Please analyze the following quotation from The Importance of Being Earnest:
Cecily: " It is always painful to part from people whom one has known for a very brief space of time. The absence of old friends, one can endure with equanimity" Act 2.
This sentence is said by Cecily to Algernon after the latter explains that he has to leave (because Jack kicked him out in a rage).
Cecily is expressing one of Oscar Wilde's most used creative licenses which is the application of epigrams and paradoxes; these consist on sayings which, in style, look like a moral of a story, but instead this moral ends up contradicting itself. The application of this license in his writings is specifically designed to cause shock, or surprise, in the audience and elicit the comedic element that it definitely incites.
What Cecily is basically saying, in typical Wilde fashion, is that the people whom you have known only briefly should be considered more importantly than the people that you have known for your entire life.
Cecily says that
...even a momentary separation from anyone to whom one has just been introduced is almost unbearable.
however, she previously says
The absence of old friends one can endure with equanimity (composure, tranquility)
These words support Wilde's central theme of superficiality; Cecily states that those whom you just happen to meet are people that you really do not know for sure. Therefore, you can put any mask that you wish since they do not know you either. As a result, one can create a scenario where we pretend to be truly heartfelt about people that we do not know, because it is the easiest thing to do.
Yet, Wilde claims that those whom you know, and who know you as well, may have enough information and knowledge about you to render them quite welcome to leave for good.
This also connects to another main central theme in the play, which is the issue of the double life. Wilde, who led a double life, had a lot of acquaintances whom he had known for many years. Since they knew about his double life, many of them provided damning evidence that Wilde would have otherwise wanted to keep secret.
This shows how it is best NOT to know people well enough that they would know things about you as well, for example, your secrets.
Therefore, Cecily's words really mean that it is easier to keep a superficial friendship where everyone lies to each other rather than build a profound friendship that involves sharing secrets or knowing about one's weaknesses. That is essentially what Wilde is trying to say using Cecily as his mouthpiece: those whom we do not know, we can pretend to care about; those who know us a little too well, are more than welcome to leave and never come back.