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Family resemblance is a concept within modern analytic philosophy and language critiques that says words are loosely defined by a combination of common, or related, features rather than a rigorous single feature. It was popularised by Ludwig Wittgenstein in his posthumously published Philosophical Investigations, using the German term "Familienähnlichkeit". "Family likeness" or "family similarity" have also been used as common English translations.
Not all features may be common to every object in the "family". For analogy, four siblings in a family can be said to resemble each other, even though only three may share the same hair colour, three may share the same eye colour, only two may have similar facial structures and so on. No single feature is common to all members of the family, so the family cannot be defined as "having a wide nose" nor "having ginger hair". Wittgenstein's go-to example of a word with resemblance is "game". In this case, things that we refer to as "games" can be very wide ranging; from chess to baseball. Similarly, what can be called a "sport" has the same problems. Not all sports require extreme physical exertion; motor racing doesn't require as much athletic ability as running and weightlifting, and snooker certainly doesn't. In a similar vein, competitive ballroom or Latin dancing requires skill, training and physical ability, fitness and co-ordination, while also being highly competitive, but is not considered as "sport" in common parlance despite some campaigns insisting that it "is" a sport. Neither must it be "defined" as being competitive, as not all sports and games have a necessary competition element, and not all competitions are sports.
Yet, the loose weave of commonalities do allow us to group things together with words. This gets troublesome at the frayed edges, where people may - while thinking of things purely in terms of "essences" - wonder if something really "is" or "is not" a particular thing. The point of more modern approaches to language and philosophy is that this "is" or "is not" question isn't particularly interesting and has few actual ramifications.
One of the main points stemming from the notion of family likeness between words, is that arguing from simple dictionary definitions is pointless, and somewhat fallacious. This can be seen in anyone trying to argue that atheism is a religion. It may have some superficial similarities if you look at particular movements or media trends, including New Atheism and Atheism+, but otherwise atheism as a raw concept has few, if any, of the commonalities found in most religions.
Family resemblance also more accurately models the way the mind actually thinks; using loose resemblance over explicit rules to categorise things. This should be obvious to most people, as this happens to be exactly how the universe functions, too. Given this, it's a wonder that it took until the mid-20th century for the idea to break into the philosophical mainstream, and still eludes Internet Debaters who think "Webster's Dictionary says atheism is..." is the height of thought.
 See also
- Family Resemblance in Wittgenstein, Philosophy Index