[see here for a brief introduction to this ongoing series]
The third letter to Lucilius is on the topic of friendship, and here Seneca advises his pal to tread carefully and make important distinctions. To begin with, he says that one cannot decouple friendship from trust: if one doesn’t trust another person, then one cannot reasonably say that that person is a friend. However, the relationship between friendship and trust is cast by Seneca in the following manner:
“When friendship is settled, you must trust; before friendship is formed, you must pass judgment.” (III.2)
That is to say, how do we know whether we can trust someone enough to consider him a friend? We must first get to know him enough to be able to make a judgment about his character. Only if we can arrive at a positive conclusion in that respect we can think of him as a friend. But once this is done, trust becomes implicit, and is not to be revoked on penalty of effectively ending the friendship:
“Ponder for a long time whether you shall admit a given person to your friendship; but when you have decided to admit him, welcome him with all your heart and soul. Speak as boldly with him as with yourself.” (III.2)
Trust, according to Seneca, is something that needs to be accorded with wisdom: both extremes — to trust everyone and to trust no one — are foolish, though he says that the first is “more ingenious” while the latter is safer.
The essay ends with this somewhat cryptic advice:
“Discuss the problem with Nature; she will tell you that she has created both day and night.” (III.5)
Which, of course, is a reminder of the Stoic dictum to “follow nature” (I love the locution “discuss the problem with Nature, which conjures the image of Nature as one’s friend or advisor). If we remember that by that the Stoics mean, as Seneca himself puts it elsewhere (De Tranquillitate Animi X.4), to apply reason to human problems, then was the author is saying is that once we do that the answer becomes as clear as if we were inquiring about obvious natural phenomena such as the alternation of day and night.