirits are simple things unaware that they are bound to a certain concept or a specific place, many are ignored as they perform the background tasks that they have maintained since the dawn of time. Some have developed awareness and goals of their own, perhaps seeking to improve the world in some way or push some agenda (such as "technology", "occultism", "capitalism", "evolution", or "monotheistic faith"). A few rare spirits have developed a degree of full self-awareness. These are the greatest allies and greatest enemies a mortal human can face.
The Accepted Beliefs
Spirits are magical. Spirits do not seem to follow the laws of physics as we understand them, they are able to fade into in, their forms may vary from opaque through translucent to transparent, some are able to render themselves insubstantial (often walking through walls or otherwise impenetrable barriers). As far as the spirits are concerned, this is natural as the laws of their native hyperspace realm surrounding our reality are very different to our own.
Spirits are Otherworldly. Yes and No. Spirits naturally exist on a plane that is interconnected with our own, their actions have always impacted on our world and our actions have had ripple effects in theirs, it has been a symbiotic relationship since the beginning of time. Jewish Kaballah has a description where the centre of the universe is a light and all of perceived reality is simply a reflection of that light. Some aspects of reality are more pure and exist closer to the light, while others exist further away. In this cosmology, our world exists in a neutral sphere around this light. The agents of the pure exist closer the to light, refracting the will of the light onto our reality. The agents of the impure try to cast shadows and prevent use reaching the light. All exists within a delicate ecosystem and our perception of the world is but a tiny part of the whole. We may not have perceived spirits before the tilt, but they were definitely a part of our world.
Spirits cannot be killed. The exorcists of various faiths have often stated that spirits are eternal, they can be repelled from certain physical locations, they can be banished back to their home realm, their consciousness can be suppressed, and they can be bound into a quantum stasis (imagine a djinni in a lamp being a parapsychological version of Shroedinger's Cat), but there is always the chance that they will resume their original form once given enough time.
Spirits are dangerous. This is basically true, in certain contexts. Unaware spirits are single minded, almost programmed with a limited set of actions, deviating them from their tasks or placing them in situations where this "programming" isn't appropriate may cause unexpected results. Aware spirits have agendas rather than limited sets of programming, such spirits become dangerous to those who oppose said agendas, or when their agendas are rendered irrelevant due to changing circumstances (correcting these circumstances is often the best way to appease spirits in this condition).
Spirits fuel themselves with the beliefs and faith of humans. Since spirits manifest according to the perception of a conscious mind, they sometimes seek to strengthen that form through the concerted effort of many minds. The more "believers" hold a spirit to it's form, the more stable that form is. This renders them unable to transform and adapt, but gives them power among those who focus their belief. Many spirits choose to "work miracles" among their believers in order to assert their authority in the physical world, this often takes the belief of many humans to bend the laws of physics within our reality.
Spirits are tied to religions. Spirits have always been a part of the unseen world, and religion and occult arts have always sought to explain the unseen world. Since spirits are defined by perception and belief this has often caused them to manifest in forms associated with religious and occult concepts. The oldest beliefs of the world are shamanistic, and thus many of the oldest spirits manifest as intelligent animals, hybrids of man and beast, or anthropomorphised elemental creatures reflecting storms, fire, water, stone or wood. Many spirits care little for the concepts of religion, especially when they exist separate to our reality; the simply fulfil their tasks in the world and prefer to be left alone.
Spirits come from dreams. The complex electromagnetic signals within the brain are like the most delicate candy to curious spirits; they are drawn to the fluctuations of emotion and symbolism. Spirits are able to manipulate and inhabit dream realms, they are even able to lure humans into their own world through their dreams. The exact connection between spirits and dreams is unknown.
(As creatures driven by an observer's perception and naturally comprised of electromagnetic radiation, the conscious brain is a playground for spirits; it's hardly surprising that some spirits get into a brain and do not leave, using the filters of the conscious mind to experience the outside world.)
Humans may become spirits. When a human dies, the echoes of their beliefs resonate for a brief moment. Among some groups it is said that in this moment of dying, a spirit may copy the memory pattern of a human, thus becoming a ghost. Other groups state that hidden reaper spirits offer a chance at immortality to those who have proven their worthiness. Only at the moment of death is this known, and those who've made the choice are unwilling (or unable) to share the truth with the living.
Spirits are demons. Spirits may be responsible for the human belief in demons, but they are just as responsible for legends of angels, faeries, saints, ghosts and totems.
Spirits are nothing without Humans. Humans may give spirits form, but without humans the spirits would still exist. They might be a swirling energy of raw chaotic potential, and would probably work to develop a new sentient race on the planet (one capable of stabilising their form once again), but they would not disappear completely