 Without this definite sense of signal circulation one is looking at a mere mesh in a network, and it makes no sense to speak of a feedback loop.A feedback loop can contain further feedback loops within itself, or it can provide a pathway inside another feedback loop, provided its 'input' and 'output' are defined.A 1993 paper, "General Systems Theory" by David S. Walonick, Ph.D., states in part, "A closed system is one where interactions occur only among the system components and not with the environment.