Heidegger argues that traditional metaphysics focuses so much on the things being illuminated (individual beings) that it forgets the light itself (the Being that makes them visible).
: Originally, the Greeks understood Being as phusis —the power that "emerges and abides". For Heidegger, "Being essentially unfolds as appearing". This "appearing" is the process of stepping out of concealment into the light of unconcealment, or aletheia (truth). Heidegger – Introduction to Metaphysics - Light...
: Metaphysical thinking owes its ability to "see" beings to the light of Being. However, because metaphysics always represents beings as beings, the source of that light remains obscured. Heidegger – Introduction to Metaphysics - Light...