Note: this is the third part of a weekly series, discussing Seneca’s Moral letters to Lucilius. A new post will be released every Thursday morning (GMT).

What is true friendship? Take a moment to think about that before reading on. Seneca thought about it, too. In his third Moral letter to Lucilius, he proposes an answer to this question. First off, he reminds Lucilius that he called someone his friend, yet did not fully trust him. Because of that, Seneca notes:

you have in the same letter affirmed and denied that he is your friend.

So when can we consider someone a real friend and how do we handle such a friendship? According to Seneca, it is all about trust: real friends are those people we can trust completely.

But if you consider any man a friend whom you do not trust as you trust yourself, you are mightily mistaken and you do not sufficiently understand what true friendship means. Indeed, I would have you discuss everything with a friend

In order to reach such a friendship, we must judge people first and trust (or distrust) them later. This means that everyone who might be a friend of yours, must first pass your test of judgement before he actually becomes one. However, at the moment you admit a given person to your friendship, you must be committed to it. You either consider someone not a friend, or you trust him completely as a friend. You should be able to tell your friend everything. This is true friendship.

when you have decided to admit him [to your friendship], welcome him with all your heart and soul. Speak as boldly with him as with yourself. (…) Why need I keep back any words in the presence of my friend? Why should I not regard myself as alone when in his company?

Seneca continues by making a few remarks about sharing the details of your life with others. Some people, he says, tell everything to everyone. Others don’t even tell the full story to their closest friend. Both of these habits are wrong, according to Seneca:

It is equally faulty to trust everyone and to trust no one.

Place your trust in a few people that you have judged to be trustworthy. Know from your head and your heart that you can trust them with anything, and that you will be able to discuss everything with them. And in all sincerity, you can call them true friends.