Martin Heidegger, answered from the texts and cited to the page.
The question asks after something that looks, at first glance, like a technical distinction — between being and beings — and yet the more one presses on it, the less it resembles any ordinary distinction at all. Every being that we encounter, every stone, every hammer, every living organism, every human being, shows up as what it is and that it is.
The sciences take beings in precisely this way — inquiring into their content, their properties, their regional structures. But there is something else at work in every such encounter, something that makes the being show up as a being at all: its being, Sein. This is not another being lurking behind the others, not some deep principle serving as their source or ground or creator — that confusion, I must say plainly, began with Thales and has run through the entire history of Western philosophy to the present day.1
Being is not a being. That is the ontological difference in its most compressed formulation. But the difference is not simply a logical distinction one draws between two items that happen to lie before us. What makes it so difficult — and so urgent — is that the distinction does not arise subsequently by merely separating two separate things; it belongs to the fundamental occurrence in which Dasein moves as such.2
The difference is not our construction imposed from outside; it is operative in Dasein's very being, in the way that Dasein already understands being, pre-conceptually, before any philosophical thematization. Ontology as a scientific discipline is nothing but the unfolding, in conceptual form, of this pre-conceptual Seinsverständnis — the understanding of being that Dasein already carries.3
Now here is what the distinction opens onto, and why it cannot simply be named and then set aside. We can distinguish, within the ontological difference, between ontic questioning — concerned with beings in themselves, with the ὄν ὡς ὄν — and ontological questioning, concerned with beings as such, with what constitutes the being of beings, the ὄν ᾗ ὄν.
Ontic truth is the manifestness of beings as they are in themselves; ontological truth is the disclosure of the being of beings. But then the question becomes all the more pressing: how does things stand with the difference itself? Is it a matter for ontic knowledge, or for ontological knowledge, or for neither — since each is already grounded upon it?4
The difference grounds both and is therefore not fully captured by either. This is why I said in the Fundamental Concepts of Metaphysics lectures that raising the thematic name "ontological difference" does not by itself amount to something thoughtful — it is merely a passageway, a necessary one, but a passageway nonetheless. What the Western tradition has consistently done is to take being as the most general and therefore thinnest rarefaction of beings — ens qua ens, ens in commune — as if being were simply the highest being, the most eminent one, in relation to which every other being is determined.5
This is precisely the confusion that defines metaphysics as onto-theology: it identifies being with the highest being — God as supreme cause, substance as the underlying, the subject as the foundational. Even after Sein und Zeit named the distinction with some care, the danger only heightened: now "being" risks being taken and elaborated as something objectively present for itself, a new entity at the top of the hierarchy.6
Careful use of terminology, however zealously pursued, testifies to nothing unless it springs from a genuine questioning of the truth of Beyng itself. The ontological difference, then, is a passageway — unavoidable if the necessity of the basic question is to be made visible, but not itself the destination.7 The destination, if one can speak of a destination on a path that has no guaranteed end, is the question of the truth of Beyng (Seyn) — Beyng understood not as the most general predicate of beings but as Ereignis, the appropriating event in which the clearing opens and beings can first show up as beings at all.
In the first beginning, this came forth as φύσις and ἀλήθεια — but was immediately forgotten in favor of the beings that ἀλήθεια first made perceivable, reinterpreted as a being, the most eminent one.8 That forgetting is not a private oversight of the early Greeks; it is the structural event that has determined Western thinking ever since.
The ontological difference names the wound in the tradition. Working through it honestly means recognizing that it cannot be healed by a better taxonomy, only by a more originary questioning — one in which Beyng itself becomes a plight, one that disposes for itself the destiny of the thinking that belongs to it.9
We must not think of being, Sein, as a being, ein Seiendes — as, for example, some deep principle behind all other beings, serving as their source, their ground, their creator. This confusion started with the beginning of philosophy in the West, with Thales, and has continued down to the present.Basic Problems of Phenomenology GA24, p. xxiii
this distinction does not arise subsequently by merely distinguishing two separate things lying before us, but in each case belongs to that fundamental occurrence in which Dasein moves as such.Fundamental Concepts of Metaphysics GA29-30, pp. 382–383
Ontology as a scientific discipline is then nothing but the unfolding, in the light proper to thought and therefore in conceptual form, of this pre-conceptual understanding-of-being, Seinsverständnis.Basic Problems of Phenomenology GA24, p. xxiii
how do things stand with regard to this distinction itself? Is it a problem for ontological or for ontic knowledge? Or for neither of these, since each is already grounded upon it?Fundamental Concepts of Metaphysics GA29-30, pp. 382–383
being (as ens qua ens — ens in commune) is only the thinnest rarefaction of beings and is itself still a being, though indeed the one that is most eminently — since every being is determined in relation to it.Contributions to Philosophy Of the Event, pp. 385–386
the danger is now heightened that being itself will be taken and elaborated as something objectively present for itself.Contributions to Philosophy Of the Event, pp. 385–386
The 'ontological difference' is a passageway which becomes unavoidable if the necessity of asking the basic question is to be made visible on the basis of the guiding question.Contributions to Philosophy Of the Event, pp. 385–386
Beyng, as the 'between' which clears, moves itself into this clearing and therefore, though never recognized or surmised as appropriation, is for representational thinking something generally differentiable, and differentiated, as being. This applies already to the way beyng essentially occurs in the first beginning, namely, as φύσις, which comes forth as ἀλήθεια but which is at once forgotten in favor of beings.Contributions to Philosophy Of the Event, pp. 385–386
beyng itself must become a plight, one which first disposes for itself the destiny of the 'thinking' that belongs to it.Contributions to Philosophy Of the Event, pp. 385–386