Modularity, redundancy and degeneracy: Cross-domain perspectives on key design principles

This article reviews and consolidates different treatments of modularity, redundancy and degeneracy across different disciplines and different system types (spanning both Systems Science and Systems Engineering). This is done with two main objectives. The first is to draw out and compare the perspectives and classifications that are available for these important concepts. The second is to point to areas where there has been a lack of precision or where more rigorous distinctions would help make discourse on these concepts more productive. For modularity and redundancy, we first consider the definitions and variants associated with these concepts. We then identify some key classificatory principles which might be used to distinguish between variants so as to illustrate the diversity across domains and contexts. Degeneracy is addressed by explicitly relating it to redundancy and modularity. We identify two key areas where lack of explicit treatment often results in confusion for the concepts of modularity, redundancy and degeneracy: (i) the level (or `granularity') of analysis; (ii) whether it is types or instances that are being referred to. We hope that by making readers aware of the confusions that can arise, it will also encourage more serious dialogue to take place between practitioners and researchers in different domains so that existing bodies of knowledge can be usefully shared.