The best day of my life — my rebirthday, so to speak — was when I found I had no head… I had for several months been absorbed in the question: what am I? The fact that I happened to be walking in the Himalayas at the time probably had little to do with it; though in that country unusual states of mind are said to come more easily… What actually happened was something absurdly simple and unspectacular: I stopped thinking… Past and future dropped away. I forgot who and what I was, my name, manhood, animalhood, all that could be called mine. It was as if I had been born that instant, brand new, mindless, innocent of all memories. There existed only the Now, that present moment and what was clearly given in it.
… It took me no time at all to notice that this nothing, this hole where a head should have been, was no ordinary vacancy… it was very much occupied. It was a vast emptiness vastly filled, a nothing that found room for everything — room for grass, trees, shadowy distant hills, and far above them snow peaks like a row of angular clouds riding the blue sky. I had lost a head and gained a world.

-D. E. Harding, “On Having No Head”, The Mind’s I (Ed. D. Hofstadter, D. Dennett) pp. 24-30

We don’t see our own heads. As D.E. Harding so humorously writes, we never experience our heads. Instead, we experience these wondrous holes in which all of our experiences, entire universes, somehow miraculously fit.

Later in the essay he notes something about movie production: When we see a memory or dream sequence that includes the person as we would see him, say, the child they once were, it lacks realism. A good producer would film the scene from the person’s perspective, placing the camera where his eyes would be. We should never see the person’s head (although perhaps a reflection of it).

An Empiricist places the most confidence in things in his physical experience that he could repeat and show others at will. Des Cartes questions that position. We can never rule out a trick of the senses or a “Deceiving Daemon”. In fact, there is only one thing he believed we can be absolutely certain of — Cogito ergo sum, I think, therefore I am. What he meant by this is that I know I exist because I’m the one doing the thinking, wondering what I could know for certain, and whether I could be certain I exist. The existence of the question itself is proof of its answer.

Returning to Harding’s idea, we are actually more sure of that wondrous vacancy than of the things we see. Whatever the truth of the things I see, the fact that I’m there seeing them is more sure to us.

In the general introduction to Alei Shur vol I (pg. 12), Rav Shelomo Wolbe zt”l writes:

We read in Berakhos 10a: “These five [passages of Tehillim that begin] “Borkhi Nafshi” (My Soul shall Bless), corresponding to what did David compose them? He didn’t say them but corresponding to HQBH and corresponding to the soul.
“(1) Just as HQBH fills the whole world, so too the soul fills the whole body. (2) Just as HQBH sees but Is not seen, so too the soul sees but is not seen. (3) Just as HQBH nourishes the whole world, so to the soul nourishes the body. (4) Just as HQBH is tahor, so too the soul is tahor. (5) Just as HQBH ‘dwells’ in the rooms of rooms (chadrei chadarim, an idiom: in a very hidden ‘location’), so too the soul dwells in the rooms of rooms.
“Let the one that has these five things, and let it give praise to He Who has these three things!”
We find that we can learn from this that the soul in particular can praise HQBH, because only it as “an aspect in common” (tzad hashaveh) with him, as it were. Only from the aspect of the soul can man serve his Creator, and in particular the “duties of the heart/mind” (chovos halvavos) which are associated with the soul — they are the essence of such service!
Also this we learn from their statement, that among the attributes of the soul is to be something that “sees but is not seen”. In this, Chazal explain to us what ruchniyus (“spirituality”) is in its entirety: it nourishes the whole world and the body and fills it; the root of every created thing in the world, and every limb in the body is in ruchniyus, and from this root life reaches them. This spirituality fills the whole existence until “there is nothing free from it”. This ruchniyus is itself tahor, it is internal, “dwelling in the rooms of rooms”…. Chazal reveal the central point, upon which we must base our avodah (service of Hashem) if we want to work in ruchniyus, and that is “Just as HQBH sees but Is not seen, so too the soul sees but is not seen.”

In an endnote (pg. 339), Rav Wolbe adds this comment from a student:

It would seem that from the words of Chazal it is not compelling that the central point of the five is in particular this one [i.e. that the soul “sees but is not seen”] of the five that features that Chazal enumerate there. However, one of the students of the yeshiva n”y found a source for it from what it says in Devarim Raba 20:26, “Let the soul come, which sees and isn’t seen, and let it call to HQBH Who sees but Is not seen.” There is doesn’t mention all five criteria, just this one — for it is in truth the central point in avodah.

This idea is the core of Harding’s observation; our soul “sees but is not seen”. The notion of “sees but is not seen” is what makes the spiritual more fundamental, the source, and the nourishing force of the physical. And, as we saw above, the observer is actually more certain and more real than the observed.