The primary body of work in the field is The Extended Mind, by Andy Clark and David Chalmers. In this paper, Clark and Chalmers present the idea of active externalism, (similar to semantic or "content" externalism,) in which objects within the environment function as a part of the mind. They argue that it is arbitrary to say that the mind is contained only within the boundaries of the skull. The separation between the mind, the body, and the environment is seen as an unprincipled distinction. Because external objects play a significant role in aiding cognitive processes, the mind and the environment act as a "coupled system.” This coupled system can be seen as a complete cognitive system of its own. In this manner, the mind is extended into the external world. The main criterion that Clark and Chalmers outline for approaching the use of external environmental objects utilized during cognitive tasks as a part of an extended cognitive system is that the external objects must function with the same purpose as the internal processes.
In The Extended Mind, a thought experiment is presented to further illustrate the environment's role in connection to the mind. The fictional characters Otto and Inga are both travelling to a museum simultaneously. Otto has Alzheimer’s Disease, and has written all of his instructions down in a notebook to serve the function of his memory. Inga is able to recall the internal instructions within her memory. In a traditional sense, Inga can be thought to have had a belief as to the location of the museum before consulting her memory. In the same manner, Otto can be said to have held a belief of the location of the museum before consulting his notebook. The argument is that the only difference existing in these two cases is that Inga's memory is being internally processed by the brain, while Otto's memory is being served by the notebook. In other words, Otto's mind has been extended to include the notebook as the source of his memory.
Robert K. Logan’s book The Extended Mind: The Emergence of Language, the Human Mind and Culture develops the theory that language extended the human brain into the mind. Building on his previous study, The Sixth Language: Learning a Living in the Internet Age (Logan 2004), and making use of emergence theory, Logan (2007) explains how language emerged to deal with the complexity of hominid existence brought about by toolmaking, control of fire, social intelligence, coordinated hunting and gathering, and mimetic communication. The resulting emergence of language, he argues, signifies a fundamental change in the functioning of the human mind – a shift from percept-based thought to concept-based thought. From the perspective of the Extended Mind model, Logan provides an alternative to and critique of Noam Chomsky’s approach to the origin of language. He argues that language can be treated as an organism that evolved to be easily acquired, obviating the need for the hard-wiring of Chomsky’s Language Acquisition Device. In addition Logan shows how, according to this model, culture itself can be treated as an organism that has evolved to be easily attained, revealing the universality of human culture as well as providing an insight as to how altruism might have originated.