The humanities have been viewed as a separate class of disciplines ever since Wilhelm Dilthey introduced his influential distinction between Geisteswissenschaft and Naturwissenschaft. Yet a comparative historical investigation of the humanistic disciplines shows a strikingly different picture: humanistic practice is characterized by an unbroken tradition of a search for patterns and principles in texts, art, languages, music, literature and theatre. This search is found in all periods and regions, and although it was dismissed by Dilthey and others, it continues to be part and parcel of humanistic practice. For example, in art history we find stylistic and iconological analyses carried out by a principle-based approach (initiated by Wölfflin, Panofsky and developed by others). The narratological, rule-based analysis of literature was initiated by Propp and Todorov. And principles and rules are also found in linguistic analysis (Chomsky, Joshi and others), musicological analysis (Schenker, Lerdahl) and theatre analysis (e.g. universal acting rules by Barba). This paper investigates the nature of the knowledge produced in the humanities, in particular with respect to the notions of patterns and systems of rules. Some rule systems aim at defining a consistent and procedural set of rules, which occurs, for example, in linguistic and musical grammars. We will refer to these systems as procedural rule systems. More often, however, the rules do not constitute a formalized set but ‘declare’ what might be called restrictions for certain literary and artistic genres. They are similar to what in the information sciences is sometimes referred to as a declarative rule system. Thirdly, there are also rule systems that only give tentative, heuristic rules and to which we will refer as a heuristic rule system. In our comparison of rule systems, we find that there is no radical distinction between the humanities, social sciences and natural sciences. Instead there is gradual course between the most ‘informal’ humanistic disciplines such as poetics, to the most formal natural sciences such as physics, where some (sub)disciplines, such as theoretical linguistics, lie closer to physics than does a biological discipline like ethology. Philosophers of the humanities were wrong in believing that there is a constitutive distinction between Geisteswissenschaft and Naturwissenschaft. The study of actual humanistic practice shows otherwise.