Is number a primary perceptual attribute?

In this article, I briefly review evidence accumulated over the past two decades that number should be considered a primary perceptual attribute. I concentrate primarily on adaptation studies, but also show how these have been reinforced with other psychophysical methods, as well as pupillometry, EEG and imaging studies. There have been at least two major challenges to number being a primary perceptual attribute, and to adaptation itself, which I discuss. I conclude speculating on the functional role of adaptation to numerosity perception, and suggesting areas requiring further research.