Evidence indicates that unless confronted by a task that requires concentration, the mind tends to generate stimulus- independent though (SIT) -- it tends to wander from the current stimulus environment (see Singer, 1966; Antrobus & Singer, 1964; Klinger, 1971; Smallwood & Schooler, 2006).
Recent functional imaging studies suggest that the "default network" (Raichle et al., 2001) -- cortical regions that are active when people are "resting", may play a role in the production of these thoughts (Christoff et al., 2003; Maguire et al., 2001; Mason et al., 2007; McKiernan et al., 2003; 2005; Mazoyer et al., 2001). It has been suggested that mind-wandering (entertaining thoughts unbound to the external world) may constitute a psychological baseline (Klinger et al., 2001; Buckner & Carroll, 2007).