My article investigates the paradoxical dualism in Kelsen's Pure Theory of Law, in which exists on the one hand a strict distinction and on the other hand a necessary relation between Is and Ought. I shall further try to answer the question whether Kelsen's pure theory tacitly assumes in the conditions for validity of the positive legal order a basic value and underlying condition, namely, that of 'social peace'. In order to answer that question, I will first sketch why Kelsen actually differentiates Is and Ought and conceives of a relation between both categories. Then I will examine if and in what way the conditio tacita of social peace is implied in the conditions ('basic norm' and 'effectiveness') for legal validity. The concluding section will suggest what this implication possibly means for the (re)valuation of the Pure Theory of Law and of its normative concept of the positive legal order.