It is confessed by all that from his youth he was of a vehement and impetuous nature, of a quick apprehension, and a strong and aspiring bent for action and great affairs. The holidays and intervals in his studies he did not spend in play or idleness, as other children, but would be always inventing or arranging some oration or declamation to himself, the subject of which was generally the excusing or accusing his companions, so that his master would often say to him, "You, my boy, will be nothing small, but great one way or other, for good or else for bad." He received reluctantly and carelessly instructions given him to improve his manners and behaviour, or to teach him any pleasing or graceful accomplishment, but whatever was said to improve him in sagacity, or in management of affairs, he would give attention to, beyond one of his years, from confidence in his natural capacities for such things. And thus afterwards, when in company where people engaged themselves in what are commonly thought the liberal and elegant amusements, he was obliged to defend himself against the observations of those who considered themselves highly accomplished, by the somewhat arrogant retort, that he certainly could not make use of any stringed instrument, could only, were a small and obscure city put into his hands, make it great and glorious. Notwithstanding this, Stesimbrotus says that Themistocles was a hearer of Anaxagoras, and that he studied natural philosophy under Melissus, contrary to chronology; Melissus commanded the Samians in the siege by Pericles, who was much Themistocles's junior; and with Pericles, also, Anaxagoras was intimate. They, therefore, might rather be credited who report, that Themistocles was an admirer of Mnesiphilus the Phrearrhian, who was neither rhetorican nor natural philosopher, but a professor of that which was then called wisdom, consisting in a sort of political shrewdness and practical sagacity, which had begun and continued, almost like a sect of philosophy, from Solon: but those who came afterwards, and mixed it with pleadings and legal artifices, and transformed the practical part of it into a mere art of speaking and an exercise of words, were generally called sophists. Themistocles resorted to Mnesiphilus when he had already embarked in politics.
In the first essays of his youth he was not regular nor happily balanced; he allowed himself to follow mere natural character, which, without the control of reason and instruction, is apt to hurry, upon either side, into sudden and violent courses, and very often to break away and determine upon the worst; as he afterwards owned himself, saying, that the wildest colts make the best horses, if they only get properly trained and broken in. But those who upon this fasten stories of their own invention, as of his being disowned by his father, and that his mother died for grief of her son's ill-fame, certainly calumniate him; and there are others who relate, on the contrary, how that to deter him from public business, and to let him see how the vulgar behave themselves towards their leaders when they have at last no farther use of them, his father showed him the old galleys as they lay forsaken and cast about upon the sea-shore.
Yet it is evident that his mind was early imbued with the keenest interest in public affairs, and the most passionate ambition for distinction. Eager from the first to obtain the highest place, he unhesitatingly accepted the hatred of the most powerful and influential leaders in the city, but more especially of Aristides, the son of Lysimachus, who always opposed him. And yet all this great enmity between them arose, it appears, from a very boyish occasion, both being attached to the beautiful Stesilaus of Ceos, as Ariston the philosopher tells us; ever after which they took opposite sides, and were rivals in politics. Not but that the incompatibility of their lives and manners may seem to have increased the difference, for Aristides was of a mild nature, and of a nobler sort of character, and, in public matters, acting always with a view, not to glory or popularity, but to the best interest of the state consistently with safety and honesty, he was often forced to oppose Themistocles, and interfere against the increase of his influence, seeing him stirring up the people to all kinds of enterprises, and introducing various innovations. For it is said that Themistocles was so transported with the thoughts of glory and so inflamed with the passion for great actions, that, though he was still young when the battle of Marathon was fought against the Persians, upon the skilful conduct of the general, Miltiades, being everywhere talked about, he was observed to be thoughtful and reserved, alone by himself; he passed the nights without sleep, and avoided all his usual places of recreation, and to those who wondered at the change, and inquired the reason of it, he gave the answer, that "the trophy of Miltiades would not let him sleep." And when others were of opinion that the battle of Marathon would be an end to the war, Themistocles thought that it was but the beginning for far greater conflicts, and for these, to the benefit of all Greece, he kept himself in continual readiness, and his city also in proper training, foreseeing from far before what would happen.