How could I know?

I was going to start off with
"i once knew this person"
but he was not just any person
nor did i ever know him

i knew him by his late night drives
rain racing down the car window
just him, his thoughts and the road
his laugh that was like quiet thunder
the songs that would run through his mind
and hum of it on his lips.

i never knew the person
who built up walls so high
that no one could touch him
whose monsters would dwell
in the darkest hollows of his mind
who lays wide awake at 4am
heart thumping against his ribcage
so wrought with abandon
he couldn't even breathe

He was living somewhere between reality and all that he had ever dreamed
This person used to be me.
I thought I knew him.
But how could I ever know someone who did not know himself.