The title "Of Mice and Men " is one of the symbols of the novel and is not meant literally, though both mice and men feature prominently in the book. Steinbeck is not suggesting an either/or scenario for humans as animals. In fact, interestingly enough, Steinbeck didn't actually coin...
The title "Of Mice and Men" is one of the symbols of the novel and is not meant literally, though both mice and men feature prominently in the book. Steinbeck is not suggesting an either/or scenario for humans as animals. In fact, interestingly enough, Steinbeck didn't actually coin the phrase "of mice and men." He took the title from a line in a poem called "To a Mouse," which was written by poet Robert Burns in 1785:
The best-laid schemes o' mice an 'men
Gang aft agley,
An'lea'e us nought but grief an' pain,
For promis'd joy!
This stanza, as well as the entire poem itself, shares common elements with the novel. The lines above suggest that our most detailed plans can, instead of "promised joy," bring us "nought but grief and pain." This is exactly what happens to Lennie and George: their plans to own a farm with rabbits where they can enjoy the fruits of their labors (promised joy) are ultimately unfulfilled and are, instead, replaced with violence, isolation, and death (grief and pain).
The title is also indicative of a power play between a man, symbolizing strength, and a mouse, symbolizing insignificance. Though one might assume that Lennie is a mouse and Curley is a man, if readers look at the personalities of each character, it becomes clear that, morally, Lennie is "large"--he is generous, caring, and hard-working, whereas Curley is "small"--he is morally corrupt, cowardly, and mean.
Though it's tempting to take the title "Of Mice and Men" literally due to the role that each plays in the novel, there are several deeper meanings to consider which can add to the enhancement of the reading experience. Reading the poem "To a Mouse" can provide a near-parallel plotline to George and Lennie's experience. Additionally, looking at the characteristics of a "mouse" and a "man" and analyzing the characters of Lennie, Curley, and even George and Candy, can provide further insights.