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In Shadowrun, a character's Essence is the statistic that essentially determines how the integrity of their spirit. As a general rule, it's fairly stable, and most everyday people and animals will have it at a stable 6. As a hard rule, it will never return to its original value once it goes down- even if the conditions that lowered it in the first place are removed.

The most common ways to affect the attribute are through cybernetic implantations- minor surgery and implantation, such as a datajack or cybernetic eyes, is minimal in effect, but more invasive surgery, such as adding armored plates to your skin, lacing your skeletal system with a ceramic weave, or installation of an expert system that gives you inhuman reflexes, will give a significant hit to your Essence. The spirit can take a good deal of infiltration by the various auras of cybernetics, but push the number to zero or below and *poof*! it kicks out and you're left with a dead hunk of metal and flesh. Best get out a new character sheet...

In the case of mages and shamans, though, Essence is a significant number- not only is it directly linked to their capacity to use magic, it also determines their abilities to stay in the astral plane. As such, very few magicians have cyberware, and even those that do have very minimal implantations.

Paranormal beings sometimes have Essence, but some don't. Stereotypically "pure" beings like unicorns have the stat as high as 8, while harpies often have it at 4. Inhabitants of the astral plane will have an Essence stat equal to their Force, as strong spirits have more presence than their weaker kin.