The idea seems quite simple. If you want to be able to tell that you can't decide what the person is doing, you need to lack something about his previous state.

The idea seems quite simple. If you want to be able to tell that you can't decide what the person is doing, you need to lack something about his previous state. Which means that even if you know everything that happened between the initial state of the universe and now, you still can't predict that person decision. So you would have to ignore some things about the whole initial state of the universe. Because the only possible things that can explain you not being able to predict that person are : 1) The decision is completely non causal... Which destroy a lot of things, because a non causal decision having impact would destroy the chain of causality. 2) Or! There is something in the initial state of the universe that you can't know. Hence the person acts are unpredictable but still causal.