The idea of family resemblance is Wittgenstein's answer to the idea of fixity of meaning. We tend to think of words as labels that we can apply to things, ideas, mental states, and so on. This leads to the notion that a word like "understanding" must have one fixed meaning, which we might identify as some sort of mental state or process. When we use the word "understanding" in different contexts, we think that both uses of the word share something in common.

In order to show the error in this way of thinking, Wittgenstein uses the metaphor of family resemblance. If we gather together five members of the same family, they probably look alike, although there is no distinctive feature that they all share in comm on. A brother and a sister might have the same dark eyes, while that sister and her father share a slightly turned-up nose. They have a group of shared features, some of which are more distinctly present in some members of the family, while some features are not present at all. Wittgenstein argues that the different uses of one word share the same family resemblance. There is no single defining characteristic of all uses of the word "understanding"; rather, these uses share a kind of family resemblance w ith one another.