Do not waste the remainder of thy life...

Do not waste the remainder of thy life in thoughts about others, when thou dost not refer thy thoughts to some object of common utility. For thou losest the opportunity of doing something else when thou hast such thoughts as these, What is such a person doing, and why, and what is he saying, and what is he thinking of, and what is he contriving, and whatever else of the kind makes us wander away from the observation of our own ruling power. We ought then to check in the series of our thoughts everything that is without a purpose and useless, but most of all the over-curious feeling and the malignant; and a man should use himself to think of those things only about which if one should suddenly ask, What hast thou now in thy thoughts? With perfect openness thou mightest, immediately answer, This or That; so that from thy words it should be plain that everything in thee is simple and benevolent, and such as befits a social animal, and one that cares not for thoughts about pleasure or sensual enjoyments at all, nor has any rivalry or envy and suspicion, or anything else for which thou wouldst blush if thou shouldst say that thou hadst it in thy mind. For the man who is such and no longer delays being among the number of the best, is like a priest and minister of the gods, using too the deity which is planted within him, which makes the man uncontaminated by pleasure, unharmed by any pain, untouched by any insult, feeling no wrong, a fighter in the noblest fight, one who cannot be overpowered by any passion, dyed deep with justice, accepting with all his soul everything which happens and is assigned to him as his portion; and not often, nor yet without great necessity and for the general interest, imagining what another says, or does, or thinks. For it is only what belongs to himself that he makes the matter for his activity; and he constantly thinks of that which is allotted to himself out of the sum total of things, and he makes his own acts fair, and he is persuaded that his own portion is good. For the lot which is assigned to each man is carried along with him and carries him along with it. And he remembers also that every rational animal is his kinsman, and that to care for all men is according to man's nature; and a man should hold on to the opinion not of all, but of those only who confessedly live according to nature. But as to those who live not so, he always bears in mind what kind of men they are both at home and from home, both by night and by day, and what they are, and with what men they live an impure life. Accordingly, he does not value at all the praise which comes from such men, since they are not even satisfied with themselves.

Marcus Aurelius Antoninus Augustus, Meditations, Book III - 4.