Keywords HCI, undo, non-determinism, dialogue, granularity, PIE model. Introduction Predictions have always been very important to humans. This happened and happens because, in order to predict something, it is necessary to know the object of the prediction, and such a knowledge is, in some sense, a form of control, of power. For a human, a prediction about something is the expression of his power on that thing. But when humans are involved in something, it is not possible to do predictions any more.
Human behaviour follows no physical law.
Since humans play a role in HCI and they are unpredictable, the resulting interaction is non-deterministic. Undo and non-determinism While interacting with a computer, any user would like to feel in control on the dialogue. Analysing the answers of the four above questions we may note that, in order to really feel in control of his dialogue, the user basically needs to know the system state and the system behaviour after any one of his actions. When using interactive systems, the knowledge on system state and behaviour may be increased by employing the undo. In fact, not only undo allows the user to repair an error, if it occurs, by reaching a past state, but it also enables the user to handle and reduce non-determinism.