Martin Heidegger said this quote

“ But how does it come about that while the ‘I think’ gives Kant a genuine phenomenal starting-point, he cannot exploit it ontologically, and has to fall back on the ‘subject’—that is to say, something *substantial*? The “I” is not just an ‘I think’, but an ‘I think something’. And does not Kant himself keep on stressing that the “I” remains related to its representations, and would be nothing without them?For Kant, however, these representations are the ‘empirical’, which is ‘accompanied’ by the “I”—the appearances to which the “I” ‘clings’. Kant nowhere shows the kind of Being of this ‘clinging’ and ‘accompanying’. At bottom, however, their kind of Being is understood as the constant Being-present-at-hand of the “I” along with its representations. Kant has indeed avoided cutting the “I” adrift from thinking; but he has done so without starting with the ‘I think’ itself in its full essential content as an ‘I think something’, and above all, without seeing what is ontologically ‘presupposed’ in taking the ‘I think something’ as a basic characteristic of the Self. For even the ‘I think something’ is not definite enough ontologically as a starting-point, because the ‘something’ remains indefinite. If by this “something” we understand an entity *within-the-world*, then it tacitly implies that the *world* has been presupposed; and this very phenomenon of the world co-determines the state of Being of the “I,” if indeed it is to be possible for the “I” to be something like an ‘I think something’. In saying “I,” I have in view the entity which in each case I am as an ‘I-am-in-a-world’. Kant did not see the phenomenon of the world, and was consistent enough to keep the ‘representations’ apart from the *a priori* content of the ‘I think’. But as a consequence the “I” was again forced back to an *isolated* subject, accompanying representations in a way which is ontologically quite indefinite.*In saying “I,” Dasein expresses itself as Being-in-the-world*. But does saying “I” in the everyday manner have *itself* in view *as* being-in-the-world [*in-der-Welt-seiend*]? Here we must make a distinction. When saying “I,” Dasein surely has in view the entity which, in every case, it is itself. The everyday interpretation of the Self, however, has a tendency to understand itself in terms of the ‘world’ with which it is concerned. When Dasein has itself in view ontically, it *fails to see* itself in relation to the kind of Being of that entity which it is itself. And this holds especially for the basic state of Dasein, Being-in-the-world.―from_Being and Time_. Translated by John Macquarrie & Edward Robinson, pp. 367-370 ”