The Histories · 3.134

So when after this by his treatment he had made her well, then Atossa instructed by Demokedes uttered to Dareios in his bedchamber some such words as these: "O king, though thou hast such great power, thou dost sit still, and dost not win in addition any nation or power for the Persians: and yet it is reasonable that a man who is both young and master of much wealth should be seen to perform some great deed, in order that the Persians may know surely that he is a man by whom they are ruled. It is expedient indeed in two ways that thou shouldest do so, both in order that the Persians may know that their ruler is a man, and in order that they may be worn down by war and not have leisure to plot against thee. For now thou mightest display some great deed, while thou art still young; seeing that as the body grows the spirit grows old also with it, and is blunted for every kind of action."…

Herodotus of Halicarnassus · trans. G. C. Macaulay, 1890
Source edition
Passage preview. Read in context for the complete text, notes and references.