Retroactively

Lacan says,

We must go pretty far along a path for the point of departure to be clarified retroactively. (p. 162)

I like this line a lot. It makes me think about how we all set out along a path heading in a direction. Or, to say this in a different way, we have an idea of what we would like to do and what we would like to happen to us.

An example could be a person getting married. This person probably believes they are on the path of spending being one half of a couple with the person who is now their spouse. In our society, this also often means that the person will refrain from having sex with other people.

At the start of the marriage, a person may want to be on this path; they may want to be committed to their spouse. However, things don't always stay this way. People will leave the path. Perhaps this takes the form of having an affair.

What Lacan's quote shows is that the moment the affair is consummated is not the point where the path was left. The path was left some time earlier when the person started to lose interest in their spouse.

Chances are, the person did not realize they had left the path when they did. They can only look back after the fact and retroactively determine a point when such and such happened, and they started to lose interest in continuing along the path they had set out on.

Lacan, Jacques, From an Other to the other: The Seminar of Jacques Lacan Book XVI. Edited by Jacques-Alain Miller, translated by Bruce Fink.