If you integrated the seemingly random "curve" of a stock graph (http://www.stockpickssystem.com/wp-.../1929-stock-market-crash-stock-chart-djia.gif), what physical value would you get out of it or does this not make any sense? On the contrary, if you took the derivative at a certain point of time you could find the rate of growth at that time. This IS a value that makes physical sense. I'm inferring that the y-axis is the index and the x-axis is the date. On the graph, just after the peak in 1929, the stock market crashed in the greatest financial disaster ever; the great depression. Guess what, today, the Dow Jones just hit its highest index in recorded history. Should that scare us somewhat?