Full Idea
Being is the positing of essence. That which is my essence is my being. The fish exists in water; you cannot, however, separate its essence from this being.
Clarification
To 'posit' something is to propose it
Gist of Idea
Being posits essence, and my essence is my being
Source
Ludwig Feuerbach (Principles of Philosophy of the Future [1843], §27)
Book Reference
Feuerbach,Ludwig: 'Principles of the Philosophy of the Future', ed/tr. Vogel,M [Hackett 1986], p.42
A Reaction
This throws a different light on later (e.g. Heidegger) discussions of 'being', which may map onto Aristotelian discussions of essences.