Causal systems are important, but t

Causal systems are important, but they are not the only systems that are of practical significance. Causality is not of fundamental importance in
• applications, such as image processing, in which the independent variable is not time;
• processing recorded data for which time is the independent variable, as often happens with speech; and
• many applications, such as in stock market analysis and demographic studies, when we want to determine a slowly varying trend in data, and when we average data over an interval to smooth out the fluctuations and keep only the trend.