What does “that inward eye” stand for?

“ For oft, when on my couch I lie In vacant or in pensive mood, They flash upon that inward eye Which is the bliss of solitude; And then my heart with pleasure fills, And dances with the daffodils. ” 
What does “that inward eye” stand for?
【正确答案】：

"THAT INWARD EYD" STANDS FOR ONE’S MIND.

【题目解析】："That inward eyd" stands for one’s mind.