 The question can perhaps be made more pointed if we consider sortals for collections that do not imply complex structure. Corporations are complex, as are educational institutions. In contrast, a flock of birds does not entail a great deal of complexity, just enough birds in one location. We can sometimes definitively say that two flocks are the same: if exactly the same birds flew over the lake this morning as did yesterday then the two flocks are in fact one. And if none of the birds that flew over the lake yesterday were in today’s flock, then they are definitively different. But suppose most but not all were the same birds?