The first chapter traces the origins and evolution of the concept of rationality in Western thought, arguing that its root lies in the Greek concept of logos. It explores early philosophical attempts to grapple with the paradox of change originated by this concept of rationality, contrasting the views of Heraclitus and Pythagoras. A pivotal moment came from the Pythagorean discovery of irrational numbers, which shifted the foundations of rational arguments towards geometry. In particular, the chapter examines the Eleatic School’s denial of the reality of change as argued by Parmenides and Zeno and the atomists’ counter-arguments developed by Democritus and Epicurus. This chapter highlights the emergence of logical consistency as a core element of epistemic rationality. The role of dialectic in extending rationality to dialogue and evolution is analysed, distinguishing between the critical dialectic of Socrates and the teleological dialectic of Plato’s later works. Finally, the chapter introduces a crucial dichotomy between rationality viewed as progressive accumulation of true knowledge versus rationality as faculty of learning. This dichotomy sets the stage for the analysis of scientific pluralism developed in subsequent chapters. This chapter shows the importance of pluralism in fostering the rapid development of early Greek philosophy and science.