In Greek, being is called τὸ εἶναι. This word εἶναι is the infinitive of the verb whose participle is τὸ ὄν. From this it becomes clear that, when the thinker thinks τὸ ὄν, he does not take this word in its substantive sense, but rather in its verbal sense. The abridged and thereby ambiguous question—what is the being?—is indeed the guiding question of thinkers. But in pursuing this question they do not ask if the being is a rock or a bone or a donkey or a triangle. Rather, the question asked by the thinker—what is the being?—means only this: what is the being of the being? What is that in and through which something that ‘is’ is? What is it that characterizes the ‘being’ as such? 
Now, what characterizes ‘the free’ as such, and what designates ‘the free’ as ‘the free,’ language calls ‘freedom.’ Similarly, justice is what makes the just the just. Correspondingly, we may be allowed to say, even if the conventional understanding rebels against it, that what characterizes the being as such is ‘beingness.’
This word, however, is only the literal translation of the Greek word οὐσία, the word that was translated by the Romans as substantia and was thereby distorted in its meaning. In Aristotle’s sense, the thinker is seeking what the being as being is, i.e., he is seeking the being of beings—or, phrased otherwise, he is seeking beingness. That is why Aristotle elucidates the first of his quotations considered here—in which the eternally sought for, but also the forever newly question-worthy, is the question τί τὸ ὄν—with an addendum that immediately follows. It reads: τοῦτό ἐστι τίς ἡ οὐσία: “this—namely, what is actually sought-for in regard to beings—is, for us, beingness.” οὐσία, being, is that whence each being as such comes: the origin of beings, γένος. In this way, Plato and Aristotle designate being in relation to beings. Because being is the origin to which each being as such owes itself, being, in its relation to every being, is τὸ κοινόν (to follow Plato and Aristotle here)—the commonality that concerns every being καθόλου (i.e., every being as a whole and generally).
If, therefore, the thinker thinks τὸ ὄν, he thinks τὸ εἶναι—the being (of beings). He thinks being as that from which all beings originate. Being ‘is,’ with respect to beings, always already the ‘older.’ When being is thought, the being is conceived as that which it already was—τί ἦν. That is why Aristotle determines what the thinker is to think—τὸ εἶναι—more precisely as τὸ τί ἦν εἶναι: being as that which, for beings, always already is, i.e., “what was.”
The way of thinking the being of beings briefly outlined above was established by Plato and Aristotle. By thinking the being in a manner  that proceeds out of beingness and is oriented toward it, this thinking moves beyond the particular being under consideration. In Greek, movement from one over to the other is designated by the word μετά. Beings—the sea, the mountains, the forests, the animals, the heavens, but also the human and the gods—which of their own accord lie before, sometimes in one way and sometimes in another, without the assistance of the human, are what-occur-in-the-fore, are the coming-forth, and are thereby the lying-before—i.e., ὑποκείμενον (i.e., what approaches the human and