Heidegger notes somewhere there was a story from the war (I don’t recall whether it was WW1 or WW2) where a report was made that a fort had been taken and in fact the officer looked at the fort in the distance and saw a friendly flag was flying. The disaster came when the soldiers approached the fort “as though” it was friendly only to find out the flag had been “mis-perceived” and the fort was still in control of the enemy. The two-fold lesson is that we deal with life according to an “as though” disposition, and the “mis-taking” of the flag “as” friendly shows we are always “taking as.” And so, the false report became what the Greeks called the “hupokeimenon” for the “mis-taking” of the flag “as” friendly in a false-seeing. Similarly, Heidegger gives the example of hearing a living thing in the forest only to look down and see you “mis-took” rustling dead leaves in the wing “as” a living thing. Our “stance-toward” the world is “taking as,” either correctly or incorrectly.

Now, the history of philosophy has taken the essence of the self as an empty self-sameness, and so we say we can’t doubt the existence of the self because our doubting implies the self’s existence, and so every time we perceive something also apperceive the self. In this way, we reduce self-knowledge to an empty vacuity, the self is self-same, which neither clarifies what this means or why this is so. Heidegger comments:

But the “oneself” and the self which is thereby determined as merely something self-same remain empty and are filled only out of what is objectively present and lying there and at the moment dealt with by the human being. The to-oneself has no decisional character and is without knowledge of the bond to the occurrence of Da-sein. Heidegger, Martin. Contributions to Philosophy (Of the Event) (Studies in Continental Thought) (p. 253). Indiana University Press. Kindle Edition.

The discovery of self-sameness in the decision to turn back on the self to discover its nature completely forgets the decision making and hence leaves this ground out of the account of the foundation of the self as merely self-same.