Many studies suggest that students have difficulties in learning acid-base concepts. This study presents some conflicts in the textbook descriptions of these concepts and proposes these to be the cause of the students' difficulties. This is especially true regarding the description of the relationship among the Arrhenius, Brønsted-Lowry, and Lewis concepts of acids and bases, which are represented schematically as a cumulatively developing figure. However, some reagents, such as NH[subscript 3], dry HCl, and H[subscript 2]SO[subscript 4], cannot be classified as acid or base according to these descriptions. A new relationship is suggested in this study based on the ontological category of scientific knowledge. Finally, it is suggested that all three concepts be taught for the purpose of understanding not only the concepts but also the developmental nature of scientific theories.