In the poem "Who Has Seen the Wind" by Christina Rossetti, the narrator asserts that while the wind is completely invisible and no one can see it, we can know that that it is there because of how the leaves on the tree "hang trembling" and "bow down their heads." Though the wind is not something we can observe directly, we see evidence in its presence all around us.
Similarly, in W. O. Mitchell's novel of the same name, the protagonist, Brian, becomes keenly aware of a force at work that causes things to come to pass in the world around him. He feels rare moments of great spiritual joy, much like the wind on his back. Though he cannot see it, he feels that there must be some greater force at work.