Book excerpt: If you need to deal with complex problems, this book will open a door and widen your possibilities to influence them. Complex systems research methodologies used until today include both intuitive and mathematical approaches to approximate solutions. The unicist approach to complexity using ontology-based logical inferences made the solution of complex problems understandable, reasonable and provable. This R&D e-book will give you an introduction to the unicist approach to complexity research. The final goal of a diagnosis is to influence reality. To do so the researcher needs to explain it, but the explanation is only a comprehension framework to exert influence on the environment under study. The research that merely tries to explain a situation becomes an end in itself and therefore tends to be fallacious. All reality that operates as a complex system needs to be approached as a unified field. The unified field is not susceptible to division into variables. Division is only possible when dealing with a non-complex system. A complex system can only be studied as a unit. A very strict methodology, forecast and validation/falsification are required to avoid falling into fallacies that will lead into erroneous diagnoses. This work provides a list of actions to design the research that would permit to diagnose complex problems. It includes the unicist ontology of complex systems research.