The paper shows the development of Gaston Bachelard’s thought from his early writings to later meditations on daydreams. Bachelard’s “scientific contribution” is characterized by his conception of applied rationalism, and his conviction that a true science must be justified by a rectification process. Theoretical rationalization must necessarily be applied in practice. Similar to an open science, the philosophy of science is open if it is able to say “no” to old scientific and philosophical experience. This “no” is not final but it is a sign of openness. Bachelard appears to be the predecessor of Popper’s fallibilism. The “second part” of bachelardian philosophy con- centrates on daydreams or reveries as a profound basis of the scientific knowledge. Our diurnal daydreams are not nocturnal dreams which are the subject of the psychoanalytical research. Daydreaming is a process of our imagination, working with “oneiric” images. A psychoanalyst investigates the source of nocturnal dreams: our unconsciousness, our relations to the world and other people, etc. This is the horizontal point of view. The phenomenology leads us, according to Bachelard, to a different approach to “oneiric” images – to a vertical and subjective point of view. The interpretation oriented on sources is transformed by daydreaming flowing from these very sources. The education is a convoluted process balancing between an exact science and a subjective reverie.