Herbert Simon is one of the seminal thinkers in the study of complexity. His 1962 article, "The Architecture of Complexity", put forward several ideas that have become core to the conceptual frameworks of people who now study social complexity. So it is worthwhile highlighting a few of the key ideas that were put forward in that article. Here is Simon's definition of complexity:
Roughly, by a complex system I mean one made up of a large number of parts that interact in a nonsimple way. In such systems, the whole is more than the sum of the parts, not in an ultimate, metaphysical sense, but in the important pragmatic sense that, given the properties of the parts and the laws of their interaction, it is not a trivial matter to infer the properties of the whole. In the face of complexity, an in-principle reductionist may be at the same time a pragmatic holist.