The ancient civilizations of Egypt, Greece, China, India, and Persia all engaged in the philosophical study of psychology. Historians note that Greek philosophers, including Thales, Plato, and Aristotle (especially in his De Anima treatise), addressed the workings of the mind. As early as the 4th century BC, Greek physician Hippocrates theorized that mental disorders had physical rather than supernatural causes.
In China, psychological understanding grew from the philosophical works of Laozi and Confucius, and later from the doctrines of Buddhism. This body of knowledge involves insights drawn from introspection and observation, as well as techniques for focused thinking and acting. It frames the universe as a division of, and interaction between, physical reality and mental reality, with an emphasis on purifying the mind in order to increase virtue and power. An ancient text known as The Yellow Emperor's Classic of Internal Medicine identifies the brain as the nexus of wisdom and sensation, includes theories of personality based on yin–yang balance, and analyzes mental disorder in terms of physiological and social disequilibria. Chinese scholarship focused on the brain advanced in the Qing Dynasty with the work of Western-educated Fang Yizhi (1611–1671), Liu Zhi (1660–1730), and Wang Qingren (1768–1831). Wang Qingren emphasized the importance of the brain as the center of the nervous system, linked mental disorder with brain diseases, investigated the causes of dreams and insomnia, and advanced a theory of hemispheric lateralization in brain function.
Distinctions in types of awareness appear in the ancient thought of India, influenced by Hinduism. A central idea of the Upanishads is the distinction between a person's transient mundane self and their eternal unchanging soul. Divergent Hindu doctrines, and Buddhism, have challenged this hierarchy of selves, but have all emphasized the importance of reaching higher awareness. Yoga is a range of