The subject matter of archaeology is the engagement of human beings, now and in the past, with both the natural world and the material world they have created. All aspects of human activity are potentially relevant to archaeological research, and, conversely, the ways in which others, especially artists and anthropologists, have investigated the world are of interest to archaeologists. Archaeological artefacts and sites are also used by groups and nations to establish identity, and for financial gain, both through tourism and trade in antiquities.