Annotations from Heidegger Explained: From Phenomenon to Thing by Graham Harman, specifically from Harman's section regarding Being and Time.
...the question of being has been forgotten since the days of ancient Greece. Being is now assumed to be something present for human view, or physically present: being is spirit, or atoms, or God, or will to power, or phenomena in consciousness.
In Being and Time, Heidegger wants to revive the forgotten question of being through an analysis of human existence. He chooses Dasein [human beings] as his topic because only Dasein can ask the question of being in the first place; to understand what the question of being means, we must first understand the structure of Dasein.
No Dasein is a mere set of properties: Dasein can only be understood as the event, act, or performance of its own being. It is never entirely visible from the outside.
...the "horizon" of the question of the meaning of being is none other than time. When he says that time is the horizon, this means that only within the concept of time can we reach any proper understanding of being at all.
In posing the question of the meaning of being, Heidegger realizes that many people consider it not to be a real question at all. The first task of his book is to reawaken the need for this question, which has withered away since the high period of ancient Greek philosophy.
...every question has three parts: that which is asked about, that which is interrogated, and that which is to be found out by the asking.
... on closer examination, we simply have our old friend the threefold temporal structure. Every question analyzes something that is already given to us (past: the interrogated), in order to find out something new (future: that which is to be found out), with the resulting combination giving us the question as a whole (present: that which is asked about).
He begins by saying that beings must be interrogated to learn about their being. But just one paragraph later, it is human Dasein alone that is hauled in for questioning. Since it is humans who ask the question of the meaning of being, he thinks that we need to clarify the being of the human questioner in order to understand the question properly. Even so, this can be only a first step. As Heidegger memorably puts it: the being of beings is not itself a being.
[comparing the quest for ever less vague understanding of the being of Dasein, Harman refers to Plato's Meno. Though the sophists think it fruitless to question the search for virtue as either we already know what it is, or, if we don't know what it is, then we have no way of recognizing what it is when we find it.]
Heidegger's response is essentially the same as that of Socrates to the Sophists: we do know what being (or virtue) is in advance, but only in a vague, partially defined way, and not yet as the rigorous concept we seek. In present-day philosophy this is often called the "hermeneutic circle," or circle of interpretation.
"Ontological," of course, refers to being. "Ontic," by contrast, pertains to specific beings.
Unlike countless other great philosophers, Heidegger does not believe that his own philosophy is the final one.
Dasein is in each case mine; "mineness" is in fact its key feature. What distinguishes Dasein from bicycles, mushrooms, or even dogs is that Dasein's own being is always an issue for it. I am constantly occupied with my own being, and with how things are going for that being. Dasein is not made up of a list of qualities: its essence is nothing but existence. Only Dasein has existence, and only Dasein has "mineness".
Heidegger insists that the proper way to gain a philosophical understanding of Dasein is not to look for a special case of it.
..we should consider Dasein in its "average everydayness."
..we must never forget that Dasein is a "who," which means an event action, action, or performance, and not a "what" that can be seen from the outside. This sweeping caveat rules our every past interpretation of human being, especially the two most influential: the Greek concept of humans as rational animals, and the Christian idea of humans as created in the image of God.
...being-in means that Dasein is immersed in the world, involved with it, permanently intertwined and occupied with it even when it feels alienated or lonely. For this very reason, Heidegger says that only Dasein can touch other entities, since only Dasein has access to them or is truly aware of them.
Too many philosophers have construed their model of human being by imagining humans as entities that know the world. Heidegger sees that knowledge is only a rare special case of the way we deal with our environment, as his tool-analysis will brilliantly show. Knowledge is not primary, because it arises from out of the world. Dasein somehow has to rise above its usual interaction with the world in order to gain anything resembling knowledge. Dasein and world are bound together closely from the start. If this seems to eliminate the traditional problem of philosophy of how human beings can know a world lying outside of them, then so much the better. For Heidegger as well as Husserl, this is a false problem that never should have existed in the first place.
The usual idea of the world as "nature" must be rejected (just as Husserl would reject it) because the concept of nature arises from our average way of encountering entities. We need to look at the environment and how Dasein deals with it. Things are not present-at-hand in this environment, but are usually encountered as equipment.