Some time ago, I read somewhere how traditional definitions fail in real world since they define precise boundaries depending on whether something follows certain condition or not; however, real world is full of edge cases where an object might seem to satisfy two (near) exclusive definitions.

The solution to this problem was to define things not in terms of boundaries but in terms of central points which will correspond with objects that follow the definitions precisely. The farther you go from the central point, objects will follow the definition less rigidly. Thus, the edge cases will be somewhere midway the two central points.

It's been a few years and now, I can't seem to find what this second type definition is called (neither do I remember where I read this). Any ideas what is this concept called?

  • This sounds like it might be fuzzy logic. If so, reply back and I'll expand into an answer. – cobaltduck Oct 25 '16 at 15:13
  • @cobaltduck Fuzzy logic seems to be the right field, however I do remember it being referred to by a single (or a couple) word which I am unable to find in the linked article. – strNOcat Oct 25 '16 at 15:21
  • prototype definitions, graded membership definitions? – GrimGrom Oct 25 '16 at 15:29
  • 1
    @Silenus Graded Membership seems to be correct, thank you. – strNOcat Oct 25 '16 at 15:34

Consider the notion of family resemblance, inaugrurated by Wittgenstein. Wittgenstein argued that

things which could be thought to be connected by one essential common feature may in fact be connected by a series of overlapping similarities, where no one feature is common to all. Games... have become the paradigmatic example of a group that is related by family resemblances.

This site describes the motivating idea behind family resemblance as the idea that

[at least some] words are loosely defined by a combination of common, or related, features rather than a rigorous single feature.

This seems to be what you describe: words defined in terms "central points" rather than precise "boundaries".

As an example, a definition of the word "game" might be given by listing many features of games (some of which might be exclusive). Something is likely a game if it satisfies many (but not necessarily all) of the conditions. Different games will satisfy different conditions. Something that only satisfies a few of the conditions might be a borderline case, that is, it's unclear whether it should count as a game or not.

You might call such definitions family resemblance definitions.

Others use disjunctive definitions, although this term is a bit misleading.

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