Foucault was very interested in the history of knowledge, but not in a conventional sense. The history of knowledge has often been thought of as a series of events such as discoveries, inventions and journeys across previously uncharted areas. It was not, however, in this sense that Foucault perceived the history of knowledge. He was interested, first, in the way in which a particular set of ideas or a world view was pre-eminent for a long period of time, only to be replaced, either gradually or suddenly, by a different set of ideas. Secondly, he was also interested in the way in which concepts change over a long time period. While the word used to represent a concept may not change, the idea or ideas represented by the concept certainly do change. In other words, Foucault was less interested in the facts associated with the changes in knowledge, as with the mechanisms and processes by which our understanding of the world alters. Moreover, he saw ideological systems as exerting great influence over our ideas, and was interested in the way in which one belief system becomes liberated from one ideology, only to find itself later constrained by a different ideology. Foucault observed that history is often presented as a series of facts or events, completely dissociated from the nature of human society, and yet for him history was always firmly embedded in the thoughts and perceptions of human beings. As history was a human creation, it was subject to different interpretations at different times.
In Foucault's view, historical events are seen as a social construction, rather than as disembodied facts. For example, when someone is removed from power, it is not seen as a specific event at a particular time, but as the product of a complex interaction between human beings. Quite apart from the event itself, the manner in which it is perceived is also understood as a social interpretation. History is seen as a series of social interactions, rather than isolated events.
Much of human intellectual endeavor has been concerned with an attempt to understand the nature of historical development in purely rational terms. In other words, scholars have tried to understand the development of history by explaining it as a logical, sequential process that ultimately could lead to a more complete appreciation of the human condition. Foucault challenged the possibility of such a rational analysis, seeing history as much more unpredictable. [MB notes: Oliver suggests Foucault's books The Order of Things: An Archaeology of the Human Sciences (1970) and The Archaeology of Knowledge (1972) as references here -- although the reader should be aware this was Foucault's early writings and his methods/perspectives/theories would change as he, and his society, changed over the years.]
Foucault described his study of the history of knowledge as 'archaeology', an apt metaphor to refer to his efforts to gradually reveal the layers of human understanding that had existed in different epochs. One of Foucault's interesting suggestions was that human beings do not specifically and intentionally create systems of thought. Rather, the latter are a product of the activities of human beings. In other words, particular ways of acting or thinking presuppose a specific pattern of knowledge, which then become characteristic of a particular historical period. (19-21)
Oliver, Paul. Foucault: The Key Ideas. Black Lick, OH: McGraw-Hill, 2010.