An eschatology of a culture in the broadest sense is "the doctrine of last things." It is concerned with final destinies, the ultimates of existence, and with the end of time and the universe. Most human religious systems and prescriptive philosophies incorporate some doctrine of destiny and purpose, so it is difficult to believe that sentient extraterrestrials capable of comprehending their condition will not be bothered by the same questions that have puzzled mankind for thousands of years: Why are we alive? What is our purpose here? Whither lies our destiny?
Xenologists generally agree that there are three major classes of eschatologies which represent basic approaches in assimilating reality: The naturalistic, the eternalistic, and the historistic.
Naturalistic forms are characteristic of "primitive" religious systems and of mass religions in higher cultures. The individual understands himself to be a part of nature, which is itself embossed with cyclical rhythms. Wrongness is experienced as an alienation from nature, whereas the Ultimate Good or final purpose is to achieve complete organic unity with nature.
Eternalistic eschatologies are grounded in a conception of time as an endless cycle of eternal recurrence. It is from this "vain repetition" that the individual must seek to escape. The "last thing" to hope for is to be delivered from the "unreal" realm of the temporal, historical and empirical to the "timeless" realm of spirit. For instance, the people of India hold to the existence of kalpas -- cosmic periods of four phases through which successive worlds appear, flourish, disintegrate and die. Hindu eschatology extends cyclicity to individuals as well as the universe at large. Notions of reincarnation and transmigration of souls effectively maintain the rigid caste system (there are today more than 2300 distinct castes in India) and a sociocultural order which is repressive -- from the Western point of view:
[Untouchables are] denied access to the interior of a Hindu temple; denied the right of using the public water supply; required to take all they need from a different point in the river; in many cases with children who cannot get access to the ordinary school; and, what is worst of all, people who do not themselves make a struggle to get out of their misery, because it is a part of their faith that their miserable lot is the punishment administered by heaven for some wrong that they may have done in a previous existence.2589
Another variant of the eternalistic eschatology appeared among the Stoics in Hellenistic times, when it was the belief that:
When the planets return, at certain fixed periods of time, to the same relative positions which they had at the beginning when the cosmos was first constituted, this produces the conflagration and destruction of everything which exists. Then again the cosmos is restored anew in a precisely similar arrangement as before. The stars again move in their orbits, each performing its revolution in the former period, without variation.1847
This eternalistic viewpoint has much in common with the "oscillating universe" hypothesis espoused by many contemporary cosmologists.
Historical eschatologies typically are founded on notions of linear time. There is a beginning and an ending to time, and at the end there will come a "final judgment," a "new world," or some other major event which signifies movement towards a fundamentally new plane of existence. Christianity, Judaism, and Islam provide classic examples of historical eschatologies among the human religious systems of Earth.
Xenologists are able to imagine many more eschatologies than the basic three displayed -by mankind. For example, a galactic civilization might adopt a kind of "thermodynamic eschatology,"3076 setting as its foremost goal the halting or reversal of entropic processes in this universe.2616 This might involve finding some way to overcome the Second Law of Thermodynamics on the scale of the universe, a feat which lies well beyond the bounds of present human science and technology. If, however, other universes exist, the Second Law might be forestalled by borrowing negentropy from those other universes. The central galactic eschatology thus may imply the achievement (at some distant future date) of a higher plane of materially immortal existence accomplished by halting the expansion of the universe and resisting the spread of entropy therein.