itself is even treated at first like a being and explained by means of determinations of beings, as at the beginning of ancient philosophy. When Thales answers the question What is that which is? by saying "Water," he is here explaining beings by means of a being, something that is, although at bottom he is seeking to determine what that which is, is as a being. In the question he therefore understands something like being, but in the answer he interprets being as a being. This type of interpretation of being then remains customary in ancient philosophy for a long time afterward, even after the essential advances made by Plato and Aristotle in formulating the problems, and at bottom this interpretation has remained the usual one in philosophy right down to the present day.
In the question as to what that which is, is as something that is—what a being is as a being—being is treated like a being. Nevertheless, although unsuitably interpreted, it is still made a problem. Somehow the Dasein knows about something like being. Since it exists, the Dasein understands being and comports itself toward beings. The distinction between being and beings is there [ist da], latent in the Dasein and its existence, even if not in explicit awareness. The distinction is there, ist da [i.e. exists]; that is to say, it has the mode of being of the Dasein: it belongs to existence. Existence means, as it were, "to be in the performance of this distinction." Only a soul that can make this distinction has the aptitude, going beyond the animal's soul, to become the soul of a human being. The distinction between being and beings is temporalized in the temporalizing of temporality . Only because this distinction is always already temporalizing itself on the basis of temporality and conjointly with temporality and is thus somehow projected, and thus unveiled, can it be known expressly and explicitly and, as known, be interrogated and, as interrogated, investigated and, as investigated, conceptually comprehended. The distinction between being and beings is preontologically there, without an explicit concept of being, latent in the Dasein's existence. As such it can become an explicitly understood difference. On the basis of temporality there belongs to the Dasein's existence the immediate unity of the understanding of being and comportment toward beings. Only because this distinction belongs to existence can the distinction become explicit in different ways. Because when this distinction between being and beings becomes explicit the terms distinguished contrast with each other, being thereby becomes a possible theme for conceptual comprehension (logos). For this reason we call the distinction between being and beings, when it is carried out explicitly, the ontological difference [die ontologische Differenz] . This explicit accomplishment and the development of the ontological difference is therefore also, since it is founded on the Dasein' s existence, not arbitrary and incidental but a basic comportment of the Dasein in which ontology, that is, philosophy, constitutes itself as a science.