suggested as a methodological norm or principle, meaning that a full theoretical account of the social construction of, say, scientific, religious or ethical knowledge systems, should itself be explainable by the same principles and methods as used for accounting for these other knowledge systems. This points to a general feature of naturalised epistemologies, that such theories of knowledge allow for specific fields of research to elucidate other fields as part of an overall self-reflective process: any particular field of research occupied with aspects of knowledge processes in general (e.g., history of science, cognitive science, sociology of science, psychology of perception, semiotics, logic, neuroscience) may reflexively study other such fields yielding to an overall improved reflection on the conditions for creating knowledge. Reflexivity includes both a subjective process of self-consciousness inquiry and the study of social behaviour with reference to theories about social relationships. History The principle of reflexivity was perhaps first enunciated by the sociologists William I. Thomas and Dorothy Swaine Thomas, in their 1928 book The child in America: "If men define situations as real, they are real in their consequences". The theory was later termed the "Thomas theorem". Sociologist Robert K. Merton (1948, 1949) built on the Thomas principle to define the notion of a self-fulfilling prophecy: that once a prediction or prophecy is made, actors may accommodate their behaviours and actions so that a statement that would have been false becomes true or, conversely, a statement that would have been true becomes false - as a consequence of the prediction or prophecy being made. The prophecy has a constitutive impact on the outcome or result, changing the outcome from what would otherwise have happened. Reflexivity was taken up as an issue in science in general by Karl Popper (1957), who in his book The poverty of historicism highlighted the influence of a prediction upon the event predicted, calling this the 'Oedipus effect' in reference to the Greek tale in which the sequence of events fulfilling the Oracle's prophecy is greatly influenced by the prophecy itself. Popper initially considered such self-fulfilling prophecy a distinguishing feature of social science, but later came to see that in the natural sciences, particularly biology and even molecular biology, something equivalent to expectation comes into play and can act to bring about that which has been expected. It was also taken up by Ernest Nagel (1961). Reflexivity presents a problem for science because