In Shenk's eyes, Lincoln went through three stages of depression. The first hit in 1835, when he was 26, and remained through the ups and downs of his early political career in Illinois. "I am now the most miserable man living. . . . I must die or be better," he wrote in 1841. A presumed love affair with a friend who died, Ann Rutledge, has generally been cited as the cause of his first breakdown. But Shenk is skeptical, as he is also skeptical that Lincoln's second breakdown was caused by a temporary breakup with his future wife, Mary Todd.

Depressives overreact to small events as much as to major ones, and Shenk discusses any number of things -- including severe political troubles, profound doubts about Mary Todd, feelings for other women and bleak weather (a frequent trigger) -- that could have been the cause. More important is what turned Lincoln from thoughts of suicide, and that was a sense of purpose, an "irrepressible desire" to achieve something meaningful.

Lincoln's marriage in 1842 to the emotionally troubled Mary Todd marks the second stage, Shenk writes. Stoic resignation (though not the most auspicious mood for a wedding) replaced the public exhibitions of despair. He maintained that reserve; in 1850, this candid chronicler of emotion barely mentioned the death of his 3-year-old son, Eddie. And though Lincoln remained an unconventional thinker, he increasingly turned to the Bible for solace.

During this period, when he won election to the House of Representatives but lost out on two Senate seats, Lincoln adapted, Shenk says, working frantically and developing the discipline, creativity and perseverance that would later serve him in his political crusade.

Finally, in the mid-1850's, Lincoln transformed his personal struggle into a struggle for universal justice. He responded to the loss of the Senate race to Stephen A. Douglas in 1858 not with suicidal musings but with resolve: "The cause of civil liberty must not be surrendered at the end of one, or even, one hundred defeats." Later, during the dark days of the Civil War, President Lincoln wrote, "I expect to maintain this contest until successful, or till I die." His experience with melancholy provided him with the creative juice that inspired his greatest writings, as well as with the religious feeling that inspired his idea of nationhood and his own role as an "instrument" of a higher power charged with a sacred trust.

At the end comes the hero's triumph: the Emancipation Proclamation fulfills his lifelong dream. Referring to his earlier rejection of suicide, Lincoln told a friend that he had indeed accomplished something meaningful: "I believe in this measure my fondest hopes will be realized."

Shenk provides some fascinating details about Lincoln and offers a sensitive portrait of his emotional state. But in the end, no psychological profile can do justice to Lincoln's life. And the speculative "may haves" and "might haves" don't stretch far enough to connect cause with effect.