Problem of Ontological Difference [402-403]

but also it has from thence that it is and what it is, in such a way indeed that the good is not itself the being-how and being-what, but even outstrips being in dignity and power." That which illuminates the knowledge of beings (positive science) and the knowledge of being (philosophical knowledge) as unveiling lies even beyond being. Only if we stand in this light do we cognize beings and understand being. The understanding of being is rooted in the projection of an ἐπέκεινα τῆς οὐσίας. Plato thus comes upon something that he describes as "outstripping being." This has the function of light, of illumination, for all unveiling of beings or, in this case, illumination for the understanding of being itself.

The basic condition for the knowledge of beings as well as for the understanding of being is: standing in an illuminating light. Or, to express it without an image: something upon which, in understanding, we have projected that which is to be understood. Understanding must itself somehow see, as unveiled, that upon which it projects. The basic facts of the antecedent illumination for all unveiling are so fundamental that it is always only with the possibility of being able to see into the light, to see in the light, that the corresponding possibility of knowing something as actual is assured. We must not only understand actuality in order to be able to experience something actual, but the understanding of actuality must on its side already have its illuminating beforehand. The understanding of being already moves in a horizon that is everywhere illuminated, giving luminous brightness. It is not an accident that Plato, or Socrates in the dialogue, explains the context to Glaucon by a simile. The fact that Plato reaches for a simile when he comes to the extreme boundary of philosophical inquiry, the beginning and end of philosophy, is no accident. And the content of the simile especially, is not accidental. It is the simile of the cave, which Plato interprets at the beginning of the seventh book of the Republic. Man's existence, living on the disk of earth arched over by the sky, is like a life in the cave. All vision needs light, although the light is not itself seen. The Dasein' s coming into the light means its attainment of the understanding of truth in general. The understanding of truth is the condition of possibility for scope and access to the actual. We must here relinquish the idea of interpreting in all its dimensions this inexhaustible simile.

Plato describes a cave in which humans have their hands, feet, and heads fettered, with their eyes turned to the cave's wall. Behind them there is a small exit from the cave, through which light falls into the cave in back of its inhabitants, so that their own shadows necessarily fall on the wall lying opposite them. Fettered and bound firmly so that they can only look ahead of them, they see only their own shadows on the wall. Behind them, between them and the light, there is a path with a partition, like the partitions puppeteers have. On this path other humans, behind the prisoners,

Basic Problems of Phenomenology (GA 24) by Martin Heidegger