When Alexander saw the breadth of his domain, he wept for there were no more worlds to conquer. (Technically a misquote, but I like the misquote better)

When Alexander saw the breadth of his domain, he wept for there were no more worlds to conquer.

"At last, he began to discourse of fortune and human affairs. "Is it meet," said he, "for him that knows he is but man, in his greatest prosperity to pride himself, and be exalted at the conquest of a city, nation, or kingdom, and not rather well to weigh this change of fortune, in which all warriors may see an example of their common frailty, and learn a lesson that there is nothing durable or constant? For what time can men select to think themselves secure, when that of victory itself forces us more than any to dread our own fortune? and a very little consideration on the law of things, and how all are hurried round, and each man's station changed, will introduce sadness in the midst of the greatest joy. Or can you, when you see before your eyes the succession of Alexander himself, who arrived at the height of power and ruled the greatest empire, in the short space of an hour trodden underfoot- when you behold a king, that was but even now surrounded with so numerous an army, receiving nourishment to support his life from the hands of his conquerors- can you, I say, believe there is any certainty in what we now possess whilst there is such a thing as chance? No, young men, cast off that vain pride and empty boast of victory; sit down with humility, looking always for what is yet to come, and the possible future reverses which the divine displeasure may eventually make the end of our present happiness.

Emotion has taught mankind to reason.