Is legal certainty a formal value?

Legal certainty is a central requirement for the rule of law. Legal systems should both enable those subject to law to predict human behaviour and institutional reactions and to prevent an arbitrary use of state power against them. The value of legal certainty is usually conceived as a formal value opposed to the values of freedom or equality (material or substantive justice). The purpose of this paper is to discuss this idea and to explore a less formal conception of the value of legal certainty. In this sense, I will argue that we need to depart from a purely formal understanding of the requirements imposed by such value and to consider the need to provide them with a substantive content. So, legal certainty is not completely independent of legal substantive justice and it implies that it is impossible to define ex ante which concrete aspects of justice should not be the law's concern.