The idea that we do observe something already indicates something irreversible. If we draw a pencil line on a paper, for instance, we have established something which cannot be undone, so to speak. Every observation is irreversible, because we have gained information that cannot be forgotten.

In: Paul Buckley and F. David Peat, Glimpsing Reality: Ideas in Physics and the Link to Biology, Werner Heisenberg (p. 12)