Essence and Existence

Essence: The what-ness of an object
Existence: The is-ness of an object

Paramenides, a pre-Socratic scholar was a huge proponent of Monism. He believed that there was only one kind of being. The argument went something like, all that exist has being. If all that separates being is empty space (like the atomists believed) or the negation of a quality (like that the Platoism posits) then there is only kind of being. It was not until Aquanis came along centuries later and pointed out the issues with monism.

1. If there is only one kind of being, then everything is one.
2. We know there  are beings of the kind which require a cause.
3. There must be at least one being that needs no cause or else all beings need a cause and there would then be no being which existed prior to the existence of any other being because all beings would then need a cause.
4. Thus, there is at least one necessary being, and at least one contingent being.
5. Therefore, there is everything is not one.

For the contingent being, it is possible for it to not exist, pending the necessary being, making its existence potential. However, a necessary being must exist, as part of what it is. In other words, this being must be without cause by its essence. Thus its essence and existence are one in the same.