In previous chapters, we showed that random process may not always necessarily be a time history operation. Time histories occurring physically in the real world have specific reasons and they are limited by certain conditions. In previous chapters, we mainly focused on time-varying development, instead of a clear understanding of why time history exists. At most, we studied several important conditions of those time histories. Yet, these studies were limited to how a process behaves, such as if it is stationary, what the corresponding statistical parameters are, as well as what frequency spectra they have listed. Generally speaking, the previous chapters were limited to the mathematical models of random processes, instead of what the causes of these models were.