Internalisation relates to the human being's ability to imagine, consider alternative approaches to a problem, perform mental simulations. Externalisation transforms an internalised action into an external one. Mediation Human activity is mediated by artefacts tools both internal and external. These tools may be signs, language, instruments or machines. They are created by people and effect control over behaviour.
Artefacts have an associated culture and history and permanence that exists across time and space.
Development Activity Theory requires that the way a human interacts with reality should be analysed within the context of development. Hence the research method is not laboratory experiments but rather formative experiments that combine active participation with the monitoring of the developmental change shown by the study participants.