Do that, dear Lucilius: assert your own freedom. Gather and guard the time that until now was being taken from you, or was stolen from you, or that slipped away. Convince yourself that what I write is true: some moments are snatched from us, some are filched, and some just vanish. But no loss is as shameful as the one that comes about through carelessness. Take a close look, and you will see that when we are not doing well, most of life slips away from us; when we are inactive, much of it—but when we inattentive, we miss it all. Can you show me even one person who sets a price on his time, who knows the worth of a day, who realizes that every day is a day when he is dying? In fact, we are wrong to think that death lies ahead: much of it has passed us by already, for all our past life is in the grip of death.
And so, dear Lucilius, do what your letter says you are doing: embrace every hour. If you lay hands on today, you will find you are less dependent on tomorrow. While you delay, life speeds by. Everything we have belongs to others, Lucilius; time alone is ours. Nature has put us in possession of this one thing, this fleeting, slippery thing—and anyone who wants to can dispossess us. Such is the foolishness of mortal beings: when they borrow the smallest, cheapest items, such as can easily be replaced, they acknowledge the debt, but no one considers himself indebted for taking up our time. Yet this is the one loan that even those who are grateful cannot repay.
Lucius Annaeus Seneca, Letters on Ethics.