Man must consider, not only that each day part of his life is spent, and that less and less remains to him, but also that, even if he live longer, it is very uncertain whether his intelligence will suffice as heretofore for the understanding of his affairs, and for grasping that knowledge which aims at comprehending things human and divine. When dotage begins, breath, nourishment, fancy, impulse, and so forth will not fail him. But self-command, accurate appreciation of duty, power to scrutinize what strikes his senses, or even to decide whether he should take his departure, all powers, indeed, which demand a well-trained understanding, must be extinguished in him. Let him be up and doing then, not only because death comes nearer every day, but because understanding and intelligence often leave us before we die. 

Marcus Aurelius, Meditations  (III,1)

We must not rivet ourselves so fast to our humours and complexions: our chiefest sufficiency is to know how to apply ourselves to divers employments.
 
The principal use of reading to me is, that by various objects it rouses my reason, and employs my judgment, not my memory. 

Montaigne

They who retire themselves from the common offices, from that infinite number of troublesome rules that fetter a man of exact honesty in civil life, are in my opinion very discreet.

Montaigne

I have a soul free and entirely its own, and accustomed to guide itself after its own fashion; having hitherto never had either master or governor imposed upon me: I have walked as far as I would, and at the pace that best pleased myself; this is it that has rendered me unfit for the service of others, and has made me of no use to any one but myself.

Michel de Montaigne

Solitude seems to me to wear the best favor in such as have already employed their most active and flourishing age in the world's service…. We have lived enough for others; let us at least live out the small remnant of life for ourselves; let us now call in our thoughts and intentions to ourselves, and to our own ease and repose…. We must break the knot of our obligations, how strong soever, and hereafter love this or that, but espouse nothing but ourselves: that is to say, let the remainder be our own, but not so joined and so close as not to be forced away without flaying us or tearing out part of our whole. The greatest thing in the world is for a man to know that he is his own. 'Tis time to wean ourselves from society when we can no longer add anything to it; he who is not in a condition to lend must forbid himself to borrow. Our forces begin to fail us; let us call them in and concentrate them in and for ourselves. He that can cast off within himself and resolve the offices of friendship and company, let him do it. In this decay of nature which renders him useless, burdensome, and importunate to others, let him take care not to be useless, burdensome, and importunate to himself. Let him soothe and caress himself, and above all things be sure to govern himself with reverence to his reason and conscience to that degree as to be ashamed to make a false step in their presence.... The stoutest and most resolute natures render even their seclusion glorious and exemplary.

Michel de Montaigne

About Filling A Commonplace Book?

I go here and there, culling out of several books the sentences that best please me, not to keep them (for I have no memory to retain them in), but to transplant them into this; where, to say the truth, they are no more mine than in their first places. 

Michel de Montaigne