The lawyer enters this bet as a young man, debating capital punishment and life imprisonment at a party. (It might be worth noting that this topic isn't one most party-goers would find a light topic of conversation.) The lawyer interjects his opinion:
Capital punishment and life-imprisonment are equally immoral; but if I were offered the choice between them, I would certainly choose the second. It's better to live somehow than not to live at all.
Thus ensues a lively conversation. Finally, the banker tells him that he'd wager $2 million that the lawyer could not stay in complete isolation for even five years. Full of ego, the lawyer raises the terms:
"If you mean it seriously," replied the lawyer, "then I bet I'll stay not five but fifteen."
When looking at stories involving money, especially ones in which the plot hinges on money, it's important to consider how much that same quantity would be worth today. $2 million in the late 1800s would have approximately $50 million of buying power today. It becomes a bit easier to see why the lawyer would take the bet.
The lawyer sticks it out. During his fifteen years, he becomes fluent in multiple languages, studies history and philosophy, and then studies solely the New Testament for an extended period of time.
Before he is released at the end of his fifteenth year, he writes a letter rejecting the money and explaining his reasoning. He says that mankind takes "falsehood for truth and ugliness for beauty." He says that people consider themselves wise and beautiful, oblivious in their daily lives that death will wipe each of them from the world. They have exchanged the promises of Heaven for the temporary pleasures of Earth. And he wants no part of it.
The lawyer runs off without his money, having decided that money is foolish and life is fleeting. He abandons this society because he no longer believes in its values.
By the end of the story, the banker's attitude has changed significantly. He is no longer motivated by the two million rubles, for example, because money holds no value nor interest for him:
"I renounce the two million of which I once dreamed as of paradise and which now I despise."
In addition, the banker no longer values books and learning. During his fifteen-year confinement, for instance, the banker read voraciously, taking up everything from the Classics to light-hearted and "sensational" novels. By the end of his confinement, however, the lawyer realizes that this knowledge is worthless because it does not create happiness nor hold any real meaning:
"And I despise your books, I despise wisdom and the blessings of this world. It is all worthless, fleeting, illusory, and deceptive, like a mirage."
Finally, the lawyer's attitude also suggests that he no longer believes solitary confinement to be a better alternative to the death penalty. As we see in his letter, death's ability to destroy everything renders life, and all of its earthly pleasures, totally meaningless:
"Death will wipe you off the face of the earth as though you were no more than mice burrowing under the floor."