In 1928, he earned a doctorate in psychology, under the supervision of Karl Bühler. His dissertation was entitled "Die Methodenfrage der Denkpsychologie" (The question of method in cognitive psychology). In 1929, he obtained the authorisation to teach mathematics and physics in secondary school, which he started doing. He married his colleague Josefine Anna Henninger (1906–1985) in 1930. Fearing the rise of Nazism and the threat of the Anschluss, he started to use the evenings and the nights to write his first book Die beiden Grundprobleme der Erkenntnistheorie (The Two Fundamental Problems of the Theory of Knowledge). He needed to publish one to get some academic position in a country that was safe for people of Jewish descent. However, he ended up not publishing the two-volume work, but a condensed version of it with some new material, Logik der Forschung (The Logic of Scientific Discovery), in 1934. Here, he criticised psychologism, naturalism, inductionism, and logical positivism, and put forth his theory of potential falsifiability as the criterion demarcating science from non-science. In 1935 and 1936, he took unpaid leave to go to the United Kingdom for a study visit.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What was the title of Popper's last book?
unanswerable