In the history of economic thought, a school of economic thought is a group of economic thinkers who share or shared a common perspective on the way economies work. While economists do not always fit into particular schools, particularly in modern times, classifying economists into schools of thought is common. Economic thought may be roughly divided into three phases: premodern (Greco-Roman, Indian, Persian, Islamic, and Imperial Chinese), early modern (mercantilist, physiocrats) and modern (beginning with Adam Smith and classical economics in the late 18th century, and Karl Marx and Friedrich Engels' Marxian economics in the mid 19th century). Systematic economic theory has been developed mainly since the beginning of what is termed the modern era.