In this short paper, which reflects on one of my contributions to the systems literature in 1992 (Pluralism and the Legitimation of Systems Science), I discuss the context at that time. Systems scientists were embroiled in a paradigm war, which threatened to fragment the systems research community. This is relevant, not only to understanding my 1992 contribution, but also because the same paradigms are evident in the complexity science community, and therefore it potentially faces the same risk of fragmentation. Having explained the context, I then go on to discuss my proposed solution to the paradigm war: that there are four domains of complexity, three of which reflect the competing paradigms. The problem comes when researchers say that inquiry into just one of these domains is valid. However, when we recognise all four as part of a new theory of complexity, we can view them as complementary. The four domains are natural world complexity, or “what is” (where the ideal of inquiry is truth); social world complexity, or the complexity of “what ought to be” in relation to actual or potential action (where the ideal of inquiry is rightness); subjective world complexity, or the complexity of what any individual (the self or another) is thinking, intending or feeling (where the ideal of inquiry is understanding subjectivity); and the complexity of interactions between elements of the other domains of complexity in the context of research and intervention practice. Following a discussion of the relevance of this theory for complexity scientists, I end the paper with a final critical reflection on my 1992 paper, pointing to some theoretical assumptions and terminology that I would, in retrospect, revise.