I've found that Franz Kafka's The Metamorphosis a particularly useful way of thinking about phenomenon of existence as it happens and the distinction between thinking theoretically about existence and practically existing. Gregor Samsa's experience of waking up and discovering that he has transformed into a "monstrous verminous bug" seems to adequately delineate both the average everyday experience of being (despite Samsa's condition being far from average or everyday) and the theoretical experience of being.
This existential delineation is established due to Samsa's transformation being incomplete. Samsa may have transformed into an insect but he still thinks like a human. A conflict exists between what he is, what he feels himself to be and what he thinks he is. He only discovers what he is (and not who) through his experience of Being-in-the-world. First, Gregor Samsa experiences himself as a monstrous sight:
"He lay on his armour-hard back and saw, as he lifted his head up a little, his brown, arched abdomen divided up into rigid bow-like sections. From this height the blanket, just about ready to slide off completely, could hardly stay in place. His numerous legs, pitifully thin in comparison to the rest of his circumference, flickered helplessly before his eyes."
Then Samsa experiences himself through the sensation of his new body trying to position itself. He is unable to move and discovers "a light, dull pain in his side, which he had never felt before." From this point onwards, and this is where it gets interesting, Samsa thinks about his life as a travelling salesman. He dwells on what is expected of him in his profession and how he can meet those expectations. He thinks about not being able to get out of bed and the passing time and being late for work (on the day that he has transformed into a huge insect). Samsa also thinks theoretically about his life as a salesmen while finding an itch in a strange new place covered with white spots and relieving the itch and feeling a secondary repulsive sensation. There is a clear disjunct between what Samsa feels himself to be and what he thinks himself to be or between the experience of being and thinking about being.
Through the course of the story Gregor Samsa slowly reveals himself to himself by Being-in-the-world and not how he thinks of himself in the world. His Being is brought forth and encountered and not delivered through the process of thinking, for Samsa (at first) does not think of himself as a bug. The phenomenon of existence as it happens provides Samsa with the new reality of his existence. He is a monstrous verminous bug and not a travelling salesman. His transformation is only completed, both mentally and physically, once Samsa accepts what is provided to him by his body existing in the world and not what he thinks of his body existing in the world.
Therefore, only by Being-in-the-world and experiencing experience as a phenomenon is Gregor Samsa able to experience himself as himself. He only uses empirical thought later to theoretically conceive of himself as he actually is. The phenomenon of experience comes first then the experience of the experience provides him with the reasoning that he is in fact a huge insect.
The phenomenon of existence as it is experienced or specifically Heidegger's articulation of Dasein or Being-in-the-world is therefore crudely delivered within the first few pages of Kafka's The Metamorphosis, and becomes a useful way, at least for me, of looking at certain aspects of phenomenology.
