Diogenes is not a serious philosopher, but his book Lives of Eminent Philosophers is a major biographical source for all classical Greek and Roman philosophers until the 3rd century ad, when it is presumed that Diogenes Laertius lived. He has been counted as making reference to 365 books by 250 authors, and it is in his quotation of lost material that he is regarded as most reliable. Nietzsche, himself a considerable classicist, wrote of Diogenes that he is ‘by accident the clumsy watchman guarding treasure whose value he does not know’.