CoachA primary source makes its own light: the Sun and the stars, flames, glowing filaments, the white-hot steel of the forge, fireflies, lightning. Its light exists whether or not anything else shines — switch off the world, and a primary source still glows.
Examples
Example 49.4 (The Moon’s file, closed)
The childhood verdict now has its formal words: the Moon is no primary source; it is a diffusing object of planetary size — gray rock scattering sunlight in all directions, our way included. So moonlight is secondhand sunshine, and the “dark side” is simply the half whose file holds, at that moment, no light to forward.