Two Ways of "Reasoning"
When we "reason" day to day, we actually do it in two very different ways.
The first is like solving a math problem: "All people are mortal; Teacher Su is a person; therefore Teacher Su is mortal." As long as the first two are true, the last one is guaranteed, no exceptions. This is deduction.
The second is like summing up experience: "Every swan I've seen is white, so swans are probably all white." This conclusion seems reasonable, but it's only very likely—the day a black swan turns up, it's overturned. This is induction.
Deduction: If the Premises Hold, the Conclusion Can't Escape
The heart of deduction is "necessity": if all the premises are true, the conclusion cannot be false.
Example: "The convenience store is closed on Sundays. Today is Sunday. So the store is closed today."
You don't actually need to go check—the first two sentences alone settle the conclusion. That's the power of deduction.
Induction: Very Likely, but It Can Backfire
Induction sums up a general rule from "one example after another." It can teach you new things, but at a cost: the conclusion is always only possible, never necessary.
Example: "I've been to this place ten times and it was delicious every time. So everything they make is delicious."
That's a reasonable induction, but the eleventh visit could easily disappoint. No matter how many examples, you can't guarantee the next one 100%.
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想真正动手做题、记进度、攒连胜？到互动课里练。
What is the key feature of deductive reasoning?
答案：If the premises are all true, the conclusion must be true
The hallmark of deduction is "necessity." The second describes induction; the third is also an induction feature; the fourth is wrong—a deductive conclusion never goes beyond what the premises entail and won't assert new facts outside them.
"There was traffic every day this past week, so the commute will probably be jammed tomorrow too." Which kind of reasoning is this?
答案：Induction, because it draws a merely "very likely" conclusion from past examples
Going from "the past few days" to "tomorrow" is reasoning from examples to a generalization, and the conclusion is only likely (tomorrow could happen to be clear)—that's induction. Whether or not "so" is used has nothing to do with deduction vs. induction.
True or false: no matter how many examples back it, an inductive conclusion can never be guaranteed 100% correct and can always be overturned by a new situation.
答案：True
This is the essence of induction. Even after seeing ten thousand white swans, you can't rule out a black one. Induction gives "very likely," not "necessarily."
"All metals expand when heated; iron is a metal; so iron expands when heated." Which kind of reasoning is this?
答案：Deduction—if the premises are true, the conclusion must be true
Applying the general rule "all metals" to the specific member "iron" means that if the premises are true the conclusion is guaranteed—that's deduction. It doesn't generalize from examples, so it isn't induction.
Reasoning that sums up a "general rule" from "one specific example after another" is called ____ reasoning.
答案：inductive
Reasoning that goes from examples to a generalization, with a conclusion that's only likely, is "induction." Its opposite—where true premises guarantee a true conclusion—is "deduction."
True or false: deductive reasoning can tell you brand-new knowledge that isn't in the premises at all.
答案：False
A deductive conclusion never goes beyond what the premises entail; it only extracts the conclusion already implicit in them and won't assert facts outside them. Discovering new rules the premises don't contain relies on induction (observing and summarizing).