Phenomena commonly referred to as illusions, fallacies, and cognitive biases have been extensively investigated as valuable tools for elucidating the characteristics of human perception and cognition. Nevertheless, since these phenomena are analyzed and utilized from an interdisciplinary standpoint, these terms are used for phenomena of different characteristics, leading to confusion regarding the definition of “illusion.” In this paper, the author introduces the concept of sakkaku-sei (cognitive multiplicity), which characterizes these phenomena. The author classifies these phenomena into three primary types: (i) multiple interpretations inside and outside a human cognitive system, (ii) multiple interpretations inside a single human cognitive system, and (iii) multiple interpretations between multiple human cognitive systems. Employing the proposed categorizations, approximately 40 types of illusions and cognitive biases phenomena are classified. This classification offers insights into what sense a specific phenomenon is regarded as illusions, fallacies, or cognitive biases, thereby facilitating an understanding of these phenomena from various perspectives. Moreover, the classification provides clues for system design intervening in human perception and cognition in domains such as human-computer interaction and virtual reality.
