‘Rational’ and ‘Irrational’ are two words in the philosophical diction which have been very confounding and controversial. For many philosophers, the philosophy of Plato is not only a great example of Idealism but also of rational thought. But for many philosophers it is not according to rational standards or even to standards of common sense. Kierkegaard is supposed to be the father and founder of Existentialism and his dissatisfaction towards reason, rationality and logic is quite obvious. Many great philosophers and thinkers like Schopenhauer, Bergeson, Nietzsche and Freud (the founder of Psycho Analysis) raised many serious questions about the validity of rational arguments and the importance of reason in the complete personality of a human being. The views of Karl R. Popper in this regard are very interesting. He is the strong upholder of Rationalism in Philosophy and Science but he does admit the limitations of a rational thought. This article is an attempt to understand the concept of a valid rational argument according to Karl R. Popper.