The theme of certainty and uncertainty of law is one of the key to legal science and practice. It is essential for both law-making and law enforcement. In the domestic and foreign theory of law can be found different understanding of these categories and different methodological approaches to the study of this problem. The purpose of this article is to analyze, in the context of the main provisions of the Hegelian "Science of Logic", "Philosophy of Law", the works of foreign and domestic legal scientists, which constitute the methodological basis of research, the dialectical process of the transition of the certainty of law in abstracto into uncertainty and formation legal certainty. In the process of research, one-sidedness of the conclusions of individual scientists is revealed that the state of certainty and uncertainty of law is manifested exclusively in the sphere of verbal formulations of articles of laws and other normative legal acts. The article formulates a judgment that, from the philosophical and legal point of view, law as a complex sociopsychological phenomenon possesses a set (system) of invariable qualitative properties inherent only to it as one of the most important social regulators of human behavior. The law is abstract in nature, and at the same time specifically, because its rules and principles are addressed to everyone and everyone. Consequently, the right in itself represents the dialectical unity of uncertainty and certainty. Uncertainty of law in concreto is revealed when there are legal facts requiring legal qualification. It is concluded that the decision of the court or other law enforcement agency to resolve a specific legal situation is considered as the moment of termination and removal of the contradiction that arose between the properties of the uncertainty of the rule of law and actual circumstances, i.e. as the achievement of certainty in concreto.