The Art of Reasoning | Audiobook: A Practical Guide to Logic & Argument
If we’re looking for a definition, why not just use the dictionary? Isn’t that what dictionaries are for? Well, yes and no. A dictionary is a good place to begin our search for a definition. But dictionaries are concerned with words. They often give nothing more than synonyms and rarely provide the full context we need to understand the concept that a word expresses. To define a concept, we usually have to go beyond the dictionary. Logicians have identified six rules for constructing a type of definition that is suitable for general purposes: A definition should include a genus and a differentia. A definition should be neither too broad nor too narrow. A definition should state the essential attributes of the concept’s referents. A definition should not be circular. A definition should not use negative terms unnecessarily. A definition should not use vague, obscure, or metaphorical language. 1.5A Genus and Differentia 1. A definition should include a genus and a differentia. The most common way to violate this rule is to leave out the genus. And the usual sign of this omission is the use of the word “when,” as in “fear is when you think you’re in danger.” What’s missing in this definition is the genus: Fear is the emotion one feels in response to the awareness of danger. The word “where” is sometimes used in the same way, as in “a denouement in a story is where the conflict is resolved and the story ends.” Here again, the genus is missing. This definition does not tell us what kind of thing a denouement is—namely, an element or a stage in the plot of a literary work. Like the differentia, the genus need not be specified by a single word. If we define an automobile as a motor vehicle intended for personal transportation, the genus is contained in the phrase “motor vehicle.” A definition of USER SESSION might be: “A user session at a website occurs when a unique user is present on a site for a specified length of time,” which distinguishes it from a hit, which occurs each time a user views a different page at a site. This definition includes a genus even though it uses the word “when,” because it describes the user session as a unit of measure for usage of a website, in contrast with another measure, a hit. From a logical standpoint, user sessions and hits are two species of the genus “unit of measure for website usage.” The definition makes this clear. When we define a concept that designates an action, we often use a verb phrase, as in “to practice is to perform an activity for the sake of improving one’s skill.” The genus here can be restated as “an activity one performs.” The English language gives us many ways to specify a genus. To identify the genus, we need to look for the element in a defining statement that refers to a wider class. A definition by genus and differentia builds on what we have learned about classification. The genus not only helps us identify the referents of a concept, but also conveys a great deal of information about them. If you don’t know what a peso is, for example, the most useful thing I can tell you is that it’s a unit of Mexican currency. In the same way, the statement that humans are animals locates our species within the biological order and conveys a vast amount of information in summary form—that we are living beings, that we are mortal, that we have specific needs for survival and reproduction, and so on. 1.5B Co-Extensive and Essential Attributes 2. A definition should be neither too broad nor too narrow. A definition is too broad if it includes things that are not referents of the concept. For example, the definition “Humans are two-legged animals” is too broad because the defining phrase “two-legged animal” includes birds as well as humans. We can represent this problem in a diagram: The diagram is titled, Two-legged Animals, with a large bracket on either side. It encloses Human, which appears with a pair of smaller brackets. A row of dots appears at the bottom of the diagram, enclosed within Humans and Two-legged Animals. Two dots enclosed within Two-legged Animals are labeled, birds. A definition is too narrow if it fails to include things that are referents of the concept. An example would be “Humans are religious animals.” This definition is too narrow because, no matter how widespread religious belief may be, some people are atheists. We can see this by drawing a diagram: “Religious animals” has a pair of brackets. “Humans” appears below it as follows. The left bracket of “Humans” appears outside “Religious animals,” while the right bracket is enclosed inside it. A row of dots appears at the bottom of the diagram, enclosed within “Humans” and “Religious animals.” Two out of the three dots enclosed within “Humans” are labeled, atheists. Being too narrow and being too broad are opposite flaws in a definition. But both involve the relation between the concept and its referents. One purpose of a definition is to identify the referents of a concept. A definition that does not pick out the right referents—one that includes too much or too little—is not doing its job. It is like an incompetent doorkeeper at a party, letting in people who weren’t invited or turning away people who were. We can tell whether a definition is too broad or too narrow by looking for counter-examples. A counterexample is a particular instance that proves a definition wrong. If a definition is too narrow, a counterexample is something that belongs in the concept but is excluded by the definition. Atheists are counterexamples to the definition of humans as religious animals. If a definition is too broad, a counterexample is something that does not belong in the concept but is included in the definition. Birds are counterexamples to the definition of humans as two-legged animals. Let’s look at a few other cases. “A college is a degree-granting educational institution.” A counterexample would be a law school. Law schools are degree-granting educational institutions, but they are not colleges. So this definition includes too much; it’s too broad. “A cigarette is a sheaf of chopped tobacco rolled in white paper.” What about the brands that are rolled in brown paper? They are counterexamples proving that the definition does not include enough; it’s too narrow. We should notice, finally, that a definition can be simultaneously too broad and too narrow. Suppose, for example, that we define murder as the act of killing another person outside a military context. By this definition, killing someone in self-defense would be an act of murder, but it isn’t. So this definition is too broad. But it is also too narrow. Suppose a soldier kills another member of his own regiment in cold blood. This would be murder, but the definition would exclude it because it occurred in a military context. So the same definition can violate the rule in both ways: It can be both too broad and too narrow. The diagram has overlapping brackets. The text, “killing another person outside a military context,” has a pair of brackets. “Murder” appears below it as follows. The left bracket of “Murder” appears outside the brackets of the above statement, while the right bracket is enclosed inside it. A row of dots appears at the bottom of the diagram, enclosed within “Murder” and “killing another person outside a military context.” One dot is enclosed between the left bracket of Murder and the left bracket of “killing another person outside a military context,” which is labeled, one soldier murders another in same regiment. Two dots enclosed between the right bracket of Murder and the right bracket of “killing another person outside a military context,” are labeled, self-defense. 3. A definition should state the essential attributes of the concept’s referents. The referents of a concept often have many attributes in common. Some are relatively superficial, some are essential. As we saw in discussing classification, “essential” means fundamental: an essential attribute causes or explains the existence of other attributes. For example, the heart makes a certain thumping noise, so we might try to define it as “the organ that goes lub-dub, lub-dub.” But the “lub-dub” sound is a superficial trait; it is merely a by-product of the heart’s essential function, which is to circulate the blood. This essential function explains many of the heart’s other properties: the way it beats, the way it is hooked up to the veins and arteries, even the sound it makes. But explanation is a one-way street. The “lub-dub” sound does not explain the heart’s function. Remember that one purpose of a definition is to condense the knowledge we have about the referents of a concept. Defining by essential attributes is the best way to achieve this purpose, because then you convey not only the particular attributes named in the definition, but also the ones they underlie and explain. The rule of essentiality applies to the genus as well as the differentia. Dogs, for example, belong to various wider groups: they are animals, they are playmates, they are a means of self-defense. But ANIMAL would be the best genus to use for general purposes in defining DOG, because a dog’s animal nature is more fundamental and explains more about it than does the fact that it can play with human beings or defend them. In regard to the differentia, the rule of essentiality will help us choose among attributes when there is more than one that would differentiate a concept from other species of the same genus. Consider the concept HUMAN. Many attributes, in addition to the faculty of reason, are common and distinctive to humans: language, social institutions, the accumulation of knowledge from one generation to the next, laws, moral codes, certain complex emotions such as reverence, a sense of humor, a brain of a certain size and complexity, a certain physical shape and posture. But reason is the common element, the underlying cause, for many of these attributes. Not for all of them—reason doesn’t seem to have much connection with our physical shape and posture. But it is reason that allowed us to develop abstract language and technology, to create social institutions based on general rules and laws, to pass along knowledge to the next generation, and so forth. Reason gives us a differentia that condenses the greatest amount of knowledge about the concept HUMAN. As with choosing an essential principle in classification, there is no hard and fast rule for determining which attributes are essential. Our view of what is essential to a class of objects may change as we acquire more knowledge about them, and it may involve controversial issues on which people disagree. The rule of essentiality means this: Pick the most essential attribute you can, given everything you know, using your best judgment. And the guidelines to follow are the ones we have already discussed: Look for the attribute that explains the most. For man-made objects, actions, and institutions, look for the basic function. For objects in nature, such as biological species or physical substances, look for underlying traits that cause and explain the more superficial attributes. 1.5C Other Rules 4. A definition should not be circular. Suppose we define ownership as the legal relation between people and the things they own. Because this definition uses the word “own,” it defines the concept OWNERSHIP in terms of itself. Instead of explaining what it means to own something, it assumes that we know this already. It tells us how the concept relates to itself, but not how it relates to other concepts or to reality. This definition doesn’t go anywhere; it just moves in a circle. The same problem arises if we use synonyms in a definition. Suppose we define ownership as the legal relation between people and things they possess. “Own” and “possess” are synonyms, different words that express the same concept. In terms of concepts, therefore, the definition is still circular: The concept OWNERSHIP is still being used to define itself. The same objection would apply if we define man as the human animal, large as the attribute possessed by something that is big, or folly as a foolish act. In each case, the italicized words are synonyms. To avoid such circularity, it is useful to ask: What contrast is the concept intended to draw? For example, what is the difference between owning a dress and borrowing it or trying it on in the store? How are humans different from other animals? What makes an action a folly as opposed to a wise action? Circularity can take an even subtler form when two different concepts are used to define each other. Suppose that we define a doctor as a professional who offers medical services to patients. So far, so good: DOCTOR and PATIENT are distinct concepts. But if we now define a patient as a person who receives the services of a doctor, then we have a circular pair of definitions. A better approach would be to define the doctor–patient relationship first; then we could define both DOCTOR and PATIENT in terms of that relationship. 5. A definition should not use negative terms unnecessarily. At the beginning of the 20th century, the automobile was described as a “horseless carriage.” That phrase certainly does describe the automobile, but it would not be a good definition. The differentia “horseless” tells us about one source of power that automobiles do not use. But there are many sources of power automobiles do not use; what we want to know is the source they do use. As another example, suppose that when we introduced the term “differentia,” we defined it as the part of the definition that is not the genus. That would not have been much help to you in understanding what a differentia is. In general, negative definitions should be avoided because knowing what a thing is not doesn’t tell us much about what it is. For that reason, a negative definition usually violates the rule of essentiality as well. Some concepts, however, are inherently negative and thus require negative terms in their definitions. A bachelor is a man who is not married; failing means not succeeding; an empty space is one with nothing in it. How do we know whether a concept is negative? In some cases, a suffix or prefix gives us a linguistic clue: immortal, worthless, asymmetric. In the absence of such clues, you will have to use your judgment. There are no hard and fast rules, but it’s a good idea to look first for a positive attribute, then fall back on a negative one only if the search fails. 6. A definition should not use vague, obscure, or metaphorical language. We might think of this as the “clarity” rule. The purpose of a definition is to clarify our understanding of a concept. At the very least, therefore, the language we use in a definition should not be less clear than the concept being defined. Unfortunately, there are too many ways of being unclear to list them all here, but vagueness, obscurity, and metaphor are the three most common. A vague definition is unclear because it does not give any precise criterion for membership in the concept. Suppose we define maturity as the stage of psychological development in which a person becomes well-adjusted. How do we tell whether a person is well-adjusted? Does adjustment mean passive acceptance of the social environment, or can it include a critical outlook? Is it primarily a set of cognitive skills, an emotional state, or both? As these questions indicate, the term “well-adjusted” is vague. It’s unclear who belongs in the class of well-adjusted people and who doesn’t; the class has highly indefinite boundaries. Of course, the concept MATURITY itself has indefinite boundaries, but a definition shouldn’t make the problem worse. An obscure definition is unclear because it uses abstract or technical language that is more difficult to understand than the concept itself. An example would be a definition of death as the cessation of one’s participation in finitude. The problem here is not necessarily one of vagueness. In the appropriate context of a philosophical theory about human life and afterlife, this definition might have a perfectly clear and definite meaning. But that’s the problem: it has a clear meaning only in a specialized context. For general purposes, the defining terms are too obscure to be useful. The same is true of many technical definitions in law, science, or other specialized areas. Such definitions may be perfectly clear to specialists, but if the concept is employed outside the specialty, then we also need a general-purpose definition that is intelligible to laymen. A metaphorical definition is unclear because it doesn’t convey the literal meaning of the concept, but only an analogy that we have to interpret. A famous Broadway musical maintained that “Life is a cabaret.” Like any good metaphor, this one uses a simple image to convey a complex thought that would take many paragraphs to explain in literal terms. But for a definition, we need the literal terms. A metaphor leaves too many questions unanswered. In exactly what ways is life like a cabaret? How far does the analogy extend? Is this the essential truth about life or just one perspective? Metaphors are valuable tools of thought and communication, but they can’t do the work of definitions. Altogether, then, there are six rules of definition. These rules give us standards for evaluating definitions proposed by other people and guidelines for creating definitions—as we will see in the next section. REVIEW Rules for Definitions * A definition should include a genus and a differentia. * A definition should not be too broad or too narrow. * A definition should state the essential attributes of the concept’s referents. * A definition should not be circular. * A definition should not use negative terms unnecessarily. * A definition should not use vague, obscure, or metaphorical language. This is a public episode. If you would like to discuss this with other subscribers or get access to bonus episodes, visit bypedram.substack.com [https://bypedram.substack.com?utm_medium=podcast&utm_campaign=CTA_1]
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