The discussion now turns to the dynamic dimension of legal certainty. The dynamic dimension concerns the problem of action in time and the ideals that must be guaranteed if law is to "assure" citizens of rights and thus act as an instrument of their protection. The chapter therefore deals with the ideals of reliability and calculability. Reliability means the ideal state in which citizens can know which changes are allowed or prohibited, avoiding frustration of their rights. Thus reliability exists only if citizens can be assured today of the effects that law assured them yesterday, which in turn depends on the inviolability of past situations, the durability of the legal order, and the irretroactivity of present norms. Calculability means the ideal state in which citizens know how and when changes can be made, so that they are not surprised. Thus calculability exists only if citizens can control today the effects law will assign tomorrow, which is the case only when they can anticipate and measure a fairly invariable range of criteria and argumentative structures that define consequences to be assigned, heteronomously and coercively or autonomously and spontaneously, to their own actions or those of others, or to facts that have actually occurred or might occur in future, whether or not they are in dispute, as well as the reasonable time frame within which the final consequences will be applied.