0. Introduction In the last years there has been an increase in the call for a more realistic poli- tical theory, or to at least rethink the relationship between practical politics and political theory (Gunnell 1986; Honig 1993; Smith 2004; Stears 2005: Williams 2005). This plea has been put forward in different arenas and regarding various concerns. For instance, there is now a fairly common objection to purely ideal theory and an insistence on the need to engage with non-ideal theory as well (Farrelly 2007). There are also complaints about the impotency, or even futility of political philosophy in relation to real life problems (cf. Goodin 1995). Final ly, there is a thread of discussion regarding the feasibility of political theories (Mason 2004). I share the general sentiment of the criticism and appreciate the common reference to empirical studies as a remedy to the lack of realism. But I believe the need for more realism is not, or not only, to be identified regarding the mentioned topics. Empirical findings, I will claim, have a far more basic task fulfil than is anticipated in the common appeal to realism in political They are vital in the very construction of a plausible political theory. In this paper, I will focus on the role that findings of the empirical sciences might play in justifying philosophical claims. Regarding this more basic aspect of the need for realism, there is very little scholarly work being done so far. There are some ex 1 I will be unspecific about the notion 'empirical findings', My main focus is on normative beliefs of real people, which can be studied empirically, but obviously this is only one aspect Others are, e.g., anthropological findings about human biology, about the functiotiing of t human mind, or "the "muddied waters of regular political discourse" (Freeden 2005, 133)