Essential Nature

In accordance with medieval philosophy, in Mythic Europe everything has an Essential Nature that Hermetic Magic cannot alter. Magic can suppress certain qualities or introduce others, but the object's essential nature is maintained.

Consider for example a lead coin magically turned into gold. The magic alters all the coin's qualities (color, softness, weight, and so on) into those of gold. The coin is still lead underneath, however, and when the magic ends will revert back to lead, reasserting all the natural qualities manifested by its essential nature.

Any quality of any thing may be part of its essential nature. A person may be crippled in his essential nature, for example, in which case it would be impossible to use Hermetic magic to heal his legs. Usually such flaws chosen at character creation represent a flaw at the character's essential nature that he has had since birth, but non-Hermetic magic or Twilight may change someone's essential nature.

In addition to the Limit of Essential Nature, Hermetic Magic is also bound by the Limit of the Divine. Additionally, Lesser Limits also create bounds on Hermetic Magic and are derived from the two fundamental limits of magic.

References

"Ars Magica Fifth Edition" pg 79