The purpose of the paper is to evaluate of Imre Lakatos' MSRP (Methodology of Scientific Research Programs). Presenting the methodology which is based on Popperian Refutationism, Lakatos intended to overcome Pluralism (, Relativism and Skepticism) and distinguishes the best theory (/program) in science. The question is that did the lakatos' secondary change in the form and content of MSRP -against some historical facts and criticisms- make some serious deficiencies in his methodology? The answer to this question is positive. One of Lakatos' changes in MSRP is to resort to a new concept of "rationality". Presenting a logical analysis, the paper shows that this change causes MSRP to be unable to distinguish the best program among others. Fur...