A membership function in fuzzy logic represents the degree of truth of the statement "

belongs to the subset

of the set

". The set

can be split into the subsets

and in ordinary set theory either

or

. In fuzzy logic, the membership function represents the degree to which

. For example, let

be the set of ages of people. Three subsets of

are

,

, and

(young, middle-aged, and old). Take some

who is 28 years old. It cannot be strictly said whether

belongs to

or

or even perhaps

. To use fuzzy logic, we might define

by: "If the person's age is

, the person is 100% middle-aged. If their age is

or

, the person is 50% middle-aged." This Demonstration plots the function used to define membership, which depends on the

and

; the center is

and the 0% member boundary is at

.