Avicenna was the first to posit the distinction between the existence of a thing and its essence, and the distinction was later refined by Aquinas – Etienne Gilson credits Aquinas as being a kind of forerunner of existentialism because of his use and development of the idea. John Duns Scotus would refine it further, developing one of his most original philosophical ideas, the formal distinction. Briefly, though the existence and essence distinction is this: a things existence (its ‘that it is’) is different from its essence (its ‘what it is’).
‘Existence, for Avicenna (at least as he was read in the thirteenth century) is not a constituent of the essence of anything, for we cannot infer from the knowledge of what a thing is the fact of its existence. Hence existence is an accessory accident which must be conferred upon a thing by an external cause in order that it may exist.’ ( Julius R. Weinberg, ‘A Short History of Medieval Philosophy,’ p. 184)
Aquinas disagreed in part, seeing that Avicenna seemed to be saying that there could be essences without existence:
‘…there is a feature of a thing, namely its essence, which can be understood without knowing whether such an essence has being in the universe of things. In the case of those things whose essences are not the same as their act of existing, the act of existing must be derived from something other than themselves.’ (ibid, p. 185)
The distinction made here between essence and existence is Scotus’ formal distinction that is, when two realities do not share a total and complete identity between them. An example might be, say, the attributes of wisdom and goodness in God (on a conception of God which holds to divine simplicity): while they aren’t two different properties (because in God all is fused into the most simple unity) there is a distinction between them. Scotus developed his formal distinction by saying that existence isn’t really different from essence, while still retaining the basic insight that existence does not follow from essence:
‘…while existence and essence are formally distinct, they are distinct to the extent that the existence of any created or caused thing cannot be deduced from its nature or essence.’ (ibid, p. 219)
Now, when it comes to knowing something, we first are aware of ‘being’ or ‘existence’ in a general way. Our knowledge of ‘what’ something is only follows after we are aware ‘that’ something is. Our awareness and of being and grasp of our existence come before our grasp of the essence of something. Our grasping of the essence is not a logical deduction – just as we cannot deduce existence from essence, we cannot deduce essence from existence:
‘As soon as it comes into touch with sensible experience, the human intellect elicits the immediate intuition of being: X is, or exists; but from the intuition that something is, the knowledge of what it is, beyond the fact that it is something, cannot possibly be deduced. The intellect does not deduce, it intuits, it sees, and, in the light of intellectual intuition, the discursive power of reason slowly builds up from experience a determinate knowledge of concrete reality.’ (Etienne Gilson, ‘The Unity of Philosophical Experience’, p. 252-253)
The common thread here is that existence and essence cannot explain each other and cannot exist without each other – a finite thing is a unity which derives its existence, or has its ground of being or however you wish to say it, in something else which gives being to finite things. Only an infinite thing in which essence and existence are the same act contains its own ground of existence – the unity of essence and existence in a pure act is an infinite of being.