When confronted with collections of stimuli and asked to determine which represent examples of 'birds', people find it easy to accept or reject things that have all or none of the properties they associate with that concept; when shown entities that share some but not all of the critical properties, people spend much more time trying to decide, and their decisions are tentative. Their responses simply aren't compatible with binary models.

In a natural environment, people's uncertainty could be uncertainty about their knowledge of the entity (does that thing really have cloven hooves? does that flying thing have feathers?), rather than about the concept (does a kosher beast have to have cloven hooves? does a bird have to have feathers?). It's possible that people's uncertainty in conditions where they are told that the beast has such and such characteristics is due to their methods of reasoning not being developed for such situations, which are rare in real life.