Rationality as Retaining "Fixed Propositions" and Replacing "Fluid Propositions"

One of the popular theories of rationality of science is rationality as foundationism according to which rationality of a scientific theory is based on sense data upon which the theory has been constructed. The issue of certain data is، however، followed by many debates. In the present article، appealing to Wittgenstein ideas about «certainty»، authors present a new understanding of certainties in a scientific theory. According to this new understanding، each and every scientific theory consists of two kinds of propositions: fixed and fluid. Based on this classification of propositions of a scientific theory، a new idea is presented concerning rationality of scientific theories according to which a theory is rational if، firstly it is consistent and، second، it retains fixed propositions of the scientific society and replaces fluid propositions by other proper ones. Presenting historical evidence، the authors try to show that this idea is efficient and realistic if it is assessed according to standards of rationality which are based on evidency of sense data.