Thucydides in the first book of his history, before enumerating the different αἰτίαι which were the prelude to the outbreak of hostilities, says: ‘but the true reason (ἰληθεστάτη πρόφασις), though it was never mentioned, was Sparta's fear of Athens' growth to power.’ The statement is repeated in other parts of this book, and we must assume that when it was written it represented Thucydides' considered judgement. These passages belong to the latest stratum of his work, and date from the time when he had formed a unified conception of what had previously seemed to him isolated events, namely the Archidamian War, the Sicilian Expedition, and the Decelean War. As he looked back after 404 they seemed parts of a whole, a Peloponnesian War, and what gave them unity was a single purpose behind them, the Spartan determination to remove from Greece the dangerous power of Athens, which she regarded as a threat to her own security. Judgements as to causes when the results are complete may in human affairs be as dangerous as illuminating, since reason insists on arranging things according to a pattern; it is our duty therefore to inquire whether Thucydides' interpretation corresponds with the facts, whether the unity which he has given to these three events is real or imaginary.