The reasonableness principle is a specific Italian feature pertaining to the explication of two overlapping basic principles of modern constitutional states: the equality principle and the proportionality principle. This principle is not specifically named in the text of the constitution of Italy, but was developed solely in the practice of the Constitutional Court. Becoming one of the basic building blocks of the constitutional structure, a sort of “objective order of values” in the Italian manner, the reasonableness principle is an unavoidable concept of serious analysis and understanding of the constitutional identity of Italy. In addition, this principle is little known in our country. In its rulings, the Croatian Constitutional Court increasingly tends to refer to the practice not only of the European Court of Human Rights, but also of courts of other countries, particularly in landmark cases (e.g. the decision regarding the “crisis tax”, the right of assembly, etc.). The paper attempts to show that the principio di ragionevolezza actually represents the essence of the modern practice of the Italian Constitutional Court, which can be identified, regardless of what we may call it, in the general direction of the development of constitutional court practice in general. Understanding this principle becomes important for the understanding of Croatian legal theory and practice in this area. It also recontextualises the well-known tension between the concepts of the “application of law” and “creation of law” at the highest institutional levels which in civil law European countries are nominally considered to be only “applying the law”. The paper provides arguments for the thesis that the reasonableness principle operates as an attempt at objectivisation of constitutional court interpretations, whose features are virtually of a quantum nature. In other words, the determination of the content of a concrete constitutional right and constitutional values in the process of their application constitutes in fact their (re)creation.