The Spartans do not ask how many are the enemy but where are they. - Plutarch

Those who receive with most pains and difficulty, remember best; every new think they learn, being, as it were, burnt and branded in on their minds.

When Darius offered him ten thousand talents, and to divide Asia equally with him, "I would accept it," said Parmenio, "were I Alexander." "And so truly would I," said Alexander, "if I were Parmenio." But he answered Darius that the earth could not bear two suns, nor Asia two kings.

if the “Know thyself” of the oracle were an easy thing for every man, it would not be held to be a divine injunction.