In "The Solitary Reaper," we know that the highland lass is alone (solitary) and we know that she is unaware of the presence of the speaker. Therefore, she thinks she's alone and thus acts naturally. This is important because of the strong connection between this lass and nature itself. She reaps and binds the grain which further illustrates her connection with nature. And her song is melancholy and so beautiful that the speaker is overwhelmed. She sings in a dialect the speaker of the poem is unfamiliar with ("Will no one tell me what she sings?"). Thus, the speaker imagines she sings of ancient battles and stories. It's important to reiterate that she doesn't know the speaker is there watching her. Had she been aware of his presence, she might have darted away like a frightened animal. Since she feels alone, she acts "naturally," again underscoring her close connection to and within "nature."