Chad Engelland explains where Heidegger found the in-between of transcendental philosophy, from "Heidegger on overcoming rationalism through transcendental philosophy".

Kant’s highest principle of synthetic judgments, then, discloses the context for the encounter of the human being and things. In the 1927 Being and Time, Heidegger calls this the temporal disclosedness of care and in his later thinking this "between" unfolds from "appropriation" or "enowning" (Ereignis). In the 1936–1938 Contributions, the “between” comes to expression again and again; Heidegger radicalizes it in a manner evocative of Plato as the place of divinities and mortals. In the 1946 Letter on Humanism, he writes, "[T]he human being in his essence is ek-sistent into the openness of being, into the open region that first clears the 'between' within which a 'relation' of subject to object can 'be'".

Kant is misunderstood, Heidegger says, when we approach him in our everyday or scientific attitude which wants to understand everything as something on-hand. Instead we must adopt the transcendental "viewpoint and position of questioning" (Blick- und Fragestellung), which brings to light the originary relation of experience and the object of experience. Kant’s breakthrough in judgment, reads Heidegger, is based on the transcendental attitude which does not analyze articulated objects themselves (as in science and everyday experience) or the form of articulations considered apart from the objects (as in logic) but thinks the prior, originary unity of the two. Kant thereby regains some measure of the logos-phenomenon unity of the ancient Greeks and of phenomenology. Heidegger thinks the union of language and thing happens within the pre-subjective and pre-objective open “between,” and he thinks Kant’s transcendental thinking discloses this domain. Both the logical and mathematical prejudices are thereby subject to critique.

Heidegger’s reading of Kant becomes even more significant when we understand it in the context of the previous semester course. Heidegger's interpretation of Kant’s highest principle of synthetic judgment echoes his interpretation of Parmenides’s saying about the reciprocal belonging-together of apprehension and being. But it is not enough for us simply to note Kant’s Parmenidean-like originality. This similarity illustrates the thesis about Kant the 1935–1936 lecture course advocates, namely, the transitional role Kant plays in overcoming the disjunction, "being and thought." We recall that in the 1935 lecture course, Heidegger specified two requirements for the overcoming of the disjunction. (a) The first was to show the limits of its inceptive truth. As we have seen, Heidegger thinks that the tradition takes its bearing from the end of the inceptive beginning. The end of the beginning saw the emergence of the logical prejudice which becomes, in modernity, the mathematical prejudice. But Kant shows the limits of the mathematical prejudice and in doing so shows the limits of inceptive truth.

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