--  作者：mike1999
--  发布时间：2003-6-10 17:35:00

--  第三天的课程 最重要！！Analysis of Arguments
第八课： Inductive and Deductive Reasoning

Many people distinguish between two basic kinds of argument: inductive and deductive. Induction is 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 deductively. Consider the following example:      Adham: I\'ve 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: That\'s Newton\'s Law. Everything that goes up must come down. And so, if you kick the ball up, it must come down.
     

Adham is using inductive reasoning, arguing from observation, while Rizik is using deductive reasoning, arguing from the law of gravity. Rizik\'s argument is clearly from the general (the law of gravity) to the specific (this kick); Adham\'s argument may be less obviously from the specific (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.

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 of an argument is inductive or deductive, because each requires different sorts of support. Adham\'s inductive argument, above, is supported by his previous observations, while Rizik\'s deductive argument is supported by his reference to the law of gravity. Thus, Adham could provide additional support by detailing those observations, without any recourse to books or theories of physics, while Rizik could provide additional support by discussing Newton\'s 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 toward sound argumentation.

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--  作者：mike1999
--  发布时间：2003-6-10 17:37:00

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第九课：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, 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, "Every time I\'ve seen a red-tinted sunset, the next day\'s 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 day\'s weather) with another (today\'s observed sunset and tomorrow\'s 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 present and future, but she could also argue inductively from those particular cases to a general one, such as "It\'s always beautiful the day after a red-tinted sunset."

The strength of such an argument depends in large part on three of its elements:

how accurate and comprehensive the previous observations are;
how strong the causal link seems to be;
how similar the two cases are.
In Mariko\'s argument, to satisfy the first element, we would want to be sure that she\'s seen many such sunsets, and that "redness" and "beauty" have been judged consistently. To satisfy the second, we would want to feel confident 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 today\'s sunset and of the previous ones. A difference in season, a difference in geographical or 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." The second premise of Mariko\'s argument, then, might read, "Today\'s 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 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.