Many legal concepts seem to be perfectly clear to neophytes but in fact prove to be highly complex when it comes to determining if not their meaning, then at least their gist. Legal certainty definitely belongs to this category. As for beauty or democracy, it is possible to sense its meaning without being able to explain it clearly.

It formed part of the collective legal unconscious for many years until it was given a firm legal basis in case law by the ECJ in 1962 (SNUPAT and Bosch judgements) and then by the ECHR in 1979 (Marckx judgement). It was not incorporated into the French legal grammar until 2006 via the KPMG judgement made by the Conseil d'Etat (Council of State). Since then, post-rationalisations have led to its recognition in the French Declaration of the Rights of Man and of the Citizen (in a clever combination of articles 2 and 7 – on certainty – and 16 – on the guarantee of rights), as an implicit constitutional “value”, or indeed as the basis of the rule of law. As for any new concept, it has led to new readings and new ways of structuring the law, which seems to be unaware of its constitution.

This late appearance of the concept in legal history could be explained by the Tocqueville effect. When a particular phenomenon declines, any reappearances of this phenomenon become increasingly intolerable. The concept of legal certainty may thus have appeared at this precise moment in history in which legal certainty is already very prevalent within modern legal systems and, in a certain sense, no longer poses problems, becoming established as an implicit requirement of a “good” legal system.

Without really seeking to theorise this notion, law, case law and their reinterpretation by the doctrine, have caused legal certainty to become a “global concept” – a concept defined by a sum of components whose completeness is never postulated: nonretroactivity, accessibility and intelligibility, normativity and quality of the law, consistency of the law and case law, and the need for transitional measures in order to cope with an instability or unpredictability of the law, etc.