Meehl was not particularly religious during his upbringing, but in adulthood during the 1950s collaborated with a group of Lutheran theologians and psychologists to write What, Then, Is Man?. This project was commissioned by the Lutheran Church–Missouri Synod through Concordia Seminary. The project explored both orthodox theology, psychological science, and how Christians (Lutherans, in particular) could responsibly function as both Christians and psychologists without betraying orthodoxy or sound science and practice. Later life and death In 1995, Meehl was a signatory of a collective statement titled "Mainstream Science on Intelligence", written by Linda Gottfredson and published in the Wall Street Journal. He died on February 14, 2003, at his home in Minneapolis of chronic myelomonocytic leukemia. In 2005, Donald R. Peterson, a student of Meehl's, published a volume of their correspondence. Philosophy of science Meehl founded, along with Herbert Feigl and Wilfrid Sellars, the Minnesota Center for the Philosophy of Science, and was a leading figure in philosophy of science as applied to psychology. Arguably Meehl's most important contributions to psychological research methodology were in legitimizing scientific claims about unobservable psychological processes. In the first half of the 20th century, psychology was dominated by operationism and behaviorism. As outlined in Bridgman's The Logic of Modern Physics, if two researchers had different operational definitions, they had different concepts. There was no "surplus meaning". If, for example, two researchers had different measures of "anomia" or "intelligence", they had different concepts. Behaviorists focussed on stimulus–response theories and were deeply skeptical of "unscientific" explanations in terms of unobservable psychological processes. Behaviorists and operationists would have rejected as unscientific any notion that there was some general thing called "intelligence" that existed inside a person's head and that might be reflected almost-equivalently in Stanford-Binet IQ tests or Wechsler scales. Meehl changed that via two landmark papers. In 1948, Kenneth MacCorquodale and Meehl introduced the distinction between "hypothetical construct" and "intervening variable". "Naively, it would seem that there is a difference in logical status between constructs which involve the hypothesization of an entity, process, or event which is not itself observed, and constructs which do not involve such hypothesization." An intervening variable is simply a mathematical combination of operations. If one speaks of the "expected value" of a gamble (probability of winning × payoff for winning), this is not hypothesizing any unobservable psychological process. Expected value is simply a mathematical combination of observables. On the other hand,