"We take the words out of their natural place in talking and assume they refer to some essence or ideal entity which we try to define. Because the word is uniform in appearance, we assume it refers to a uniform entity about which we can generalise. We forget the application of the word.
Take the word "good". What is common between a good joke, a good tennis player, a good man, feeling good, good will, good breeding, good looking, and a good for nothing? There is no one common property which the word good refers to.
We cannot analyze the word so that we reach some essence or element from which the concept is built up. But there are resemblances between the various meanings of the term - like family resemblances."John Heaton (paraphrasing Ludwig Wittgenstein) Introducing Wittgenstein pp.126-127
In his essay on The 'Uncanny' (1919), Freud attempts to distill a definition (or 'essence') of the term "uncanny" by analysing examples of its application. His acknowledgement of the difficulties of this approach are an example of the problem that Wittgenstein's theory of family resemblances describes:
"... the word [uncanny] is not always used in a clearly definable sense, so that it tends to coincide with what excites fear in general. Yet we may expect that a special core of feeling is present which justifies the use of a special conceptual term. One is curious to know what this common core is which allows us to distinguish as 'uncanny' certain things which lie within the field of what is frightening.
In his study of the 'uncanny' Jentsch quite rightly lays stress on the obstacle presented by the fact that people vary so very greatly in their sensitivity to this quality of feeling.
Either we can find out what meaning has come to be attached to the word 'uncanny' in the course of its history; or we can collect all those properties of persons, things, sense-impressions, experiences and situations which arouse in us the feeling of uncanniness, and then infer the unknown nature of the uncanny from what all these examples have in common."Sigmund Freud The 'Uncanny' in An Infantile
Neurosis and Other Works pp.219-220
Mike Kelley also describes collecting a group of examples of the uncanny in order to better define the meaning of the word, in the opening of his essay Playing with Dead Things: On the Uncanny (1993):
"What I'm after is a group of objects that, like the original collection of images I pinned to my wall, share an 'uncanny' quality. What this quality is, precisely, and how it functions, are difficult to describe."Mike Kelley p.26 The Uncanny
Their feeling that the word uncanny is ill-defined and provisional is indicated by the fact that both Freud and Kelley frequently place it between inverted commas.