Reading Heidegger is difficult, which is why secondary literature is often necessary. Graham Harman is widely regarded as an authority in this field. His great achievement in Heidegger Explained is that he clearly explains the different phases of Heidegger’s thought while consistently holding on to Heideggers “single great thought” = "Being is not presence."
Being is not Presence
==> Harman clarifies this through Heidegger’s distinction from his teacher Husserl. Whereas Husserl in phenomenology seeks Essence and A Priori structures – "das Seiende als Seiendes", i.e. through appearances to consciousness
==> Heidegger shifts the focus to existence: Dasein.
Dasein, the being that is “thrown” into the world, inseparable from its environment, and for whom truth emerges in this very situation. In this way we can link Dasein as an Event able to experience Truth.
The classical science/ concept of Truth (the search for essences and a priori structures) has, for Heidegger, no validity, since there is always something unknown, a dimension of concealment. This difference with Husserl, and the way it is explained, is one of the book’s strengths. To truly grasp this, understanding of 'Dasein' as Heidegger conceives it is essential, yet this is usually not well explained in secondary literature. Can it even be explained at all?
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Truth as an Event
For Heidegger, truth is not a fixed property but an event (different from 'Essence' or 'What is'): Sein/Being shows itself and withdraws, in a play of presence and absence. “Truth arises only from Dasein being in the world.”
Truth is an event, an act, a performance, and not a mere mirroring from the outside (p. 33). Since there is always also fallenness (ruinance linked to well explained concepts as The They, Fear/Angst, Curiosity ... ), philosophy must form a counter-movement (p. 30–31).
Ontological Difference and Freedom
This ultimately leads to Heidegger’s core concepts: the ontological difference and transcendence. As he writes: “Dasein transcends itself.” (p. 51).
Transcendence means freedom:
“Another name for transcendence is freedom … Dasein is a creature of Distance. Only through distance do we gain a true nearness to things … Only because Dasein transcends and nihilates can Dasein ever ask ‘Why?’ about anything.” (p. 53).
From this it follows that truth and knowledge are always dependent on Dasein’s capacity to relate to what reveals and conceals itself. “All objectifying presupposes transcendence … there would be no truth without Dasein.” (p. 75).
For me, this is the essence of Heidegger. For even greater depth one is better off reading Heidegger’s shorter works such as What is Metaphysics? or Gelassenheit (Gelatenheid in the Dutch version).That is my personal opinion, since Being and Time is virtually unreadable on a first reading unless one is already familiar with Dasein – a concept not even mentioned in What is Metaphysics? The rest of Harman’s book delves into the various concepts across Heidegger’s oeuvre.
What I especially appreciate is Harman’s clear explanation of the difficult concept of being-towards-death in just one page (p. 71).
==> As Dasein is constantly tranquilized by “the They” through absorption, fallenness, and endless distractions, always saying “not yet”), it cuts itself off from genuine Angst which leads to Alienation.
==> In accepting this Angst, however, we become free to exist authentically, taking full responsibility for our own choices.
Same link with Guilt/Conscience which can never belong to 'The They' ... "The call of conscience gives no specific information but simply calls Dasein into its own possibilities, whatever they may be." ..." ... the call of conscience strike us uncanny" (p. 72)
conclusion
Harman offers one of the most accessible and clear introductions to Heidegger available. His strength lies in maintaining focus on Heideggers's core idea (being is not presence) throughout all phases of Heidegger’s thought. So you are able to grasp it. It is unconcealed!
In the rest of the book Harman’s focus is tool-being themes, which is understandable given his own OOO project. For me personally, the existential-humanist aspect of Heidegger remains the most compelling, but I certainly regard Harman’s work as a valuable guide for gaining a better understanding of Heidegger.