A note on rigor, since this piece is about rigor. I deliberately left out the juicy stuff you'd expect: no "the founder secretly disagreed" narrative (unfalsifiable), no invented success-rate percentages, no quotes I couldn't trace to a real source. Every claim rests on his published method and the documented expertise of his subjects.

Sources: Csikszentmihalyi, Beyond Boredom and Anxiety (1975); Csikszentmihalyi & Larson, "Validity and Reliability of the Experience-Sampling Method," J. Nervous & Mental Disease 175(9), 1987 (journal); Csikszentmihalyi, Flow (1990); Ericsson, Krampe & Tesch-Römer, "The Role of Deliberate Practice," Psychological Review 100(3), 1993 (review).