Mistake of law and legal uncertainty regarding the pursuit of and the defence against claims

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Whether recourse to a mistake of law can protect against legal disadvantages is a classic question of jurisprudence. The Roman legal phrase "error iuris nocet" – the mistake of law is harmful – is widely known. The problem is often examined in the light of criminal law, but the issue is also of fundamental importance in private law. The concept of “mistake of law” needs to be clarified. The work presented here demonstrates that the term is poorly chosen as it does not necessarily cover cases of (deliberate) legal uncertainty. However, it is precisely in this area that, at least in the recent past, the actual practical relevance has become apparent.German private law treats mistakes of law and legal uncertainty very differently in different contexts. The spectrum ranges from leniency to great strictness. These discrepancies irritate many observers. The habilitation thesis shows that the prevailing approach is by no means as contradictory in its results as it may seem at first glance.To prove this, the thesis develops a new doctrinal framework for the field of mistakes of law. This is divided into four "quadrants". On a first level, a distinction is made between the assumption of being in the right oneself and an underestimation of one's own rights. On the second level, a distinction can be drawn between the pursuit of and the defence against claims. The erroneous failure to pursue an existing claim and the erroneous defence against third-party claims regularly lead to considerable disadvantages for the erring party (prescription, liability). On the other hand, mistakes of law oftentimes prevent disadvantages when it comes to the erroneous pursuit of aclaim or the failure to defend oneself against non-existing claims.As is demonstrated, these different results can be explained, to a great extent, by uniform aspects. These can be found above all in the law of civil procedure – a field that has so far been largely neglected in the context of mistakes of law. In particular, there is a general interest in the clarification of unresolved legal questions. In this respect, it is crucial to provide sufficient incentives in cases of legal uncertainty.Following this doctrinal analysis, the thesis develops concrete standards to facilitate the practical handling of mistakes of law. In conclusion, it is shown that the current system can still be optimised in many respects – not least in view of the future significance of smart contracts and artificial intelligence.