In algebra, a line is defined by the equation of any two terms, each of which is the product of a constant and the first power of a variable. Plotting the possible values of the two variables by means of Cartesian co-ordinates, the result is a line that is perfectly straight. In Western societies, straight lines are ubiquitous. Indeed the straight line has emerged as a virtual icon of modernity, an index of the triumph of rational, purposeful design over the vicissitudes of the natural world. Once, however, the straight line comes to connote a moral condition, it sets itself apart from lines of every other sort in very much the same way as, in the history of Western thought and science, humanity came to be distinguished from animality. The hegemony of the straight line is a phenomenon of modernity, not of culture in general.