The Impossibility of Planning
The most famous quote from this poem, "The best laid schemes o’ Mice an’ Men/Gang aft agley," resoundingly states that nobody, from the simplest creature to the most advanced, is capable of controlling their fate perfectly. The mouse and the farmer have no intention of harming one another, but their lives intersect in a way that creates disaster for the mouse and inconvenience for the farmer. This coincidental encounter demonstrates that, despite each one's careful quest for safety and desire not to harm others, they impact each other simply by virtue of existing on the same planet. This is true despite the fact that the farmer, as a human, is capable of advanced planning that the mouse isn't—as he notes ruefully, the complexity of his plans only creates greater heartbreak when they go "agley."
The Protection of the Innocent
Many of Robert Burns's fellow Romantic poets were deeply concerned with the status of children and the weak in industrializing societies. The speaker of "To a Mouse" uses a doting, parental tone when addressing his subject, and his worries about the safety and vulnerability of the mouse reflect these broader concerns about the vulnerabilities of children and childhood itself. The farmer concludes that, given his power over the mouse, he actually is responsible not merely for avoiding doing harm to it but for making small sacrifices in order to protect it. Like a child relying on a parent, the mouse cannot survive without the farmer's supply of food, and the farmer considers it his obligation to share. Meanwhile, the farmer's use of a Scottish dialect as he expresses compassion with the mouse suggests that this relationship between the strong and the weak can exist, not only between species or between adults and children, but between societies—in this case, between dominant English speakers and their Scots-speaking counterparts.
The Uniqueness of Humanity
The speaker of "To a Mouse" reflects that being born human has granted him certain privileges, but that these privileges aren't entirely positive—they come with burdens for others, as well as additional responsibilities for the human. Humans, he notes, hold power over the natural world and other creatures (especially given the early-industrial revolution era of the poem's writing). As a result, they're able to ensure their own well-being, while other animals like the mouse must fight to survive. At the same time, this creates an unsatisfying and even alienating rift between the speaker and the mouse, and between humans and the natural world. The farmer feels guilty and even sad when he notices that the mouse is afraid of him, and realizes that this is a result of human dominance. At the same time, the intelligence that allows the speaker to live securely and safely also makes him unhappy—it's the same intelligence that lets him remember and plan ahead, and those abilities bring him regret, anxiety, and uncertainty that the mouse will never face.
