 A useful way of making this point invokes Segal’s (1996) idea of diachronic modularity (see also Scholl &amp; Leslie, 1999). Diachronic modules are systems that exhibit parametric variation over the course of their development. For example, in the case of language, different individuals learn to speak different languages depending on the linguistic environment in which they grew up, but they nonetheless share the same underlying linguistic competence in virtue of their (plausibly innate) knowledge of Universal Grammar. Given the observed variation in how people see the Müller-Lyer illusion, it may be that the visual system is modular in much the same way, with its development is constrained by features of the visual environment. Such a possibility seems consistent with the claim that input systems are modular in Fodor’s sense.