Meditations 8.57

The sun appears to be poured down, and in all directions indeed it is diffused, yet it is not effused. For this diffusion is extension: Accordingly its rays are called Extensions [aktines] because they are extended [apo tou ekteinesthai]. But one may judge what kind of a thing a ray is, if he looks at the sun's light passing through a narrow opening into a darkened room, for it is extended in a right line, and as it were is divided when it meets with any solid body which stands in the way and intercepts the air beyond; but there the light remains fixed and does not glide or fall off. Such then ought to be the outpouring and diffusion of the understanding, and it should in no way be an effusion, but an extension, and it should make no violent or impetuous collision with the obstacles which are in its way; nor yet fall down, but be fixed, and enlighten that which receives it. For a body will deprive itself of the illumination, if it does not admit it.

Marcus Aurelius, Meditations 8.57, translated by George Long

The sun's light is diffused down, as it seems, yes, and in every direction, yet it does not diffuse itself away. For this diffusion is an extension. At any rate the beams of the Sun are called Extension rays, because they have an extension in space. And what a ray is you may easily see, if you observe the sun's light entering through a narrow chink into a darkened room, for it extends straight on, and is as it were brought up against any solid body it encounters that cuts off the air beyond. There the ray comes to a standstill, neither slipping off nor sinking down. Such then should be the diffusion and circumfusion of the mind, never a diffusing away but extension, and it should never make a violent or uncontrollable impact against any obstacle it meets with, no, nor collapse, but stand firm and illuminate what receives it. For that which conducts it not on its way will deprive itself wilfully of its beams.

Marcus Aurelius, Meditations 8.57, translated by C. R. Haines