a1 Department of Psychological and Brain Sciences, Johns Hopkins University, Baltimore, MD 21218 Halberda@jhu.edu http://www.psy.jhu.edu/~halberda/ Feigenson@jhu.edu http://www.psy.jhu.edu/~feigenson/
Rips et al. consider whether representations of individual objects or analog magnitudes are building blocks for the concept natural number. We argue for a third core capacity – the ability to bind representations of individuals into sets. However, even with this addition to the list of starting materials, we agree that a significant acquisition story is needed to capture natural number.