Some theoreticians on the non-Communist left have emphasized the structural or systemic character of "imperialism". Such writers have expanded the period associated with the term so that it now designates neither a policy, nor a short space of decades in the late 19th century, but a world system extending over a period of centuries, often going back to Christopher Columbus and, in some accounts, to the Crusades. As the application of the term has expanded, its meaning has shifted along five distinct but often parallel axes: the moral, the economic, the systemic, the cultural, and the temporal. Those changes reflect—among other shifts in sensibility—a growing unease, even great distaste, with the pervasiveness of such power, specifically, Western power.