This paper discusses the American scientist and philosopher Charles S. Peirce's (1839–1914) classification of the sciences from the contemporary perspective of interdisciplinary studies. Three theses are defended: (1) Studies on interdisciplinarity pertain to the intermediate class of Peirce's classification of all science, the sciences of review (retrospective science), ranking below the sciences of discovery (heuretic sciences) and above practical science (the arts). (2) Scientific research methods adopted by interdisciplinary inquiries are cross-categorial. Making them converge to an increasing extent with the sciences of discovery, especially the methodeutic of normative logic, is one of the future challenges for studies on interdisciplinarity. (3) The overall structure of Peirce's classification, were it to be applied in today's situation, would not, in any major respect, be radically different from what it was designed to reflect a hundred years ago, in spite of the virtually exponential creation and production of new domains and the massive increase in investment in research and scientific publication. Accordingly, charges that the sciences of discovery are becoming ever more fragmented are not new.