Steven Galt Crowell on thematizing being phenomenologically.
Here the essentially reflective character of philosophy’s cognitive comportment must be recognized, for Heidegger is quite explicit that to thematize being is neither to posit a metaphysically supreme or ineffable entity, nor to identify a universal or “highest" region (a kind of "real predicate" of entities), but rather to proceed transcendentally, phenomenologically, "with reference to the way in which such 'being' is comprehensible: the meaning of being" (GA 61:58). To say that philosophy is concerned with the meaning of being is to say that its very Gehaltssinn (the "towards which" of its comportment) includes reference to being’s being "comprehended," thus to Dasein and to the specific sort of Vollzugssinn in Dasein's philosophizing comportment whereby the comprehension of beings as being is accomplished. Thus, the full definition of philosophy implicates a moment of reflection, since the being who philosophizes must concern itself with its own being as being: "Philosophy is principled cognitive comportment toward beings as bring (meaning of being) in such a way, indeed, that in the comportment and for it the being (meaning of being) of the having of the comportment is always decisively also at issue (GA 61:60). If Heidegger now designates philosophy as "ontological phenomenology," the order of terms is significant: Philosophy is ontology, but ontology must be understood as an adjectival modification of phenomenology. Phenomenology is the "authentic Vollzugsinn" of philosophy (GA 61:60).