nieutracony through the censors was one of the major reasons Lem decided to focus on the less-censored genre of science fiction. Nonetheless, most of Lem's works published in the 1950s also contain—forced upon him by the censors and editors—various elements of socialist realism as well as of the "glorious future of communism". Lem later criticized several of his early pieces as compromised by the ideological pressure. Lem became truly productive after 1956, when the de-Stalinization period in the Soviet Union led to the "Polish October", when Poland experienced an increase in freedom of speech. Between 1956 and 1968, Lem authored seventeen books. His writing over the next three decades or so was split between science fiction and essays about science and culture. In 1957, he published his first non-fiction, philosophical book, Dialogues, as well as a science fiction anthology, The Star Diaries (Dzienniki gwiazdowe), collecting short stories about one of his most popular characters, Ijon Tichy. 1959 saw the publication of three books: Eden, The Investigation (Śledztwo) and the short story anthology An Invasion from Aldebaran (Inwazja z Aldebarana). 1961 saw the novels: Memoirs Found in a Bathtub (Pamiętnik znaleziony w wannie), Solaris, and Return from the Stars (Powrót z gwiazd), with Solaris being among his top works. This was followed by a collection of his essays and non-fiction prose, Wejście na orbitę (1962), and a short story anthology Noc księżycowa (1963). In 1964, Lem published a large work on the border of philosophy and sociology of science and futurology, Summa Technologiae, as well as a novel, The Invincible (Niezwyciężony). 1965 saw the publication of The Cyberiad (Cyberiada) and of a short story anthology, The Hunt (). 1966 is the year of Highcastle (Polish title: Wysoki Zamek), followed in 1968 by His Master's Voice (Głos Pana) and Tales of Pirx the Pilot (Opowieści o pilocie Pirxie). Highcastle was another of Lem's autobiographical works, and touched upon a theme that usually was not favored by the censors: Lem's youth in the pre-war, then-Polish, Lviv. 1968 and 1970 saw two more non-fiction treatises, The Philosophy of Chance (Filozofia przypadku) and Science Fiction and Futurology (Fantastyka i futurologia). Ijon Tichy returned in 1971's The Futurological Congress Kongres futurologiczny; in the same year Lem released a genre-mixing experiment, Doskonała próżnia, a collection of reviews of non-existent books. In 1973 a similar work, Imaginary Magnitude (Wielkość urojona), was published. In 1976, Lem published two novels: