Sarah Bakewell in The New York Times:
If the proof of a pudding is in the eating, and the proof of a rule is in the exceptions, where should we look for the proof of a philosophy? For Friedrich Nietzsche, the answer was obvious: to test a philosophy, find out if you can live by it. This is “the only critique of a philosophy that is possible and that proves something,” he wrote in 1874. It’s also the form of critique that is generally overlooked in the philosophy faculties of universities. Nietzsche therefore dismissed the professional discipline as irrelevant, a “critique of words by means of other words,” and devoted himself to pursuing an idiosyncratic philosophical quest outside the academy. As for texts, he wrote, “I for one prefer reading Diogenes Laertius” — the popular third-century Epicurean author of a biographical compilation called “Lives of the Eminent Philosophers.” If the proof of philosophy lies in life, then what could be more useful than reading about how the great philosophers have lived?
As James Miller shows in his fascinating “Examined Lives,” choosing Diogenes Laertius over more rigorous treatises was provocative because it challenged an idea already predominant in Nietzsche’s time: that a philosophy should be objectively valid, without the need to refer to particular quirks or life experiences on the part of its originator. Diogenes Laertius represents an older tradition, which sees philosophy not as a set of precepts but as something one learns by following a wise man — sometimes literally following him wherever he goes, listening, and observing how he handles situations. The “Lives” offers its readers a vicarious opportunity to try this with a number of philosophers, and see whose way works best.