Peptic ulcers, including duodenal and gastric ulcers, were currently considered an infectious disease caused by Helicobacter pylori. However, this etiology cannot explain the major characteristics and observations/phenomena of the disease. To address the challenge, a Complex Causal Relationship with its accompanying methodologies was applied to analyze the existing data. Peptic ulcers were identified as a psychosomatic disease triggered by psychological stress, whereas Helicobacter pylori plays a secondary role in the late phase of peptic ulceration. This etiology addresses all the characteristics, observations/phenomena, controversies, and mysteries of peptic ulcers in a series of 6 articles. This third article discusses the etiology in-depth, painting a complete picture of the pathogenesis of peptic ulcers. The early phase is a long-term abstract psychopathological process, starting from early life when the impacts of multiple psychosomatic factors are transformed into hyperplasia and hypertrophy of gastrin and parietal cells and/or negative life-views via superposition mechanism. These pre-existing abnormalities potentiate the individual’s response to current psychological stress in the intermediate phase, resulting in hypersecretion of gastric acid and/or pre-ulcer lesions. The late phase is a corrosion process caused by gastric acid, Helicobacter pylori, and/or NSAIDs, resulting in duodenal and/or gastric ulcers. This complete picture highlights the importance of past life experience in peptic ulcers and visualizes the non-causal roles of gastric acid, Helicobacter pylori, and NSAIDs. Significantly, the holistic view provided by the complete picture suggests that life phenomena and diseases are highly complex, and an advanced way of thinking is indispensable for life science and medical studies.