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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Definition | 3/4 | https://en.wikipedia.org/wiki/Definition | reference | science, encyclopedia | 2026-05-05T07:23:46.235974+00:00 | kb-cron |
At least one thing is stated to be a member of the set being defined; this is sometimes called a "base set". All things bearing a certain relation to other members of the set are also to count as members of the set. It is this step that makes the definition recursive. All other things are excluded from the set For instance, we could define a natural number as follows (after Peano):
"0" is a natural number. Each natural number has a unique successor, such that: the successor of a natural number is also a natural number; distinct natural numbers have distinct successors; no natural number is succeeded by "0". Nothing else is a natural number. So "0" will have exactly one successor, which for convenience can be called "1". In turn, "1" will have exactly one successor, which could be called "2", and so on. The second condition in the definition itself refers to natural numbers, and hence involves self-reference. Although this sort of definition involves a form of circularity, it is not vicious, and the definition has been quite successful. In the same way, we can define ancestor as follows:
A parent is an ancestor. A parent of an ancestor is an ancestor. Nothing else is an ancestor. Or simply: an ancestor is a parent or a parent of an ancestor.
== In medicine == In medical dictionaries, guidelines and other consensus statements and classifications, definitions should as far as possible be:
simple and easy to understand, preferably even by the general public; useful clinically or in related areas where the definition will be used; specific (that is, by reading the definition only, it should ideally not be possible to refer to any other entity than that being defined); measurable; a reflection of current scientific knowledge. Certain rules have traditionally been given for definitions (in particular, genus-differentia definitions).
A definition must set out the essential attributes of the thing defined. Definitions should avoid circularity. To define a horse as "a member of the species equus" would convey no information whatsoever. For this reason, Locke adds that a definition of a term must not consist of terms which are synonymous with it. This would be a circular definition, a circulus in definiendo. Note, however, that it is acceptable to define two relative terms in respect of each other. Clearly, we cannot define "antecedent" without using the term "consequent", nor conversely. The definition must not be too wide or too narrow. It must be applicable to everything to which the defined term applies (i.e. not miss anything out), and to nothing else (i.e. not include any things to which the defined term would not truly apply). The definition must not be obscure. The purpose of a definition is to explain the meaning of a term which may be obscure or difficult, by the use of terms that are commonly understood and whose meaning is clear. The violation of this rule is known by the Latin term obscurum per obscurius. However, sometimes scientific and philosophical terms are difficult to define without obscurity. A definition should not be negative where it can be positive. We should not define "wisdom" as the absence of folly, or a healthy thing as whatever is not sick. Sometimes this is unavoidable, however. For example, it appears difficult to define blindness in positive terms rather than as "the absence of sight in a creature that is normally sighted".
=== Fallacies of definition ===