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Perceptions in poetry are very subjective as poetry speaks to many people in many different ways. These are my personal reflections.
In "Lost in the forest..." by Pablo Neruda, the themes of memory and loss are brought to the reader with the narrator's "voice"—that enable to reader to better recognize and find the meaning of his thoughts, or at least of the tone of the stanzas.
The speaker notes that he is lost, and the whisper of twig from the forest could be one of three voices:
...of the rain crying, / a cracked bell, or a torn heart.
As the poem continues, it seems that the speaker is lost in a dream. Each sound of the "three voices" present a sound of pain: crying rain, a dissonant, cracked bell, or the figurative noise created by pain from a torn or broken heart.
In the second stanza, that which seems "deep and secret" to the speaker "shouts" to be heard, and it has the power to break through the leaves while "hidden by the earth":
Something from far off it seemed
deep and secret to me, hidden by the earth,
a shout muffled by huge autumns...
Perhaps that which is hidden by the earth is actually the clutter of memories that have buried some heartache that will no long reman silent but rise to the surface as with a "shout."
As the speaker wakes in the third stanza, a sense of this secret knowledge at first allows him to sing "under his tongue," a much more positive use of the voice. Upon waking, perhaps it is much the way we feel when we awake and realize, "Thank God, it was only a dream." This may allow his "singing" to be lifted up.
However, with waking, the speaker is now also very much aware of what does not disappear with wakefulness: his lost childhood—as his roots there "cry" to him, and his sense of loss is apparent as he tries to grasp perhaps an elusive scent, a memory, from the past that drives home the pain he is feeling.
Neruda lost his mother soon after he was born. His father remarried, but perhaps Neruda was trying to reconnect to his birth mother. The use of "voice," in one way or another, may simply symbolize his attempt to verbalize a pain that he may believe he was too young to understand as a baby, but now can perceive all too easily. The use of "voice" throughout the poem provides a perception based upon each "voice" used, as it runs through the poem.