If you look more closely at the Wikipedia.org page on Activity Theory (AT), you'll see that the diagram of Engestrom's extended AT (Engeström, 1987) was provided by me
For a fuller explanation of Vygotsky's notion of mediation, here's an extract from Mind in Society:
"We call the internal reconstruction of an external operation internalisation.
A good example of this process may be found in the development of
pointing. Initially, this gesture is nothing more than an unsuccessful
attempt to grasp at something, a movement aimed at a certain object
which designates forthcoming activity. The child attempts to grasp an
object placed beyond his reach; his hands, stretched toward that object,
remain poised in the air. His fingers make grasping movements. At this
initial stage pointing is represented by the child's movement, which
seems to be pointing to an object - that and nothing more.
When the mother comes to the child's aid
and realises his movement means something, the situation changes
fundamentally. Pointing becomes a gesture for others. The child's
unsuccessful attempt engenders a reaction not from the object he seeks
but from another person. Consequently, the primary meaning of
that unsuccessful grasping movement is established by others. Only
later, when the child can link his unsuccessful grasping movement to the
objective situation as a whole, does he begin to understand this
movement as pointing. At this juncture there occurs a change in that
movement's function: from an object-oriented movement it becomes a
movement aimed at another person, a means of establishing relations. The grasping movement changes to the act of pointing.
As a result of this change, the movement itself is then physically
simplified, and what results is the form of pointing that we may call a
true gesture. It becomes a true gesture only after it objectively
manifests all the functions of pointing for others and is understood by
others as a gesture. Its meaning and functions are created at first by
an objective situation and then by people who surround the child.
As the above description of pointing illustrates, the process of internalisation consists of a series of transformations:
(a) An operation that initially represents an external activity is reconstructed and begins to occur internally.
Of particular importance to the development of higher mental processes
is the transformation of sign-using activity, the history and
characteristics of which are illustrated by the development of practical
intelligence, voluntary attention, and memory.
(b) An interpersonal process is transformed into an intrapersonal one.
Every function in the child's cultural development appears twice:
first, on the social level, and later, on the individual level; first between people (intrapsychological), and then inside the child (intrapsychological).
This applies equally to voluntary attention, to logical memory, and to
the formation of concepts. All the higher functions originate as actual
relations between human individuals.
(c) The transformation of an interpersonal process into an intrapersonal one is the result of a long series of developmental events.
The process being transformed continues to exist and to change as an
external form of activity for a long time before definitively turning
inward. For many functions, the stage of external signs lasts forever,
that is, it is their final stage of development. Other functions develop
further and gradually become inner functions. However, they take on the
character of inner processes only as a result of a prolonged
development. Their transfer inward is linked with changes in the laws
governing their activity; they are incorporated into a new system with
its own laws.
The internalisation of cultural forms of
behaviour involves the reconstruction of psychological activity on the
basis of sign operations. Psychological processes as they appear in
animals actually cease to exist; they are incorporated into this system
of behaviour and are culturally reconstructed and developed to form a
new psychological entity. The use of external signs is also radically
reconstructed. The developmental changes in sign operations are akin to
those that occur in language. Aspects of external or communicative
speech as well as egocentric speech turn "inward” to become the basis of
The internalisation of socially rooted
and historically developed activities is the distinguishing feature of
human psychology, the basis of the qualitative leap from animal to human
psychology. As yet, the barest outline of this process is known.” (Vygotsky, 1978, p. 56-57)
It's worth noting that in higher education especially, learners have already had at least 17 years of these ZPD episodes and so have internalised a vast number of thinking processes and ideally should already be highly independent learners capable of mediating at least most their own learning and each others'. However, we don't live in an ideal world and our education systems appear to place more value on conformity, compliance, and standardised test scores than on learning. This may be partly why colleges and universities complain that their intake are often poorly prepared for the kinds of studying, higher-order and critical thinking that's expected of them.
Engeström, Y. (1987). Learning by Expanding: An Activity - Theoretical Approach to Developmental Research. Retrieved February 10, 2015, from http://lchc.ucsd.edu/mca/Paper/Engestrom/expanding/toc.htm
Vygotsky, L. S. (1978). Mind in Society: The Development of Higher Psychological Processes. Harvard.