What is the Essence of a thing?

"We, then, following the holy Fathers, all with one consent, teach men to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood, truly God and truly man, of a reasonable [rational: psuches logikes] soul and body; consubstantial with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, Only-begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence [en prosopon kai mian upostasin] not parted or divided into two persons, but one and the same Son, and only-begotten, God the Word, the Lord Jesus Christ, as the prophets from the beginning [have declared] concerning him, and the Lord Jesus Christ himself has taught us, and the Creed of the holy Fathers has handed down to us." - The Creed of Chalcedon


"In order to understand the creedal statement, one must begin somewhere. There must be a pou sto, a point d'appui, a basic agreement on a fundamental term. Otherwise we shall drown in an ocean of ambiguity. No choice is better than the Nicene term hypostasis. This earliest of all creeds affirms that the Godhead is mia ousia and tres upostaseis. The term ousia is a participial noun from the verb to be; the translation commonly is being. A dog is a being, a rock is a being; but if ousia is simply a form of the verb am, are, is, be, then since there are dreams, dreams are: They are beings. Sometiems the term reality is used. But dreams, especially bad dreams, are real: They are real bad dreams. Plato claimed that his Ideas were realities: The Idea of Justice, the Idea of Man, the Idea of Horse, and the Idea of Number are realities. Therefore the term ousia is not a so good a basic term as hypostasis.
Another form of the verb 'to be' is esse or essence. The term can be useful, if defined. To illustrate: Someone sees a new gadget, or an animal he has never seen before. He asks, 'What is it?' His friend replies, 'It is this sort of gadget or this sort of animal.' In ordinary conversations the answer is usually incomplete, but if the friend is at all knowledgeable he gives a part of the definition. The full answer to the question 'What is it?' is the definition. Now, if the theologians had been content, or able to use the term unambiguously, a great deal of confusion would have been avoided. The essence is the definition. The essence of a plane triangle is an area bounded by three straight lines. The phrase 'it is of the essence. . .' means it is a part of the definition. Unfortunately many theologians do not explicitly say this, but often by usage deny it. In an historical survey, however, the author must reproduce the blunders of those whom he quotes."

Gordon H. Clark, Incarnation, Pg. 5-6