 Mameli and Bateson maintain that it is an empirical question whether any of these properties cluster together. Moreover, they propose that in the absence of a known unifying causal process, we should not assume that they do cluster. Mameli (2008) has dubbed this the “clutter hypothesis” (in contrast to “cluster hypothesis”). Their work has inspired some recent philosophical discussion over which hypothesis (clutter or cluster) is best supported by available evidence and theory. For instance, Nicholas Shea (2012) argues that recent developments in evolutionary and developmental biology suggest that i-properties do not cluster. In contrast, Muhammad Ali Khalidi (2016) argues that innateness properties do cluster provided that we focus strictly on psychology and not biology (assuming that such a distinction can be drawn). This question remains an important area for future research.