In brief: A load-bearing one — but not the last. In Hegel difference is no lack of unity but that through which anything is at all; without it there would be only the night in which all cows are black. At the same time it is not the end of the analysis: it drives on to contradiction. And that is exactly where twentieth-century criticism takes hold.
Its place in the Logic
In the logic of essence four determinations stand in a row: identity, difference, opposition, contradiction (Encyclopaedia §§ 115–120). Identity is the poorest of them: „A = A“ says nothing. Difference has several stages — indifferent diversity (two things simply are different), opposition (each is what it is only through the other), and finally contradiction, in which the determination founders on itself.
That this last step is no misfortune Hegel says as plainly as it can be said:
[Identity] is merely the determination of the simple immediate, of dead being; but contradiction is the root of all movement and vitality; only in so far as something has a contradiction within it does it move, does it have drive and activity.
Science of Logic II, Contradiction
Difference as the principle of the modern world
In the philosophy of right difference has a name: particularity. And Hegel takes it to be what separates modernity from antiquity:
The right of the particularity of the subject to find itself satisfied, or — what is the same thing — the right of subjective freedom, constitutes the turning and central point in the difference between antiquity and modern times.
Groundlines, § 124 Remark
From this follows the standard by which Hegel measures orders. Where particularity has no place, it appears as destruction (§ 206 Remark); where it has its place, it becomes „the principle of all animation of civil society“. And § 260 expressly requires that the principle of subjectivity be allowed „to complete itself into the self-subsistent extreme of personal particularity“. A unity that keeps difference small is, in Hegel, no unity.
And the limit
Difference nevertheless remains for Hegel a moment. It is to be sublated — not effaced, but taken up into a unity that bears it. That is precisely what thinkers of the twentieth century have disputed who set difference against Hegel: what cannot be taken up is not a remainder the system has yet to grasp, but the first thing. For the philosophy of right this means: whether § 260 keeps what it promises, or whether in the end the universal weighs more than the particular after all, is not a question of interpretation but is to be tested against § 272–§ 320.
It remains notable that Hegel’s first publication — The Difference Between Fichte’s and Schelling’s System of Philosophy (1801) — carries the word in its title. The reproach that he does not know difference does not, at any rate, hit the beginning.
Sources. Science of Logic II, First Section, First Chapter (identity, difference, contradiction); Encyclopaedia (1830) §§ 115–122. – The Difference Between Fichte’s and Schelling’s System of Philosophy, 1801. – Groundlines § 124 Remark, § 182–§ 187, § 206 Remark, § 260. – Literature. As a counter-position Gilles Deleuze, Différence et répétition, Paris 1968, and Jacques Derrida, Marges de la philosophie, Paris 1972; in defence Robert B. Pippin, Hegel’s Idealism, Cambridge 1989. – In the glossary: contradiction, determinate negation, concrete universality.