Kay

If the subordination is both permanent and comprehensive, it is not merely ‘a’ function.
Essence:
1. the characteristic or intrinsic feature of a thing, which determines its identity; fundamental nature
2. the most distinctive element of a thing the essence of a problem
3. a perfect or complete form of something, esp a person who typifies an abstract quality he was the essence of gentility
4. Philosophy
a. the unchanging and unchangeable nature of something which is necessary to its being the thing it is; its necessary properties
b. the properties in virtue of which something is called by its name
c. the nature of something as distinct from, and logically prior to, its existence