A few days ago I met a simple example that clearly illustrates what I mean, which I’ll quote extensively:
This beer can example is a simple version of something that everybody does: making a note on a piece of paper in order not to forget something we have to do: A shopping list, a note in a diary for an appointment, a list of tasks we have to do. In its simplicity the beer can example shows the essence of what it means that a part of our mind is outside our head. If our mind would be only inside our head, we should have to mark somewhere in our memory that we have to buy beer. But then we have the risk to forget it. Therefore we need something that triggers this marking. If we would make a marking-2 in our memory, again we should have the risk that we forget it. Therefore, we do something else: we make marking-2 outside our head and we make a tag within our head that gets activated as soon as it meets the marking outside our head that belongs to it: the beer can on the mat in our case. We move marking-2 to the outside world, put it on a striking place and replace it in our head by a tag. And so a part of our mind has found its place in the world around us now.
Consider the following scenario: You have to remember to buy a case of beer for a party. To jog your memory, you place an empty beer can on your front doormat. When next you leave the house, you trip over the can and recall your mission. You have thus used … a … trick … exploiting some aspect of the real world as a particular substitute for on-board memory. (Andy Clark, Being there, The MIT Press, Cambridge, Mass. etc, 1997; p. 75)
However, this tagging and marking-2 has a significance that is wider than simply being a memory aid. Once we understand it, it makes clear, for instance, why books are such valuable objects: When we destroy a book, we destroy not only a bunch of paper, but we destroy a part of a human mind, a part of what we as humans are.