The Uncertainty Principle

Just as the Doppler Effect has a human component, so to does Heisenberg's Uncertainty Principle. The quantum physicist notices that, at the sub-atomic level, the very manner in which we observe a phenomenon affects the phenomenon. Observing one aspect of a particle may alter another aspect of that particle. In short, we cannot say anything purely objective about reality; we affect it by our very presence.

In the same way, we cannot make objective statements about other people, because how we observe them and interact with them fundamentally changes who they are or who they can hope to become.