Throughout life, all people are engaged in activities – practical or mental – trying to solve problems, activities that themselves give rise to problems. To solve these problems, people need knowledge. They can acquire this personal knowledge in two main ways. First, they can interrogate the world – natural and social – by means of closer observation, deeper analysis, controlled experiment, all forms of cognitive interaction. Second, they can enrich their personal knowledge by communicative interaction with the stock of public knowledge that mankind has built up over the millenia, thus acquiring what we may call information. The activities in which people engage also produce two other kinds of knowledge: that embodied in people (skills) and that embodied in their artefacts.