1、Inductive and Deductive Reasoning实用文档Inductive and Deductive Reasoning(文档可以直接使用，也可根据实际需要修改使用,可编辑 欢迎下载）Critical ThinkingExcerpted from :/ sjsu.edu/depts/itl/7/index.htmlInductive and Deductive ReasoningMany people distinguish between two basic kinds of argument: inductive and deductive. Induction is
2、usually described as moving from the specific to the general, while deduction begins with the general and ends with the specific; arguments based on experience or observation are best expressed inductively, while arguments based on laws, rules, or other widely accepted principles are best expressed
3、deductively. Consider the following example: Adham: Ive noticed previously that every time I kick a ball up, it comes back down, so I guess this next time when I kick it up, it will come back down, too. Rizik: Thats Newtons Law. Everything that goes up must come down. And so, if you kick the ball up
4、, it must come down. Adham is using inductive reasoning, arguing from observation, while Rizik is using deductive reasoning, arguing from the law of gravity. Riziks argument is clearly from the general (the law of gravity) to the specific (this kick); Adhams argument may be less obviously from the s
5、pecific (each individual instance in which he has observed balls being kicked up and coming back down) to the general (the prediction that a similar event will result in a similar outcome in the future) because he has stated it in terms only of the next similar event-the next time he kicks the ball.
6、 As you can see, the difference between inductive and deducative reasoning is mostly in the way the arguments are expressed. Any inductive argument can also be expressed deductively, and any deductive argument can also be expressed inductively. Even so, it is important to recognize whether the form
7、of an argument is inductive or deductive, because each requires different sorts of support. Adhams inductive argument, above, is supported by his previous observations, while Riziks deductive argument is supported by his reference to the law of gravity. Thus, Adham could provide additional support b
8、y detailing those observations, without any recourse to books or theories of physics, while Rizik could provide additional support by discussing Newtons law, even if Rizik himself had never seen a ball kicked. The appropriate selection of an inductive or deductive format for a specific first steps t
9、oward sound argumentation. Introduction to Induction As covered in the section on Inductive and Deductive Reasoning, inductive arguments are usually based on experience or observation. In effect, then, inductive arguments are all comparisons between two sets of events, ideas, or things; as a result,
10、 inductive arguments are sometimes called analogical arguments. The point of those comparisons, or analogies, is to establish whether the two sets under consideration, similar in a number of other ways, are also similar in the way of interest to the argument. Consider this example: Mariko says, Ever
11、y time Ive seen a red-tinted sunset, the next days weather has been beautiful. Today had a red-tinted sunset, so tomorrow will be beautiful.Essentially, Mariko is comparing one set of events (observed red-tinted sunsets and each following days weather) with another (todays observed sunset and tomorr
12、ows predicted weather). These sets are similar in an important way (red-tinted sunsets), and the inductive argument is that they will also be similar in another way (nice weather on the following day). In this case, Mariko is arguing from particular cases in the past to a particular case in the pres
13、ent and future, but she could also argue inductively from those particular cases to a general one, such as Its always beautiful the day after a red-tinted sunset. The strength of such an argument depends in large part on three of its elements: 1.how accurate and comprehensive the previous observatio
14、ns are; 2.how strong the causal link seems to be; 3.how similar the two cases are. In Marikos argument, to satisfy the first element, we would want to be sure that shes seen many such sunsets, and that redness and beauty have been judged consistently. To satisfy the second, we would want to feel con
15、fident that there is a strong correlation between weather patterns on successive days. To satisfy the third, we would want to know whether there are any significant differences between the observation of todays sunset and of the previous ones. A difference in season, a difference in geographical or
16、topographical location, a difference in climate, or any other significant variation might affect the comparability of the two sets of observations. In fact, we should always understand the second premise of an inductive argument to contain a claim like there is otherwise no significant difference. T
17、he second premise of Marikos argument, then, might read, Todays sunset was red-tinted (and there were no significant differences between this and previous red-tinted sunsets). Keeping such a disclaimer in mind is important, because this is where many inductive arguments are weakest. Because we argue
18、 inductively from the particular to the general, such arguments are often called generalizations, or inductive generalizations. Other kinds of arguments with a similar format include causal arguments. Exercises for Induction 1. Which of the following claims would be best expressed by inductive reaso
19、ning? Your first quiz grade usually indicates how you will do in the course. The final exam accounts for 30% of the course grade. Late papers will not be accepted. Gravitys Rainbow is required reading in your course. 2. Every time Jorge has seen a baseball game between the Giants and the Dodgers at
20、Candlestick Park, the Giants have won. Tomorrow, the Giants play the Dodgers at Candlestick. Which of the following is least significant when arguing that the Giants will win tomorrow? Jorge has only seen the Giants play the Dodgers twice. Both teams have many new players. Jorge wont be going to the
21、 game tomorrow. The field at Candlestick will be unusually muddy tomorrow. Introduction to Causal Arguments One of the most important uses for inductive reasoning is to argue causation. Consider the following example: A bicyclist moves into the traffic lane in order to pass a truck illegally parked
22、in the bike lane. The driver of a car approaching from the rear slams on her brakes in order to avoid hitting the bicycle. A following car fails to stop in time, and smashes into the back of the first. The insurance companies disagree about who should be held responsible, and they go to court to dec
23、ide who caused the accident. What arguments are likely to be made in court? The bicyclists lawyer will probably claim that the illegally parked truck caused her client to swerve into the lane of traffic. The lawyer for the driver of the first car will probably claim that the bicyclists actions cause
24、d her client to slam on the brakes. The lawyer for the second driver will probably claim that the first cars sudden stop caused his client to smash into its back. None of these claims seems to fit the pattern of an inductive argument, because none of them seems based on observation or experience. Bu
25、t, in fact, they do fit that pattern. The bicyclists lawyer, for example, is actually arguing that: Normally the bicyclist would have continued in the bike lane, but in this instance he swerved into the lane of traffic. The only significant difference between normally and in this case is the presenc
26、e of the illegally parked truck. Therefore, the truck caused the bicyclist to swerve.The lawyers for the drivers are making similar arguments: the first, that the only significant difference was the swerving bicycle; and the second, that the only significant difference was the suddenly braking car.
27、Like inductive reasoning, then, these causal arguments are based on observed instances. (In this case, no observations are needed to convince us that the bicyclist would not normally have swerved or the first driver would not normally have braked suddenly. But if, for some reason, observations were
28、necessary, we could design a study of automobile and bicycle traffic on that street, or survey drivers and bicyclists about their experiences, or in other ways provide evidence to verify the part of the premise describing the normal pattern of traffic. These causal arguments, then, follow the form o
29、f an inductive argument with one important exception: whereas an inductive argument carries as part of its second premise the implication that there is otherwise no significant difference, these causal arguments carry the implication that there is only one significant difference: for the bicyclist,
30、the truck; for the first driver, the bicycle; for the second driver, the first car. How can we know that there is really only one significant difference? In real-life situations, we cannot usually be certain of that, since the world in which we live is a very complicated and intricate place. If, how
31、ever, there is a strong likelihood of causation and there are no other apparent causes in evidence, then the argument will seem convincing. Two rules to remember in dealing with causation are: 1.The cause must precede the event in time. On one hand, arguments that have the effect before the cause ar
32、e examples of the relatively rare fallacy of reverse causation. One the other, arguments whose only proof of causation is that the effect followed the cause are examples of fallacious post hoc reasoning. 2.Even a strong correlation is insufficient to prove causation. Other possible explanations for such a strong correlation include coincidence, reversed causation, and missing something that is the cause of both the original cause and its purported effect.In the trial, for example, the second drivers lawyer co
