We meet Tom Joad at the beginning of John Steinbeck's The Grapes of Wrath, just as he is coming home to his family after being released (on parole) from prison. Because of the Great Depression and the takeover of property by the banks, the Joads and thousand of other families have been forced out of their homes and are preparing to leave for California--the Promised Land full of opportunity. Or so they hoped.
Since he has been away, Tom knows none of this until he arrives. He should not go with his family because he is on parole; but he believes that the future is going to happen as it will, no matter what, so he travels with his family. His rather fatalistic attitude is demonstrated in his belief that one voice is powerless to effect any kind of change.
Over the course of the novel, however, the Joads experience hardships and injustices which begin to reshape Tom's thinking. The migrant workers are treated worse than animals, food is wasted rather than used to feed starving families, and violence is the most effective method used by landowners and authorities to maintain control. When Tom kills the man who killed Jim Casy, someone who has been fighting to unite the workers in hopes of improving their circumstances, Tom is forced to stay in hiding, giving him time to reflect on the injustices which surround him.
By the end of the novel, Tom is able to articulate the beginning of his new philosophy to his mother, though he does not yet have a concrete plan. When she asks where he plans to go and what he hopes to do, Tom can only tell her he has decided that maybe a
"fellow ain't got a soul of his own, just little piece of a big soul, the one big soul that belongs to everybody, then...it don't matter. I'll be all around in the dark - I'll be everywhere. Wherever you can look - wherever there's a fight, so hungry people can eat, I'll be there. Wherever there's a cop beatin' up a guy, I'll be there. I'll be in the way guys yell when they're mad. I'll be in the way kids laugh when they're hungry and they know supper's ready, and when the people are eatin' the stuff they raise and livin' in the houses they build - I'll be there, too."
Tom's view that no one person has the ability to change the world has been transformed, because of his family's experiences and Jim Casy's death, into his belief that he is part of a greater cause and can effect change by standing with others against injustice.