Visual illusions raise a simple but fundamental question: how can perception differ so systematically from the physical world? For this reason, illusions have played a central role in vision research for more than a century and have been linked to perceptual mechanisms, neural structure, personality, and development. Many of these approaches implicitly assume that illusion susceptibility reflects a general property of the visual system, sometimes even a common “illusion factor”.
However, results across illusion studies are surprisingly inconsistent. While illusion magnitudes typically show good test–retest reliability within a given illusion, correlations between different illusions are often weak (Grzeczkowski et al., 2017; Cretenoud et al., 2019). This pattern challenges the idea of a single common illusion factor and leaves open whether visual illusions share underlying structure or instead reflect largely independent perceptual processes. Addressing this question requires moving beyond single-illusion approaches toward systematic batteries of illusions, allowing emerging factors to be identified, compared, and tested across individuals, tasks, and contexts.