The paper The Extended Mind by Andy Clark and David Chalmers (1998) is a seminal work in the field of extended cognition. 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 list for classifying the use of external objects 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 directions down in a notebook to serve the function of his memory. Inga is able to recall the internal directions 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. The notebook qualifies as such because it is constantly and immediately accessible to Otto, and it is automatically endorsed by him.
Going further, the authors ask and answer their own question about the role of enculturation:
"And what about socially-extended cognition? Could my mental states be partly constituted by the states of other thinkers? We see no reason why not, in principle."
They bring up the recurrent theme of the role of language:
"The major burden of the coupling between agents is carried by language...Indeed, it is not implausible that the explosion of intellectual development in recent evolutionary time is due as much to this linguistically-enabled extension of cognition as to any independent development in our inner cognitive resources."
The "extended mind" is an idea in the field of philosophy of mind, often called extended cognition, which holds that the reach of the mind need not end at the boundaries of skin and skull. Tools, instrument and other environmental props can under certain conditions also count as proper parts of our minds. Closely related topics often conjoined with the idea of "extended mind" are situated cognition, distributed cognition, and embodied cognition.
In a book with the title The Extended Mind: The Emergence of Language, the Human Mind and Culture Robert K. Logan (2007) develops the thesis that verbal language extends the brain into a mind capable of conceptualization and hence the mind = brain + language. The human brain before it acquired verbal language according to this thesis is considered to be a percept processor. With language the mind is capable of conceptualization and hence able to consider things that are not immediately available in the here and now. The first words of verbal language are concepts associated with all the percepts associated with those words. For example the word water brings to mind the water we drink, the water we cook with, the water we wash with and the water we find in rivers, lakes and oceans and the water we find when it rains or when snow melts.