In this paper the author analyzes commonalities and differences between scientific inquiry and diagnostic assessment, to highlight the thought and method rigorousness implied in the nursing diagnosis. The first part of the paper is dedicated to explore the relationship between the subjectivity of the heuristic attitude and the objectivity of the scientific methodology. In the second part, the use of inductive and deductive approaches in diagnostic inquiry are compared and the validity of their outcomes is discussed. Finally, the last section describes concepts and methods mostly used and is completed by an analysis of criteria for classification useable in nursing diagnosis.