Architect Hans van der Laan spent his lifetime studying human perception of proportion. His work led to the development of the plastic number system. In this system every step represents a group of fractions that describe objects of similar sizes. Indeed, each step is defined as the ‘representative size’ of a group of sizes that are perceptually indiscriminate from one another. Over decades of research, Van der Laan conducted a series of experiments to test his system. The clarity with which he approached the perception of proportion provides a suitable ground for testing the plastic number by scientific means. To take the first step in that direction, it is useful to consider Van der Laan’s approach within the context of sensory psychophysics and Gestalt psychology. Highlighting these connections suggests specific directions for future research of the plastic number and opens a new chapter in studies on proportion in architecture. This is a postprint of the published article.