It’s what we work with as photographers.
We observe light with our eyes.
We think about our composition with our mind.
We record light light with our camera.
Without light we have nothing to photograph.
As photographers we should practice seeing everything as a light source.
To be more specific we should learn to observe light sources that illuminate, or transmit light, and light sources that reflect light.
An example of an illuminating/transmitting light source is the sun. Everything that the light of the sun falls on becomes a reflective light source.
Reflective light sources do not visually exist without a transmitting light source.