The RUP works in a fashion similar to Heisenberg’s Uncertainty Principle in quantum physics. The idea is simple: at any given time in a two-person relationship, you cannot simultaneously know each party’s feelings for the other and how those feelings are changing.

Perhaps an example would help explain it.

Suppose Alice and Bob are in a platonic relationship. Bob, infatuated with Alice, professes his love to her, but she tells him he’s “only a friend.” Each party now knows the other’s feelings — but neither party knows how the other’s feelings have been changed by this epiphany. Upon learning how the other’s feelings have changed, each person will inevitably experience a change in their own feelings.

So there’s an element of uncertainty in all human relationships. We have to operate blindly, essentially.

When I first wrote the RUP, I was only semi-serious. It made sense in the world of my contrived example (different than the one above, which is actually hypothetical — really!), but I didn’t really apply it to the real world. Only today did I stop and think about it, and wow, I think the Principle fits perfectly.