Let’s talk about definitions.  Many people seem to be confused about them.

We use words to talk about things. We use language or λόγος to attempt to understand and say what things are.  There are thus two major kinds of definitions, those which are the definitions of words (or terms) and those which are the definitions of the things talked about by means of words.  Thus, definitions are divided into nominal definitions and real definitions.

  1. A Nominal Definition tells you how a given word is used in a given language or linguistic community.
  2. A Real Definition attempts to state in words the nature or reality of a thing in question.

Nominal Definitions come in two kinds:

  1. A Lexical Definition (a “dictionary definition”) tells you how a given word or term is typically used within a given linguistic community.
  2. A Stipulative Definition tells you how a given writer or speaker is going to use a word; i.e. he stipulates that he uses a word with a particular meaning that he specifies.

The main thing to note about nominal definitions is that they cannot be correct or incorrect. There is simply no sense in saying that a dictionary definition is “right” or “correct”—unless it fails to reflect how a given word is actually used in a speech community; but there is no sense in which the definition can be right or wrong or correct or incorrect in itself.  Words may be used in any way that convention dictates; and no one dictates “correct” linguistic convention other than actual usage. Nominal definitions often define words by means of synonyms, examples, or etymology, things generally to be avoided in real definitions, as we will see.

It is even more nonsensical to say that a stipulative definition is incorrect.  When someone stipulates that he will be using a certain word in a certain way, it is his right to do so. The most that can be said is he may be acting tendentiously, by attempting to rhetorically load the definition of a term in order to obtain an unfair advantage.  A stipulative definition can be confusing or misleading and is more likely to be tendentious when it directly contradicts the common lexical definitions.  On the other hand, lexical definitions are almost always far too vague for philosophical purposes, so a stipulative definition which draws a distinction or is precisifying of a vague term is often very helpful.  For example, neither in ordinary English nor in philosophy is there an established distinction between morality and ethics. German, however, contains the words Moralität and Sittlichkeit, making it often useful for a translator of (say) Hegel to specify that he will translate Moralität as Morality and  Sittlichkeit as Ethicality (or whatever word he chooses).

Real Definitions also come in two kinds:

  1. An Essential Definition is the gold standard of definition. It states the essence or whatness or nature of thing, thereby telling you what it actually and essentially is.
  2. An Nonessential Definition does not state the essence of a thing but allows one to arrive at some understanding of the thing in question by referring it to something that belongs to it, such as its properties, causes, effects, or accidents.

St. John Damascene defines “definition” as follows

A definition is a concise statement setting forth the nature of the thing in question, that is to say, such expresses in brief the nature of the thing in question.

The function of a definition is allow someone to understand the thing in question.  This is done by “locating” the thing, so to speak, among all the things that are, by narrowing down what one is talking about to precise limits.  Hence to “de-fine” is to “make finite” to  “de-limit.” The Greek word for definition is ὁρισμός which means “boundary” and whence our English word “horizon.” To “define” something is to “draw a boundary around it (in speech) marking it off from everything else.” From this function follow certain rules of definition.

First, and most importantly, a proper definition must be coextensive with the thing defined. That is, in definition a thing T, one’s definition should be neither too broad, catching things that are not T, nor too narrow, leaving out things that are T.  A well-formulated definition is convertible with the thing defined, such that for all T, the definition will apply and will not apply to anything that is not T.  For example, one of my students once defended the moral permissibility of murder by defining “murder” as “killing.”  Although it was easy for her to prove that “murder” (so defined) is sometimes justified, she didn’t really prove her point, since in fact, not all killing is murder e.g. when an animal kills another animal or a human being kills someone is justified self-defense. I sometimes tell my students to imagine a pond filled with fish of all colors; a good definition will be like a net, in which one catches (say) the red fish; if the definition is successful, it will (1) catch every red fish, and (2) catch no non-red fish.

Second, a definition should be clear, concise, and literal. Since the purpose of a definition is to aid understanding, to make what a thing is clear, it is evident that a definition which defines a thing in terms that are either equally obscure or more obscure than the term being defined defeats this purpose.  Similarly, while poetic metaphors and figurative language have many legitimate uses, e.g. in poetry, they work against a clear understanding, and so should be avoided in philosophical definitions; a definition should be made in terms that are literal. Finally, a definition should be concise; there are books, and libraries written about human beings in all their variety and complexity, but in a definition, one wants to understand something “in one handful,” so to speak.  A definition which goes on too long also defeats the purpose of bringing about a clear understanding of a thing; a concise definition brings the entire thing in question before our eyes; we can see it “in one look”, as a whole. And if it the it is clear and literal, this “one, clear look” is enough to allow us to understand what it is we are talking about.  Once we have it clearly in view, sighted in on, so to speak, we can begin to examine it in greater detail, zooming in to look at its parts or zooming out to look at its relations.  But we need to begin with that one, clear idea.  (ἰδέα means “look”.)

Third, a definition should be positive, not negative, and non-circular.  To take the second first, to define A as B, and then define B as A obviously adds no clarity of understanding; similarly, to define A as A adds no clarity at all. People often fall into the trap of giving a definition by synonym, which can be acceptable in a lexical definition, but not in a real definition, where it is apt to be circular.  For example, if I do not know the noun “affect”, it may be perfectly fine to tell me that “an affect” is “an emotion”. Assuming I know the word “emotion,” I will now know what the word “affect” means  and how to use it.  But if I am a philosopher and I am interested in the question of what the affects (or emotions or passions or feelings) are, just giving me another name for the same thing doesn’t help me in any way to understand what it is.  And then, since a definition is meant to be a statement of what something is, it should be positive, and say what it is, and not what it is not.  There is an important exception to this principle, however: a negative definition must be used when defining negative or privative realities; e.g. “cold” must be defined negatively as the absence of heat, since cold has no real or substantial being of its own; or again, blindness must be defined in terms of a privation of sight, since it too has no positive being of its own, but is a word which denotes an absence or lack of a real power, namely, sight.

A real, essential definition is traditionally given in terms of genus and specific difference; one “locates” a being by stating the broader region or class of beings it falls under, its genus, and the essential characteristic (or characteristics) that set it apart from all other kinds of being in that class.  E.g. the traditional definitions of “man” or “human being” is “the mortal living being that has λόγος.”

The fact that humans are the only rational animals marks us off from all other animals; it is also our essential property, since everything distinctly human follows from our having λόγος or being intelligent; whatever in us that belongs to us qua animal, we share with other animals.

We could also define human beings in terms of a property. A property is a characteristic that, while not itself the essence, follows from the essence of a thing so that it properly applies to all members of the species (kind) in question.  A property definition of human being might be “The animal with a sense of humor” or “the animal that asks questions.” Both a sense of humor and the ability to ask questions are consequences of our having reason or λόγος, but not identical with it. Nevertheless a property would succeed in marking of a being from all other beings.

One can also define something by an accident. An accident is a characteristic that applies to a thing that is unconnected to its essence, and so belongs to it “by accident” (in traditional usage).  A accidental definition of human being might be “The intelligent animal than lives on earth” or “the most numerous primate on earth.” As it happens, all human beings do presently live on earth, and so the definition succeeds in marking off “human being” from all other things; but one day human beings may colonize Mars, and this accidental definition would no longer apply. Similarly, human beings do happen to outnumber all other primates, but it is possible that was not always the case, and could cease to be the case in some hellish Planet of the Apes future. (“Get your hands off me, you damn dirty ape!”)

One can also define a thing by one of its causes.  Of the four Aristotelian causes, formal, final, material, and efficient, the formal cause is the same as the essence or nature or whatness, so that is just another term for a real, essential definition. One could define a thing in terms of its material composition, e.g. water could be defined as “two parts hydrogen chemically bonded to one part oxygen.” One could define a thing in terms of its efficient cause or what brings it about, e.g. a solar eclipse as “an astronomical event caused by the moon blocking the sun’s light by coming between it and the earth.” Similarly, since things often have innate causal or dispositional powers, one can define a thing by its characteristic effects, e.g.  “a carcinogen is something that tends to cause cancer” or “Bacillus anthracis is the etiologic agent of the disease anthrax.” And finally, one can define a thing by its final cause, its goal, purpose , or function; this is particularly useful in the case of artifacts, since they are almost always made for specific purposes, e.g. “a clock is a device for telling time,” “a pen is an instrument for writing,” “a hairbrush is a tool for brushing hair” (note that this definition looks circular, but is not; many artifacts are named in such a way as to “contain” their definitions e.g. “hairbrush,” “screwdriver,” “washing machine,” “pencil sharpener,” etc.).

It is reasonable to begin a discussion by defining one’s terms.  As with most reasonable things, however, what begins as a reasonable request can become an unreasonable demand (one advanced under the false pretext of being reasonable) if taken to excess. Philosopher Peter Geach comments:


I am tempted to add another type of definition, the humorous definition, which while not a proper definition form, can often reveal something true or profound about something, e.g.


If you are not familiar with Ambrose Bierce’s The Devil’s Dictionary, go read around in it. It’s one of the funniest things ever written.

Even the great Samuel Johnson couldn’t resist a few humorous definitions in his 1755 A Dictionary of the English Language, defining “lexicographer” (a writer of dictionaries) as “a harmless drudge.”

To bring this to a close, I hope you get two main take-aways:

  1. Lexical Definitions, i.e. dictionary definitions, are nothing but records of word usage and DO NOT SAY ANYTHING SUBSTANTIVE ABOUT REALITY.  Noah Webster was NOT SOME SORT OF DEMIGOD.  Do not treat the dictionary as if it were Holy Scripture.  Attempting to argue whether a dictionary definition is “correct” or “incorrect” is always pointless (although most of them do suffer from severe vagueness) and will virtually guarantee you will have a pointless argument about words, in which you will never get to an actual examination of the subject matter in question. To argue about words is pointless, because words are conventional; the point is to direct the inquiry to reality, by means of words, because reality is not conventional.
  2. Real Definitions, as the name implies, refer to reality, and so can be correct or incorrect, and their test is very similar to the test of whether or not a proposition is true: their touchstone is reality, to which they must correspond.  It is correct to define gold as “a metal heavier than lead” because, in reality, gold is much heavier than lead (although this does not differentiate gold from other metals heavier than lead, so it is not a good definition).  It is incorrect to define a bird as “an animal that can fly”, since in reality, there are both flightless birds and flying animals such as bees and bats that are not birds.  Nothing, however, prevents there being a word, such as the ancient Hebrew “oit” which does mean “flying animal.” This is why, by the way, the Old Testament seems to classify bats as “birds.” It doesn’t; it classifies them as “oits,” as “flying animals,” which of course, they are—in the 17th century, the English “bird” could still do that work, so it was used in the King James Version of the Bible, although it now reads strangely to contemporary English speakers. Similarly, the word “fish” used to mean (more or less) “any animal than lives in the water,” and so whales and dolphins were “fish.” Today, we usually do not call whale or dolphins “fish” because our language has been somewhat altered by the classifications of modern biology, but it is utterly simpleminded to think that the people of Chaucer’s time were using the word “fish” wrongly in calling whales “fish.”   They weren’t.*

*Footnote on “fish.” Apparently there were three major variants of the word in English, “fisk,” “fiks” and “fish” (which won, obviously).  This is apparently a common thing in the history of English where a word oscillates between a competing -sk and -ks (= -x). Another case is “ask” vs “aks” (or “ax”).  Have you ever heard anyone use the expression “aks a question” rather than “ask a question”? While “ask” is more common, and came to be regarded as the proper pronunciation in the English upper classes, “aks” never went away among the English working classes, nor in some American dialects, particularly in Black English Vernacular.  Chaucer prefers “aks” to “ask,” which is a pretty weighty recommendation.  At any rate, “to aks a question” is not “wrong”. It’s just nonstandard.  Please refrain from “correcting” people who use it. I have no problem with people aksing me questions.


One comment on “Definitions

  1. Excellent read.

    In a way it is the difference between a definition reflecting reality vs. using a definition to shape reality.


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