If this historicality remains hidden from Dasein, and as long as it so remains, Dasein is also denied the possibility of historiological inquiry or the discovery of history. If historiology is wanting, this is not evidence against Dasein's historicality; on the contrary, as a deficient mode1 of this state of Being, it is evidence for it. Only because it is 'historical' can an era be unhistoriological.
On the other hand, if Dasein has seized upon its latent possibility not only of making its own existence transparent to itself but also of inquiring into the meaning of existentiality itself (that is to say, of previously inquiring into the meaning of Being in general), and if by such inquiry its eyes have been opened to its own essential historicality, then one cannot fail to see that the inquiry into Being (the ontico-ontological necessity of which we have already indicated) is itself characterized by historicality. The ownmost meaning of Being which belongs to the inquiry into Being  as an historical inquiry, gives us the assignment [Anweisung] of inquiring into the history of that inquiry itself, that is, of becoming historiological. In working out the question of Being, we must heed this assignment, so that by positively making the past our own, we may bring ourselves into full possession of the ownmost possibilities of such inquiry. The question of the meaning of Being must be carried through by explicating Dasein beforehand in its temporality and historicality; the question thus brings itself to the point where it understands itself as historiological.
Our preparatory Interpretation of the fundamental structures of Dasein with regard to the average kind of Being which is closest to it (a kind of Being in which it is therefore proximally historical as well), will make manifest, however, not only that Dasein is inclined to fall back upon its world (the world in which it is) and to interpret itself in terms of that world by its reflected light, but also that Dasein simultaneously falls prey to the tradition of which it has more or less explicitly taken hold.2 This tradition keeps it from providing its own guidance, whether in inquiring or in choosing.
1 'defizienter Modus'. Heidegger likes to think of certain characteristics as occurring in various ways or 'modes', among which may be included certain ways of 'not occurring' or 'occurring only to an inadequate extent' or, in general, occurring 'deficiently'. It is as if zero and the negative integers were to be thought of as representing 'deficient modes of being a positive integer'.
2 '...das Dasein hat nicht nur die Geneigtheit, an seine Welt, in der es ist, zu verfallen and reluzent aus ihr her sich auszulegen, Dasein verfällt in eins damit auch seiner mehr oder minder ausdrücklich ergriffenen Tradition.' The verb 'verfallen' is one which Heidegger will use many times. Though we shall usually translate it simply as 'fall', it has the connotation of deteriorating, collapsing, or falling down. Neither our 'fall back upon' nor our 'falls prey to' is quite right: but 'fall upon' and 'fall on to', which are more literal, would be misleading for 'an ... zu verfallen'; and though 'falls to the lot of' and 'devolves upon' would do well for 'verfällt' with the dative in other contexts, they will not do so well here.