Discussed in this essay:

Philosopher of the Heart: The Restless Life of Søren Kierkegaard, by Clare Carlisle. Farrar, Straus and Giroux. 368 pages. $28.

Throughout its history, philosophy has been marked by figures who sought to demolish the prevailing intellectual systems of their moment—to practice “philosophy with a hammer,” as Friedrich Nietzsche put it—in order to look with fresh eyes at the most urgent human problems. As depicted in Plato’s early dialogues, Socrates railed against professional Sophists who charged fees to engage in what amounted to rhetorical games. By contrast, he merely wandered the agora, posing pointed questions about…