A soldier and historian's own memories and defense of Socrates -- the other primary source for the historical Socrates, alongside Plato.
Xenophon knew Socrates personally, but unlike Plato he was a soldier and historian rather than a philosopher by temperament, and his Socrates comes across differently -- more practical, more concerned with everyday ethics and household management than with the abstract metaphysics Plato's Socrates often pursues. Written partly as a direct defense of Socrates against the charges that led to his execution in 399 BCE, the Memorabilia gives modern readers their only substantial alternative to Plato's portrait -- and the differences between the two accounts are exactly what make reconstructing the actual historical Socrates so difficult.