Chaotic systems are defined in terms if extreme dependence on initial conditions. Very small changes in initial conditions result in large scale variations "downstream". The implication is that if we know the initial conditions exactly, we can know the system's behavior exactly as it evolves. However, for what physical system can initial conditions be known exactly? I know that variations in system behavior can be observed to remain within certain boundaries according to Chaos Theory, but how are we justified in saying that such behavior is "deterministic" even in principle?