After unpacking the sense in which Heidegger uses the term "contradiction," the chapter reviews his use of it in the strictly logical, “injunctive” sense as the “principle of non-contradiction” (PNC) and “law of thinking,” particularly as he wields it in the course of exposing what otherwise appear to be cases of vagueness or ambiguity. It then reviews his tendency in some contexts to align the PNC with a metaphysics that restricts being to being on hand (vorhanden) and the dilemma that this ontological interpretation presents, given his apparent adherence to the principle, even as he proposes a broader understanding of being. The chapter then suggests that his analysis of attunement in the 1929/30 lectures introduces a more expansive reading of the PNC and that this reading is corroborated by his existential interpretation of the principle in the Winter Semester of 1933/34. The interpretation is a ringing endorsement of the PNC and the sameness of reference it enjoins as a condition for being-with-one-another. The chapter concludes by probing the implications of this ontologically broader – or post-ontological – interpretation of the PNC for thinking and speaking of being itself, riddled with “nothingness” as it is.

Difference is a condition of being separate and dissimilar, on the basis of which we are capable of drawing distinctions. We typically distinguish between entities within the same or approximately the same domain (e.g., black and white, house and garden), between entities in different domains (e.g., God and the number five), and between different domains of entities (e.g., chemical and psychological agents). Not to be confused with any of these differences, though underlying all of them, is the most essential difference: the difference between entities and being.

Situating Cassirer in a historical perspective, Daniel D. Dahlstrom's chapter casts light on prominent lines of convergence and divergence between Husserl’s phenomenological analyses and Cassirer’s philosophical studies. The general topic of the first line of convergence is logical theory, as Husserl and Cassirer both argue for the autonomy of logic, the promise of set theory, and the intensionality of concepts.Other lines of agreement include their common rejection of empiricist accounts of abstraction and universals, their embrace of a Kantian philosophical legacy, and their respective commitments to the primacy of meaning and self-described versions of idealism. Nevertheless, the philosophies of Husserl and Cassirer diverge from one another in significant ways, primarily in view of the thematic range of their investigations and their respective insistence upon intuition and the sign or symbol as the basis of human consciousness and cognition. Dahlstrom focuses on differences in Husserl and Cassirer's analyses of intuitions and perceptions that Cassirer himself also pronounced.

This essay elaborates the impact of idealism on Heidegger's thinking, in the context of the early transcendental and metaphysical phase of his thinking and in the context of his subsequent attempt to think being historically and non-metaphysically. Part I details how Heidegger in that first phase leaves the door open for characterising his own existential analysis and fundamental ontology as a new form of idealism, albeit an idealism with stark similarities to Kant's transcendental idealism. In the second phase, however, in a fit of self-criticism and criticism of Western thinking from its beginning, he rejects idealism tout court, seeing it as a form of Platonism that is oblivious to the historical character of being. After reviewing Heidegger's rendition of how Plato's interpretation of the idea sets the stage for modern idealism, Part II glosses his critical accounts of the pre-absolute and absolute forms of the latter. The third and final part of the essay reviews some implications that Heidegger draws from what he regards as the impact of idealism on Western thought.

I

In SZ, Heidegger's existential analysis, undertaken for the sake of fundamental ontology, has a certain kinship with ‘idealism’, as he understands the concept. This understanding departs from the way the concept was typically understood in his day, namely, as an epistemological doctrine, one that forms the counterpart to realism. Both epistemological doctrines are said to arise from the so-called problem of the reality of the external world. Since any sense of reality presupposes being-in-the-world, Heidegger rejects as senseless any attempt to question or to prove the reality of the external world, or – in the case of Dilthey – merely to explain the source of belief in that reality. These attempts confuse the world-phenomenon (characteristic of Dasein's ‘being-in’) with the being of inner-worldly entities (SZ 202, 206f.). Whether the problem of reality is conceived along strictly epistemological lines or via phenomenological improvements of the concept of subject or consciousness, it fails because the corresponding conceptions of knowing or consciousness lack an appropriate existential analysis.