William Blake once wrote these words to his friend, Thomas Butts:
"Now I a fourfold vision see,
And a fourfold vision is given to me:
'Tis fourfold in my supreme delight
And threefold in soft Beulah's night
And twofold always, may God us keep
From single vision and Newton's sleep!"
Fourfold vision is to single vision as ordinary sight is to blindness. We all experience single and threefold vision. It's twofold and fourfold vision that one must consciously work to achieve.