Since the amplitude to travel from x to y after a time t + t ' can be considered in two steps, the propagator obeys the composition identity, ∫ ∫ K ( x − − y ; t ) K ( y − − z ; t ′ ) d y = K ( x − − z ; t + t ′ ) , which can be interpreted as follows: the amplitude to travel from x to z in time t + t ' is the sum of the amplitude to travel from x to y in time t, multiplied by the amplitude to travel from y to z in time t ', summed over all possible intermediate states y. This is a property of an arbitrary quantum system, and by subdividing the time into many segments, it allows the time evolution to be expressed as a path integral.