I wonder what "it" of "it is not that" indicates. "it" is in the last third line from the bottom.
To the poet the world appears still more beautiful as he gazes at flowers that are doomed to wither, at springs that come to too speedy an end. The loveliness of May stirs him more deeply because he knows that it is fading even as he looks at it. It is not that the thought of universal mortality gives him pleasure, but that he hugs the pleasure more closely because he knows it cannot be his for long.

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I wonder what "it" of "it is not that" indicates. "it" is in the last third line from the bottom.

Question

I wonder what "it" of "it is not that" indicates. "it" is in the last third line from the bottom.
To the poet the world appears still more beautiful as he gazes at flowers that are doomed to wither, at springs that come to too speedy an end. The loveliness of May stirs him more deeply because he knows that it is fading even as he looks at it. It is not that the thought of universal mortality gives him pleasure, but that he hugs the pleasure more closely because he knows it cannot be his for long.

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I wonder what "it" of "it is not that" indicates. "it" is in the last third line from the bottom.

Answer

It = "not that the thought of universal mortality gives him pleasure"

The basic sentence structure is N-LV-N in which the predicate is the same as the subject.  In this case, the predicate noun is a relative clause.  This sentence can be rewritten to say:  The thought of universal mortality does not give him pleasure but he hugs the pleasure more closely because he knows it cannot be his for long.