Doctrine of Essence is significantly important to clarify understanding of dialectics and especially to explore some of the concepts as explanation, time, freedom, contingency, causality etc., which are acutely necessary for any scientific work. For instance Althusser deviates from Hegel and thus from Marx introducing different meaning of essence. Interpretations of C. L. R. James (Notes on Dialectics, in this site) and Andy Blunden (Essence1.htm) were very instructing for me. I crucially recommend newcomers to read them. Here, I have just present my introductory study for apprehending Doctrine of Essence as I could do. I am sure that by the foregoing discussions it will be better.
"In itself" (an sich) means being separated from others, being apart from connections with others, being absolute, self-referent, not dependent on anything else, immediate. Thing-in-itself is such a thing. For Kant, thing-in-itself is, what it is when it is not being perceived. The question of "What is thing-in-itself?" for Kant cannot be answered, because we can grasp thing-in-itself only by its appearance, semblance, show, seeming and by our subjective ideas. Kant is the pupil of Hume. As Hume had thought the world external to us could not give us knowledge of relations but only of particulars and contingencies, Kant asserted that any supposed knowledge of universality and necessity had to be a priori.
"The various forms of idealism, Leibnizian, Kantian, Fichtean, and others have not advanced beyond being as determinateness, have not advanced beyond this immediacy, any more than scepticism did." (GL, 396)
This difference corresponds to the difference of Being and Essence in the Science of Logic. In the Doctrine of Being, Hegel thinks Being immediately as it is, is in itself. The particulars and contingencies, or better, individuals constitute the sphere of Being.
"Prices go up and down, politicians have scandals, strikes, take-overs, elections, etc. In so far as we are talking about "what is going on", these are "immediates", news items, things which appear and disappear, but "mean nothing to us". This is Being." (A. Blunden)
There is a relation between the categories of Being which are necessarily involved in the attempt to think it. Nevertheless, there is not order between individual things and all relations are external.
Determinations of individuals, in the first instance, are either quantitative or qualitative. Neither quality nor quantity is, however enough in themselves for individuality, there is a third determinations in their unity, which is measure. Quantity passes over to quality and quantity passes over to quantity.
"These examples find a parallel in the story of the peasant who, as his ass trudged cheerfully along, went on adding ounce after ounce to its load, till at length it sunk under the unendurable burden" (SL, 108n)
If I bite an apple, it changes from apple to non-apple. If I take another bite, and later another, so on apple pass to nothing. In being, things flow smoothly or abruptly to non-things.
"In the sphere of Being, when somewhat becomes another, the somewhat has vanished." (SL §111n) "The ordinary consciousness conceives things as being, and studies them in quality, quantity, and measure. These immediate characteristics, however, soon show themselves to be not fixed but transient; and Essence is the result of their dialectic." (SL, §111)
"All immediate being comes out of Nothing, stays as being for a while and goes back to nothing." (C. L. R James) In the movement from nothing to nothing, quality and quantity opposing to each other in their unity is measure. Coming to be and ceasing to be, requires permanency, therefore, measure is implicitly Essence. We thereby enter second sphere of Objective Logic. In essence, being is non-being. Contrary to Being, Essence is the sphere of relations.
Essence of a thing is the property of it such that if it ceased to have, thing would ceased to exist or it would have changed into something else.
"Everything, it is said, has an Essence; that is, things really are not what they immediately show themselves. There is something more to be done than merely rove from one quality to another, and merely to advance from qualitative to quantitative, and vice versa: there is a permanence in things, and that permanence is in the first instance their Essence." (SL, §112n)
Permanent properties are reflected determinations, while temporary properties are determinations of immediate being. Since both what permanent is and what temporary is, are properties, essence is the result of being.
"(T)he concepts we use to perceive Being have themselves their origin in Being. We recognise a cad because "we've met his type before". Thus essence is the movement of Being itself, rather than something external to, foreign to Being." A. Blunden
Essence is sublated being-in-and-for-itself, not immediately but is a result of that movement. C. L. R. James, clearly summarize Hegel's elusive expressions:
"Ordinary being is the movement of nothing to being-for-other and going on, or maybe, just becoming and disappearing, and that's that. But Essence tries again. So that the being in which Essence tries to find itself is pure Show; it does not become a quality, which becomes a quantity, which becomes a Measure, etc. No, sir. Pure Show. Absolute Negativity." (Essence)
Essence is, firstly absolute indifference that is indifferent to every determinateness of being, thereby it is pure being-in-itself. Also essence is what it is through this negation of everything finite and determinate; essence is itself this negativity, infinite movement. As it is itself movement, it is not a transition. As such, it is indeterminate essence. The limit or determinateness or determinate being of essence are not external but "is present only as posited (gesetztes) by essence itself". Determinations of essence are produced by essence itself.
Reflection or positedness is the negativity of essence. Being, in its negative relation with essence, is illusory being (Schein). This illusory being is essence's own positing, reflection or production. What was before Being, now produced as illusory being. Essence transforms being to nothing. Illusory being is the negative of Essence, but produced by Essence as its own negative.
"Being is illusory being. The being of illusory being consists solely in the sublatedness of being, in its nothingness; this nothingness it has in essence and apart from essence, illusory being is not. It is the negative posited as negative." (GL, 395)
If you answer the question of "What for a thing is that?" as "for-itself", that would mean its product is not for other but for itself. For self (Für sich) means self-relation. Only through its product, some thing can be for itself. Moreover, since there is nothing only for itself, or it is impossible to produce only for itself, being-for-self is a determination in relation to other. Essence is self-relation in relation to other. Essence (absolute negativity) is the production, and illusory being is the product (reflected immediacy) which would be consumed by the production (implicit negativity). The raw material of this production is being (immediacy).
". . . the reflected determinations are of a kind different from the merely immediate determinations of Being. Of the latter it is easily admitted that they are transitory and merely relative, related to something other, while the reflected determinations have the form of Being-in-and-for-Self. They accordingly assert themselves as essential, and instead of passing over into their opposites, they appear rather as absolute, free, and indifferent to one another. They therefore stubbornly resist their movement: their Being is their selfidentity in their determinateness, according to which, while presupposing one another, they yet preserve themselves as absolutely separate in this relation." (GL, 405-External Reflection. My emphasis)
On the one hand, Essence issues from being which is illusory being, on the other hand, illusory being is the result of Essence, which is sublated being. The difficulty of understanding Essence is because of this circularity. Essence is being-in-and-for-itself only in relation to other, in a specific reference to illusory being. On the one hand, illusory being is not something external to essence; on the other hand, illusory being is the reflection of essence. Therefore, the distinction of essential and unessential is externally posited, that is, it requires a third. This third is Reflection as thought process. How do we know essence? Marx implicitly answers this question:
"Relations can be established as existing only by being thought [gedacht], as distinct from the subjects which are in these relations with each other." (Grundrisse, p. 143)
In the sphere of essence, Reflection enters the scene, as both logically and causally independent category. We can comprehend the identity or essence of a thing through its different appearances and in the process of its passing over from nothing to thing and again to nothing, by the activity of our thought's reflective activity. Andy writes:
"We are only capable of recognising things that we already know from the past. A new concept of something cannot spring right out of immediate perception, it must be mediated through a whole process through things we already know about, and this process is Essence. To ascertain the essence of things, what lies behind the immediate, we begin by recognising things, the concepts of which we already have in our brains. But here we have not just images that appear and pass away again, but what it is, so to speak." (My emphasis)
Reflection, understand that essence and appearance are not separate beings, but are moments of any individual.