Philosophy of mind
Dualism is a set of views about the relationship between mind and matter (or body). It begins with the claim that mental phenomena are, in some respects, non-physical. One of the earliest known formulations of mind–body dualism was expressed in the eastern Sankhya and Yoga schools of Hindu philosophy (c. 650 BCE), which divided the world into purusha (mind/spirit) and prakriti (material substance). Specifically, the Yoga Sutra of Patanjali presents an analytical approach to the nature of the mind.
Twin Earth thought experiment
Twin Earth is a thought experiment by philosopher Hilary Putnam, first in his paper "Meaning and Reference" (1973), and then in his paper "The Meaning of 'Meaning (1975), to illustrate his argument for semantic externalism, or the view that the meanings of words are ultimately not purely psychological. The "Twin Earth" thought experiment was one of three, the other two being the "Aluminum-Molybdenum", and the "Beech-Elm" cases. Since the publication of these cases, philosophers have proposed numerous variations on the experiment.