" In the olden days (half a century ago), programs were big, undifferentiated masses of code and data. Control could flow from anywhere to anywhere. Data could be accessed from anywhere. Calculations, the original purpose of computers, occurred with (relatively speaking) lightning speed and perfect accuracy. Then people discovered an awkward fact: programs are written as much to be changed as to be run. All this control jumping around and self-modifying code and data accessed from everywhere was great for execution, but it was terrible if you wanted to change the program later. And so began the long and halting road to find models of computation so a change here doesn’t cause an unanticipated problem there. "

― Kent Beck , Implementation Patterns