What does william wordsworth mean by "inward eye" in the following bolded line?

i wandered lonely as a cloud
that floats on high o'er vales and hills,
when all at once i saw a crowd,
a host, of golden daffodils;
beside the lake, beneath the trees,
fluttering and dancing in the breeze.

continuous as the stars that shine
and twinkle on the milky way,
they stretched in never-ending line
along the margin of a bay:
ten thousand saw i at a glance,
tossing their heads in a sprightly dance.

the waves beside them danced; but they
out-did the sparkling waves in glee:
a poet could not but be gay,
in such a jocund company:
i gazed—and gazed—but little thought
what wealth the show to me had brought:

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.

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