The subject-matter of this article is the “systemacity of law” concept and its methodological feedback. Continuing a series of articles on this subject, the author focuses on the internal rationality of claims about systemic character of law. This rationality is embedded in the legal thinking of Modernity and reveals itself in the belief in rational nature of law. According to this style of legal thinking, such internal rationality impedes law from being chaotically or randomly organised and structured. Therefore, law shall have a reasonably organized structure, even if in reality it does not have such a structure. In this way, the belief in an internal rationality of law transforms itself into the requirement for the rational organization of law. These two elements — belief in an internal rationality and the requirement of rational organization of law — are the pillars of the dogmatic conception of law which was established in Begriffsjurisprudenz of the 19th century and which still holds sway over contemporary continental legal thinking