[[Ficheiro:Simple_Feedback_02.png|thumb|Feedback exists between two parts when each affects the other. Template:Predefinição:Rp]] thumb|A feedback loop where all outputs of a process are available as causal inputs to that process Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to feed back into itself. The notion of cause-and-effect has to be handled carefully when applied to feedback systems:
- "Simple causal reasoning about a feedback system is difficult because the first system influences the second and second system influences the first, leading to a circular argument. This makes reasoning based upon cause and effect tricky, and it is necessary to analyze the system as a whole."