Let me start by explaining what I mean by an isolated concept cluster. It is often remarked that you can't define any moral term without using other moral terms. For example, you can define obligation in terms of right and wrong, or you can define right and wrong in terms of obligation, but you need to use moral terms to define moral terms. Morality is an example of an isolated concept cluster. It is a cluster of concepts which cannot be defined or understood entirely in terms of concepts outside of the cluster.
I used to think that there were just four or five isolated concept clusters, but it turns out that there are a lot. Here are some proposed examples:
cause/laws of nature
states of affairs
Let me also try to distinguish this notion of an isolated concept cluster from the notion of an ontological category, because some of the things I listed are also ontological categories. In this question, I'm not concerned with whether one category can be reduced to another category. All such reductions lose part of the concept. For example, many philosophers try to reduce morality to some form of practicality. I'm not going to argue against that, but just point out that you have lost the fundamental concept, understood by most people, of moral obligation, and turned it into something else like pragmatic obligation.
Or, for perhaps a less contentious example, consider the common practice in mathematics of reducing a straight line to a set of points. You can define what features a set of points must have to be a straight line, but you can't communicate the concept of a straight line without drawing a straight line or referring to some related concept such as curvature or the shortest distance between two points.
Has anyone investigated this area? Are there any lists of what I'm calling isolated conceptual clusters? And what are they called in the literature?