The economy of ancient Egypt is a difficult area of study due to the lack of preservation of much data (especially quantitative data); it is also a controversial subject on which widely divergent views have been expressed. It is certain, however, that the principal production and revenues of Egyptian society as a whole and of its individual members was agrarian, and as such, dependent on the yearly rising and receding of the Nile. Most agricultural producers were probably self-sufficient tenant farmers who worked the fields owned by wealthy individuals or state and temple estates. In addition to these, there were institutional and corvée workforces, and slaves, but the relative importance of these groups for society as a whole is difficult to assess. According to textual evidence, crafts were in the hands of institutional workforces, but indications also exist of craftsmen working for private contractors. Trade was essentially barter with reference to fixed units of textile, grain, copper, silver, and gold as measures of value. Coins were imported and produced in the Late Period, but a system close to a monetary economy is attested only from the Ptolemaic Period onward. Marketplaces were frequented by private individuals (including women) as well as professional traders, both native and foreign. Imports were secured by conquests and military control in the Levant, from which silver, oil, and wine reached Egypt, and in Nubia, rich in its deposits of gold.