The Enchiridion (7-8)

As in a voyage, when the ship is at anchor, if you go on shore to get water, you may amuse yourself with picking up a shell-fish or a truffle in your way, but your thoughts ought to be bent towards the ship, and perpetually attentive, lest the captain should call, and then you must leave all these things, that you may not have to be carried on board the vessel, bound like a sheep; thus likewise in life, if, instead of a truffle or shell-fish, such a thing as a wife or a child be granted you, there is no objection; but if the captain calls, run to the ship, leave all these things, and never look behind. But if you are old, never go far from the ship, lest you should be missing when called for.

Demand not that events should happen as you wish; but wish them to happen as they do happen, and you will go on well.