This account has been criticized as vacuous because it fails to say what activities are (Psillos 2004), to account for the relationship of causal and explanatory relevance (Woodward 2002), and to mark an adequate distinction between activities and correlations (Psillos 2004), though see Bogen (2005, 2008a) for a response. Glennan (forthcoming) argues that these problems can be addressed by recognizing that activities in a mechanism at one level depend on lower-level mechanisms. (See also Persson 2010 for a criticism of activities based on their inability to handle cases of polygenic effects.)