Existence is a standing like Being. Existence itself has no essence. Things have essences which are constraints on their attributes that determine their whatness. Standings are attributed to things by us, while an essence is intrinsic to a thing. Existence means something may be found by us. Being is a projection onto what we find. Being implies existence but it is possible to have a projection without its target existing. For instance, unicorns have Being but do not Exist.
http://www.quora.com/What-is-existence-per-se-What-is-the-essence-of-existence
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