Rationality is by far the luckiest philosophical concept in so far as its controversial status is concerned. It is also one of the terms most liable to rejection, criticism and analysis. Hence, our paper is an attempt to unscramble the controversy relating to tuning the term of rationality into a concept that denotes multiple significations in the era of modern philosophy, and which are considered the real foundation of what today's world has reached in its intellectual, scientific and social aspects. According to this terminological and conceptual context, we have tried in this paper to shed special light on the genealogy of the concept of rationality, its origins and evolution as a trend in modern European thought, given that it is a signifier that denotes the priority of the mind, and which derives from various meanings that are complementary at times, yet contradictory at others, whereby carrying multiple significations, that vary in accordance with its field of use. With a certain degree of special attention, we have also tried to glean some of the most basic meanings associated with the term of rationality. We hope that this way we may be able to clarify the obscurity that engulfs the term, something which has caused its ambiguity, and thus our confused understanding of the philosophy of modernity.