Getty, Alice (1914). The gods of northern Buddhism, their history, iconography, and progressive evolution through the northern Buddhist countries, Oxford: The Clarendon press, pp. 4-6.
Welwood, John (2000). The Play of the Mind: Form, Emptiness, and Beyond. Source: http://www.purifymind.com/PlayMind.htm (accessed: Saturday January 13, 2007)