Heathens commonly adopt a cosmology based on that found in Norse mythology—Norse cosmology. As part of this framework, humanity's world—known as Midgard—is regarded as just one of Nine Worlds, all of which are associated with a cosmological world tree called Yggdrasil. Different types of being are believed to inhabit these different realms; for instance, humans live on Midgard, while dwarfs live on another realm, elves on another, jötnar on another, and the divinities on two further realms. Most practitioners believe that this is a poetic or symbolic description of the cosmos, with the different levels representing higher realms beyond the material plane of existence. The world tree is also interpreted by some in the community as an icon for ecological and social engagement. Some Heathens, such as the psychologist Brian Bates, have adopted an approach to this cosmology rooted in analytical psychology, thereby interpreting the nine worlds and their inhabitants as maps of the human mind.