In the 20th and 21st centuries, economists with a methodological lineage to the early Austrian School developed many diverse approaches and theoretical orientations. For example, Ludwig von Mises organized his version of the subjectivist approach, which he called "praxeology", in a book published in English as "Human Action" in 1949. In it, Mises stated that praxeology could be used to deduce "a priori" theoretical economic truths and that deductive economic thought experiments could yield conclusions which follow irrefutably from the underlying assumptions. He wrote that conclusions could not be inferred from empirical observation or statistical analysis and argued against the use of probabilities in economic models.