Solar illumination
The Sun is what makes the world intelligible (νοητός1) by its illumination. It appears very bright—too bright to see with the eyes. When looking directly at the Sun, it is void-black: it looks like the absence of itself. What creates the light that makes other things visible is not itself visible.
The planet Hermes (Mercury) is difficult to see in the sky, because it is always close to the Sun. This god is associated with the faculty of nous that mediates between human and divine consciousness; see also the Corpus Hermeticum.