Spinoza's Ethics: II.D2: Essence As Singular

This definition builds off of "essence," a term that may be contrasted with "existence." At the beginning of the Ethics, Spinoza focuses on "essence" in defining self-caused as that whose essence involves existence. Here it pertains to the essence of the self-caused thing (God) to have (an item of) existence. Existence cannot be removed without removing God and vice versa. As the Ethics develops, a distinction is made between substance, where essence and existence are joined, and modes, where essence is permanent but existence is temporary. This definition applies equally to substance and modes and seems almost to qualify an an axiom.

Due to potential errors in understanding, I have translated id as "item" rather than "it." In a Platonic sense, Spinoza focuses on id or "item" that is essential to the definition of res or "thing." But this is not as Platonic as it appears because each res or "thing" is singular. For example, infinite squares may be drawn within a given circle. To define a specific square within that circle means to define the essence of the specific square as different than the other infinite possible squares within that circle. The differentiating items - the collection of four dots on the circle - are a group. This unique group or "item" is key to the specific square. Likewise, to identify the unique group of dots or "item" requires the square. In this way, the id or "item" that pertains to the essence of the res or "thing" is very particular and forces the articulation of the vice versa phrase in which the id or "item" is described as equally dependent on the res or "thing." This articulation may seem redundant because there is slight difference between the id or "item" and the res or "thing." 

It appears that Spinoza puts this definition early in the second section of the Ethics because of the general mental tendency towards abstraction. Rather than defining essence as a generalizing function, defining essence becomes rooted in the identification of specifics.

Ad essentiam alicujus rei id pertinere dico quo dato res necessario ponitur et quo sublato res necessario tollitur; vel id sine quo res et vice versa id quod sine re nec esse nec concipi potest.

Translated as,

I say that an item pertains to the essence of some thing in which the item is provided, the thing necessarily exists and in which the item is removed, the thing is necessarily removed; or that item without which, the thing is able to neither be nor be conceived and, vice versa, that item, without which thing, is able to neither be nor be conceived.