 McConwell and Currie (2017) have added further complexity to the discussion by arguing that there is an important difference between abstract, source-independent accounts of contingency and source-dependent accounts. The notion of sensitivity to initial conditions is a good example of what they mean by a source-independent account: it gives an abstract characterization of historical or biological processes without saying what the source of contingency is. Does the source have something to do with genetic drift? With mutational ordering? Or something else entirely? McConwell and Currie argue that in order to establish the autonomy of macroevolutionary theory, a source-dependent account is required.