Continuing with Seneca's first letter to Lucilius, we read:
"Fac ergo, mi Lucili, quod facere te scribis, omnes horas complectere. Sic fiet, ut minus ex crasino pendeas, si hodierno manum inieceris. Dum differtur, vita transcurrit. Omnia, Lucili, aliena sunt, tempus tantum notrum est. In huius rei unius fugacis ac lubricae possessionem natura nos misit, ex qua expellit quicumque vult. Et tanta stultitia mortalium est, ut quae minima et vilissima sunt, certe reparabilia, imputari sibi, cum impetravere, patiantur; nemo se iudicet quicquam debere, qui tempus accepit, cum interim hoc unum est, quod ne gratus quidem potest reddere."
"Therefore, Lucilius, do as you write to me that you are already doing: hold every hour in your grasp. Lay hold of today's task, and you will not need to depend so much on tomorrow's. While we delay, life passes us by. All things are on loan to us, Lucilius, time alone is ours. Nature entrusted us with the possession of this one thing, so fleeting and slippery that anyone who wishes can take ownership from us. And such is the foolishness of mortals, that they allow to be counted as theirs the smallest and cheapest things, easily replaced, once they have them, but no one judges himself to be in debt when he has received some time, which meanwhile is the one thing that even a grateful recipient cannot repay."
Time is the one thing that we have - that, and the moral choices we make within time. Our bodies, our possessions, our loved ones - these are all "on loan" from fortune, and must be repaid whenever fortune calls for them. But we cannot be called upon to relinquish our time - we give it up one second at a time, and cannot be made to live life any faster or slower - so we must make the most of every moment. Our choices from moment to moment are the only thing we truly possess.