Recently the term “interdisciplinary” is becoming more and more popular. Despite this, there is no satisfactory definition of “interdisciplinary studies.” That is why both scholars and grant-funding institutions have no good criteria for distinguishing essential usages of this term from inessential ones. The first part of this paper aims to provide an approximate analysis of the concept of interdisciplinarity, that is a definition indicating some essential examples of interdisciplinarity and listing some exemplifications of research which are not interdisciplinary in any essential sense of the word. Some problems of interdisciplinary research, including the assumed division and hierarchy of sciences as well as the need for a specific language of interdisciplinary research is discussed. In the second part of the paper, Kazimierz Twardowski’s research is discussed as an example of interdisciplinarity in one of the essential senses of the word. His investigations are contrasted with the investigations of those of his students, namely Jan Łukasiewicz and Władysław Witwicki, who became “intradisciplinary” researchers.