The history of economic thought is the study of the philosophies of the different thinkers and theories in the subjects that later became political economy and economics, from the ancient world to the present day in the 21st century. This field encompasses many disparate schools of economic thought. Ancient Greek writers such as the philosopher Aristotle examined ideas about the art of wealth acquisition, and questioned whether property is best left in private or public hands. In the Middle Ages, Thomas Aquinas argued that it was a moral obligation of businesses to sell goods at a just price.