Many many many many thanks for your help, Ronbee and Tdol!
Tdol, but I feel it's not any general state of sensory confusion, since the imagined event and the real/actual event are known. And it's clear from the poem which two events he can't tell the difference between. Chancing upon the daffodils is the first event (that is mentioned in the poem), and imagining the encounter with the daffodils when he lies on his couch (that's also mentioned.), is the second event. I feel that using indefinite articles insinuates that he isn't able to make the difference between any imagined event and any real event(not the events which took place in the poem),when he thinks of the daffodils.
Please please please again look at the poem for me, and tell me which articles are suitable there.

I am very very very very confused.

I am again putting the poem down here, and the answer as well.
I Wandered Lonely as a Cloud
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 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.
Q. How can the heart dance? Ans. Imagining the daffodils vividly enough brings his senses and emotions into play until he cannot tell the difference between an/the/that
imagined event and a/the
real one. And his heart starts dancing.