The aim of this research is to address the question of how false beliefs about the law should be dealt with by the criminal law. While there has long been discontent with the current position, I argue that proposals to deal with this issue in relation to the mens rea are inadequate, and that a more consistent approach to this problem can only be developed by exploring the place of knowledge of the law as an autonomous concept from mens rea. This approach thus aims to complement the traditional model of criminal responsibility based exclusively on volitional states (mens rea) with one based also on cognitive states. In doing so, this thesis develops a meaningful and operative account of the cognitive or epistemic conditions of criminal responsibility, analytically scrutinising the question of how the criminal law should treat those who act in ignorance of criminal prohibitions and who unwittingly break the law. Part I of the thesis looks at the conditions for the establishment of criminal responsibility from the perspective of an institutional theory of the criminal law. The first two chapters develop a critical account of the two current ways a false belief might be recognised as a defence – mistake of fact and mistake of law – and the ways that different theoretical accounts, characterised as legal positivism and legal moralism, have sought to address this problem. These chapters also survey the ways that a mistake of law defence might be currently recognised and examines the proposals of those authors willing to expand the conditions under which ignorance of law should exculpate. After that, in chapter III, the dissertation begins to develop a fresh approach based on an institutional framework for the criminal law. The chapter explores the connections between criminal law and interpersonal and institutional trust. The last chapter of part I argues that both volitional and epistemic/cognitive conditions are key in a correct account of deliberation or practical reasoning processes before action. Thereafter the Epistemic Condition on Criminal Responsibility (ECCR) is proposed as an algorithmic test, able to distinguish culpable from non-culpable ignorance. Part II of the thesis, rejecting the classical dichotomy of mistake of fact versus mistake of law, introduces a new kind of approach based on distinguishing between brute facts, institutional facts and institutional commands. Chapter 5 discusses and defends the feasibility of the distinction between brute and institutional facts, arguing that the perception process of both is dissimilar. Later, the chapter applies the ECCR to existing cases in common law jurisdictions related to false beliefs about brute facts. Chapter 6 scrutinises the practical distinction between false mistakes about institutional facts and false mistakes about institutional commands and applies the ECCR to false beliefs about institutional facts. The last chapter puts the ECCR into practice to address false beliefs about institutional commands. Finally, a general point about false beliefs is considered: when the duty to seek legal advice becomes a right to rely on a trustworthy source.