  The notion of "closed systems" in Quantum Mechanics is discussed. For this
purpose, we study two models of a quantum-mechanical system $P$ spatially far
separated from the "rest of the universe" $Q$. Under reasonable assumptions on
the interaction between $P$ and $Q$, we show that the system $P$ behaves as a
closed system if the initial state of $P \vee Q$ belongs to a large class of
states, including ones exhibiting entanglement between $P$ and $Q$. We use our
results to illustrate the non-deterministic nature of quantum mechanics.
Studying a specific example, we show that assigning an initial state and a
unitary time evolution to a quantum system is generally not sufficient to
predict the results of a measurement with certainty.
