Main Sequence sapient life-forms can be broadly divided into Organic, Built (or Artificial or Mechanical) groups, as well as Quantum and Memetic sapience. It is held --at least among H-2 and O-2 species (all of which are Organic) --that Organic life-forms have a higher right to existence than Built forms. It is also well known that many ethicists --not least Terragen and Tymbrimi Clan philosophers --consider the Organic-Built distinction to be not only invidious, but also absurd.
There are also three concepts that cut across this classification scheme. The first concept-continuum is "Empirical" (or not-hypothetical) as opposed to "Hypothetical." Simply put, empirical life-forms are attested to in the documentary record. Examples either exist, or have existed in the past. Hypothetical life-forms could --in theory --exist, but have never actually been documented. The most famous example of hypothetical life-forms are Plasma Beings. Demonstrating the existence of plasma life would be very difficult, at best. One would probably need to descend into the coronas of stars --since the corona itself would drown out any trace signal from Plasma Beings. Unfortunately, wise Galactics are certain that entering a stellar corona is impossible; the Library knows of no apparatus having ever been built to perform such a voyage, or for any similar set of engineering stressors.
The other cross-cutting criteria are the concepts of sentience and sapience. These are not mutually exclusive. Speaking plainly, sapience is the ability to solve problems: cold intelligence. On the other hand, sentience is the ability to feel or perceive, or (more deeply put) awareness. When an entity has high levels of both sapience (intelligence) and sentience (awareness), it is most properly referred to as a "sophont."
Organic life-forms, in turn, fall into two divisions, Terrestrial and Gas Giant, usually referred to as "Oxygen-breathers" and "Hydrogen-breathers." Hydrogen breathers are by far the more numerous order due to the much higher frequency of suitable biospheres. However, the advantage of many more biospheres is offset to a degree by the immense size of any H-2 individual relative to even the largest of O-2 sapients, and the much slower bio-rhythms of most H-2 life-forms. Indeed, while O-2 civilization tends to regard H-2 civilization as a potential threat because of its immense demographic and economic advantage, H-2 species tend to regard O-2 civilization as a potential threat because of the much faster rhythm of O-2 life. H-2 anxiety about O-2 speed is only compounded by the fact that H-2 sapients seldom use more rapid means of interstellar travel --such as A-level hyper-space, jump-line travel, F.T.L. telekinesis, or probability drive. Instead, most H-2 species putter around the galaxies in D-level hyperspace or in normal space at near-light-speed. (See Interstellar Travel, Hyper-space, Light-Speed.)
Nor are the mutual security concerns misplaced. Disastrous wars have been fought between H-2 and
O-2 civilization in the distant past (See O-2/ H-2 Wars. See also,
"Comparative Galactic-Terragen Time Line."), despite the
fact that H-2 and O-2 species do not directly compete for most resources. The major reason for the
existence of the Galactic Institute for Migration is to limit
contact --and hence conflict --between the two great orders of Organic life. Though after billions of
years of research, communication between O-2 and H-2 species remains quite difficult, the two orders do
engage in trade (See O-2/ H-2 Communication and O-2/ H-2
Trade). Many (but by no means all) trade practices resemble "silent
trade" procedures used on Earth in the past.
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