The evidence for this is the procedure (still customary today) of setting up knowing as a 'relation between subject and Object'—a procedure in which there lurks as much 'truth' as vacuity. But subject and Object do not coincide with Dasein and the world.
Even if it were feasible to give an ontological definition of "Being-in" primarily in terms of a Being-in-the-world which knows, it would still be our first task to show that knowing has the phenomenal character of a Being which is in and towards the world. If one reflects upon this relationship of Being, an entity called "Nature" is given proximally as that which becomes known. Knowing, as such, is not to be met in this entity. If knowing 'is' at all, it belongs solely to those entities which know. But even in those entities, human-Things, knowing is not present-at-hand. In any case, it is not externally ascertainable as, let us say, bodily properties are.1 Now, inasmuch as knowing belongs to these entities and is not some external characteristic, it must be 'inside'. Now the more unequivocally one maintains that knowing is proximally and really 'inside' and indeed has by no means the same kind of Being as entities which are both physical and psychical, the less one presupposes when one believes that one is making headway in the question of the essence of knowledge and in the clarification of the relationship between subject and Object. For only then can the problem arise of how this knowing subject comes out of its inner 'sphere' into one which is 'other and external', of how knowing can have any object at all, and of how one must think of the object itself so that eventually the subject knows it without needing to venture a leap into another sphere. But in any of the numerous varieties which this approach may take, the question of the kind of Being which belongs to this knowing subject is left entirely unasked, though whenever its knowing gets handled, its way of Being is already included tacitly in one's theme. Of course we are sometimes assured that we are certainly not to think of the subject's "inside" [Innen] and its 'inner sphere' as a sort of 'box' or 'cabinet'. But when one asks for the positive signification of this 'inside' of immanence in which knowing is proximally enclosed, or when one inquires how this 'Being inside' ["Innenseins"] which knowing possesses has its own character of Being grounded in the kind of Being which belongs to the subject, then silence reigns. And no matter how this inner sphere may get interpreted, if one does no more than ask how knowing makes its way 'out of' it and achieves 'transcendence', it becomes evident that the knowing  which presents such enigmas will remain problematical unless one has previously clarified how it is and what it is.
1 'In jedem Falle ist est nicht so äusserlich feststellbar wie etwa leibliche Eigenschaften. The older editions have '... nicht ist es ...' and place a comma after 'feststellbar'.