If, as we confront some task, a part of the world functions as a process which, were it done in the head, we would have no hesitation in recognising as part of the cognitive process, then that part of the world is part of the cognitive process.
Gist of Idea
If something in the world could equally have been a mental process, it is part of our cognition
A Clark / D Chalmers (The Extended Mind , §2)
In some sense they are obviously right that our cognitive activities spill out into books, calculators, record-keeping. It seems more like an invitation to shift the meaning of the word 'mind', than a proof that we have got it wrong.
-: 'Analysis' [-], p.2