Light passes through windows. This is a puzzle. This is a complicated story, a story that no one understands, in four words.
Beams of sunlight pass through the library windows, making patterns on the wall.
Sitting where I sit, among the shelves and the piles of books, I see beams of sunlight passing through the library windows, making patterns on the wall.
My evidence for the existence of beams of sunlight is (at least) in two parts: I see dust motes dancing (dancing? what is it to dance? what kinds of things can dance?) in the sunbeams, visible (to me) in the light, where in other parts of the (dusty) library air, there are no (to me) visible dust motes dancing (or otherwise) in the air, and one explanation (is it the best one? what is best?) is that there is a sunbeam, or at least a beam of light, passing through the air there. (How does a beam of light pass through the air, let alone through the class of the windows?)
The second part of my evidence is the patterns on the wall. I know, or I remember (in memories that I have now, at this moment, the only moment, the only entity, that exists) that the wall is really (what could that possibly mean?) more or less uniform in color; vaguely white, and the same vague whiteness more or less all over. But what I see, or what I see at some level of awareness (what are levels of awareness? what is awareness? who is aware?) is a complex pattern of light and dark, triangles and rectangles and more complex figures laid out and overlapping, and I theorize (automatically, involuntarily, whether or not I intend to) that these brighter shapes and triangles are where the sunbeam (passing through empty space, and then the air, the window, the air again) strikes the wall, and the darker areas, the ground to the sunbeam’s figure, are simply the rest, the shadows, where the sunbeam does not strike, or does not strike directly.
(Or the dark places are where the frames of the window and the edges of shelves and chairs, things outside and things inside, cast their shadows, and the light places, the ground to the shadows’ figure, is the rest, where the shadows do not fall; figure is ground and ground is figure.)
Can we avoid all of this complexity, if we hold Mind as primary? I am. Or, no, as generations of philosophers have pointed out, feeling clever to have caught Descartes in an error, not “I am” but only something along the lines of “Thought is”. If there is a thought that “thought is”, that can hardly be in error (well, to first order). But if there is a thought “I think”, that could be an error, because there might be no “I”, or it might be something other than “I” that is thinking.
Second attempt, then! Can we avoid all of this complexity, if we start with “Thought is”? Or “Experience is”?
Experience is. There is this instant of experience. In this instant of experience, there is a two-dimensional (or roughly two-dimensional) surface. Whether or not there is an understanding of dimensions and how to count them, there is either way still a two-dimensional surface, here in this experience. In some places, the two-dimensional surface is bright. In some places, it is dark; or it is less bright.
Whether or not there is any understanding of brightness and darkness, what might lead to brightness and darkness, anything about suns or windows or how they work, there is still this brightness, in this single moment of experience, and there is still this darkness.
(Whether the darkness is actually bright, just not as bright as the brightness, whether the surface is really two-dimensional in any strong sense, or somewhere between two and three dimensions, or whether dimensions are ultimately not a useful or coherent concept here, there is still, in this singular moment of experience that is all that there is, this experience, this experience, which is what it is, whether or not the words that might be recorded here as a result of it (and whatever “result” might mean) are the best words to describe it.)
And whether it is described at all, whether description is even possible, does not matter; the main thing, dare I write “the certain thing”, is that this (emphasized) is (similarly emphasized).
So.
From this point of view, we may say, Mind is primal. Mind, or whatever we are attempting successfully or unsuccessfully to refer to when we use the word “Mind”, does exist, or at any rate has whatever property we are attempting to refer to when we say “does exist”. Except that “refer to” and “property” are both deeply problematic themselves here.
This is why the ancient Zen teachers (who exist, in this singular present moment of experience, only as memories of stories, memories that exist now) are said to have, when asked deep and complex questions through the medium of language, and impossibly concerning language, have responded with primal experience, with blows or shouts or (more mildly) with references to washing one’s bowl, chopping wood, carrying water.
We can remember that there is this. So, what next?
Language (this language, many other languages, those languages that I am familiar with, that I know of) assumes time.
Without the concept of time, it is difficult to speak, to write, to hypothesize.
Let alone to communicate.
To communicate!
The impossible gulf: that you and I both exist (what does it mean to exist?) and that by communicating (whatever that might be) one or both of us is changed (otherwise why bother?). But change, like time (because time), is an illusion.
So!
Is it necessary (categorically, or conditionally) to participate in the illusion? Or pretend? To act (or to speak, as speech is after all action) as though time and change were real?
The sun comes through the windows and casts shadows on the wall. Is there someone at the door?
ceoln 
Hm?