This statement, taken from Ludwik Fleck's classic book of 1935, might be read as the guiding slogan of his whole enterprise. If the theory of knowledge is to bear fruit, he tells us, it must not be founded on some Phantasiebild of Science: some a priori definition, or "demarcation criterion", like that which Karl Popper has always insisted on. (Popper's Logik der Forschung had appeared in the previous year.) Any epistemological theory developed on an a priori basis alone faced insurmountable problems: it would do no more than explore the consequences of some arbitrary initial conception, selected to indicate what Science must be, if it was to fit the prejudices of the individual philosopher in question.