It seems that people, especially those who have a certain idea of what philosophy is or should be, are quick to dismiss Heidegger and those who came after him as uttering nonsense, that what they are doing is performing fiction, not philosophy. This certainly appears to be so in the case of Carnap that Critchley pointed out. But Gadamer clearly believes that these people are missing out on a crucial philosophy, philosopher, and way of thinking about the world that should not go ignored. One gets the sense from Gadamer that what Heidegger was doing was a task so momentous and painstakingly necessary, that it is all but impossible to truly grasp his overall purpose without getting sidetracked. Perhaps Heidegger himself was aware of this.
Heidegger is concerned with "Being" - that which exists. Why does anything exist? How does a thing come into Being? How are beings related to each other and to Being, and what are the spiritual implications of these relations? Why, or How, did the Ancient Greeks comprehend this notion, and what has led us to fall astray from their teachings, and how should we read these teachings? At a deeper level, though, we could ask why it is that Heidegger cares so much - what is it that Heidegger and Gadamer see that Carnap and others don't?
Dasein... this notion seems to be almost a religious one. It seems almost like the gnostic version of the soul - a divine being trapped in a material realm. For Heidegger, it seems like Dasein is the beingness of a human, the manifestation of existence and consciousness; the ability to critique, reflect upon and negate that consciousness as well. It is a sort of force, not the self, but a way of being in the world unique to humans. This notion seems to be one of, if not the, major motivator and idea in Heidegger's thought - the human Dasein and it's relation to beings and Being.
Why yes, it has been years
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