If we can estimate the state of a system from its output, in a finite time interval, the system is said to be observable. To be more precise, a system is observable at to, if for any state at time to, there exists a finite ti > to such that the knowledge of the input and the output over the time interval to < t < ti suffices to determine the state. If we can steer the state of a system from the input, in a finite time interval, the system is referred