The article considers an important for the Anglo- Saxon legal system theme, i.e. the concept of reasonableness in law. Reasonableness is often marked as a source of Anglo-Saxon legal system. But neither research literature nor course books give the comprehensive definition of this source. During the transformation of various spheres of legal validity to ensure a stable and effective legal regulation it is topical to use comprehensive concepts developed on the basis of general learned knowledge. Thus, there has recently been an increased interest in the concept of reasonableness that is topical not only for legal theory, but for legal practice. But a comprehensive understanding of the essence of reasonableness in legal thought has not yet been developed. It has been shown that in English research literature, there is a variety of opinions concerning the definition of “reasonableness”. In attempts to establish a common concept of reason as a source of law and a clearer presentation of it in some cases, the researchers see it as a kind of reasonable means to fill gaps in statute law, in others see it as everyday life, the essence of common law.