ABSTRACT
Defining can take a variety of forms: for instance, pointing to examples, enumerating all the things that fall under the term at issue, or legislating how a term will be used. One type of definition, sometimes called “essential” or “real” definition, has special power as an analytic tool. A real definition of something, X say, would identify a set of properties such that each and every X has all the properties that make up the set and only Xs have that set of properties. A real definition specifies a group of properties each of which is necessary for something’s being an X and which, taken as a group, are also sufficient for something’s being an X. In other words, a definition of X characterizes what all Xs and only Xs have in common. For instance, a widow is a woman who has lost her husband by death and who has not married again. In this case, there are three necessary conditions that together are sufficient for someone’s being a widow. Several points should be noticed. Sometimes a person may be able to identify and
refer to Xs without being able to define what makes something an X. For instance, she might acquire a working mastery of the relevant concept as a result of being introduced to a range of typical examples. People could identify water successfully long before science revealed its essential molecular structure. And conversely, someone who knows how Xs are defined might not be able to apply that definition to settle in an uncontroversial way the status of borderline or otherwise “hard” cases, or even to identify ordinary instances. For example, I can know that a person is bald if his scalp wholly or partly lacks hair, yet not be sure of a particular man whether he is bald. Finally, it need not be the case that a thing’s defining essence reveals anything about how and why it is important to us. For instance, speeding is legally defined as exceeding the maximum rate of progress specified for a given route, but this tells us nothing about why we care to set such limits.
