The following excerpts are from a paragraph in Book 3 of The Meditations by Marcus Aurelius. He starts this paragraph with what is today common advice:

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.

"Don't care what others think of you" seems like household advice, but Marcus Aurelius took it further, saying to not even think of others in meaningless ways. He says to not wonder what they are wondering about or doing, and his advice is given a reason: spending time meaninglessly thinking about others takes away from you being able to do something meaningful.
Instead, our thoughts are to be purposeful and useful:

 

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;

Having cleared away the chaff, he now describes what the wheat should be. What kinds of thoughts should we have?

 

...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.

Fill your mind with thoughts that would be appropriate if they were to be spoken aloud in real time. If anyone ever asks what you are thinking about, you should simply be able to say, "I'm thinking about a song I'm writing." You shouldn't have to come up with another thing and lie about it due to shame. Your thoughts become your character, and your character should be kind and true.

 

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;

This next part is one of my favorite lessons from Meditations. Here, Marcus Aurelius says to focus on what has been given to you. Set your thoughts on your own property, your own tools, your own home. Don't worry about other homes. Your home takes care of you, and you take care of your home. You have been given things to use, and so think about them; focus on them, because it is those things -- not some other guy's things -- that you must take care of, and which also will take care of you.

 

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.

A man who is concerned with acquiring new things is not concerned with managing his own things. Each of us has been given certain gifts, talents, and physical property to use and care for. Why should I be concerned with things I don't have? They aren't mine. They won't help me. The things I do have will help me. Conversely, I can't fix something I don't own. Only the owner can. So why should I concern myself with it?

 
This advice isn't meant to be taken selfishly:

 

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 first describes that if we consider what we have been given, we also consider our brothers and sisters. And that means that we have also been given "all men" to care for. It is in our nature to "care for all men."

 
But he slaps a disclaimer on the "all men" label: "only who confessedly live according to nature." I don't think spraying pesticides on food is living according to nature. Men who poison our food are not caring for others. They are behaving in a manner contrary to nature for the sake of profit. They are driven by a desire for things they do not have; they need money for status, items, and toys. Being concerned with things they do not have, they have abandoned their natural mind and become perverse. From 2 paragraphs later:

 

Never value anything as profitable to thyself which shall compel thee to break thy promise, to lose thy self-respect, to hate any man, to suspect, to curse, to act the hypocrite, to desire anything which needs walls and curtain