It would be hard to deny the truth of Burns' poem that the best-laid plans of mice and men can go awry. Whether or not we've personally seen this happen in our own lives, or in the lives of people we know, it occurs in general in human affairs—politics, statesmanship, war, and the economy. But perhaps the more important element of Burns' message, as Steinbeck uses it in his story, is in these lines:
But thou art blessed compared wi' me,
The present only toucheth thee,
But och! I backward cast my e'e
On prospects drear,
And forward, though I canna' see,
I guess, and fear !
The speaker seems to assume that an animal, as opposed to a person, has no sense of the past or the future. A little mouse has built her nest in the field, and is likened to a man in the sense that her work, destroyed in a moment by the plow, is vulnerable to destruction no matter how carefully it was planned. But Burns' final point is that this catastrophe doesn't have the devastating effect on her that a similar one would have on a human.
Obviously we can't know exactly what goes on in the mind of a mouse or any other animal. In using the famous quote from Burns, Steinbeck is implicitly drawing the same analogy about "plans," as they relate in the novella to the dream of a homestead in which George, Lenny and Candy will "live off the fat o' the land!" The deeper analogy, though, concerns Lenny himself. A mentally handicapped person has the same dreams as anyone else. By marginalizing and mistreating people like Lenny, the society of that time (and unfortunately probably still today to some extent) fails to recognize that (although it seems to trivialize it to put it this way) "everybody has feelings." Burns' message, as used by Steinbeck, is partly an ironic one. The very focus upon the mouse belies the speaker's apparent belief that man and mouse aren't so different after all. For all we know, the mouse goes through the same mental processes of regret, guessing, and fearing, as the human does. Empathy for all living beings is the ultimate theme of both Robert Burns and John Steinbeck.