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It seems, when reading the three lines of poetry in "The White Horse," by D.H. Lawrence, that the reader does not have much to work with in trying to understand its meaning. However, it is in the specific questions of "before" and "after" this moment, and reading between the lines, that a reader may find personal meaning in this brief verse.
D.H. Lawrence felt he had included enough information to convey meaning to his reader, so we need to look more closely. Symbolically, the horse is white, which is the color of purity. The person standing next to the horse is "the youth," someone presumably that does not have a great deal of experience with the world. The horse does not move or make a sound as the youth moves to place a halter on the horse: the silence, in fact, transports both to "another world."
On a very basic level, I would assume that the horse belongs to the youth as the horse is stoic in the face of the halter and the young man. We can assume that this action has taken place many times. Perhaps because the person in the poem is a "youth," we can assume also that he is not rushing but able to enjoy the moment. There seems to be trust between the two in this silence. And the silence takes on deeper meaning, as does the poem, when the reader looks beyond the simple description.
The most important part of the poem, then, seems to be the summation of the details that lead to the last line, where we find our meaning:
They are so silent they are in another world.
Because of the repetitive nature of the actions described and the peace that lies between the lines of the poem, it is easy to imagine a boy and his horse having a true understanding of one another based not only on repetition of these actions, but by nature of the time they have spent together in the past. And the silence may reflect a recollection of the many times they have ridden before, the anticipation of more time to be spent together, or a peaceful slice of life that exists on its own, separate from the whirling world around them.
For these reasons, I believe the poem speaks to the magical camaraderie that exists between the "youth" and the horse.