Family resemblance, a concept introduced by philosopher Ludwig Wittgenstein, describes how category members don't necessarily share one common feature. Instead, they share overlapping sets of features, like members of a family who share different combinations of facial features without any single universal feature.
This explains why many natural categories have fuzzy boundaries. Consider 'game'. Chess, tag, soccer, and video games are all games, but what single feature defines all of them? No single necessary and sufficient feature exists. Instead, they share overlapping characteristics like rules, competition, or entertainment value.
Fuzzy boundaries mean some items are prototypical members while others are marginal. This framework explains categorization difficulties people face in real life. When you study concepts and categories, remembering that boundaries are often fuzzy rather than sharp lines helps you understand why psychological categorization is flexible and context-dependent.