思辨能力培养 lecture 3 - Deductive Arguments
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Premise : The sun has risen every morning during the entire history of the earth. Conclusion : The sun will rise tomorrow morning.
Argument 1
All Americans are capitalists. Roger is an American. Therefore Roger is a capitalist.
Premise 1 : It is Tuesday.
Premise 2 : Glenn always wears a grey waistcoat on Tuesday.
Conclusion : Glenn will be wearing a grey waistcoat today.
Premise 1 : All men are mortal.
true
A valid deductive argument is a truthpreserving (维护事实) argument.
In other words, in a valid deductive argument the truth of the premises is preserved (维护) in the truth of the conclusion. If a deductive argument is valid (and the premises are true), we can be 100% certain of the conclusion. If it is invalid, however, it proves nothing and is completely useless!
Argument 2
All Americans are capitalists. Roger is a capitalist.
Therefore Roger is an American.
The sun is coming out so the rain should stop soon. If Tom goes to the party, Dana will also go. Tom is at the party. Therefore Dana is at the party. Either Lu Meng is in class or she is in her dormitory. She is not in class, therefore she is in her dormitory. I once ate a 糖葫芦 and got sick. Therefore, if we want good health, we should avoid eating 糖葫芦. The dog would have barked if it had seen a robber. The dog didn’t bark so it didn’t see a robber. No one in Paris understands me, so either my French is poor or Parisians are idiots.
Premise 2 : Socrates is a man.
Conclusion : Therefore Socrates is mortal.
A deductive argument guarantees its conclusion
i.e. if the premises are true, the conclusion MUST be
premises are all true.
Since Chengdu is west of Wuhan and Urumqi is west of Chengdu, Urumqi must be west of Wuhan. Witches (女巫们) float because witches are made of wood and wood floats.
Know the difference between deductive and inductive arguments Be able to recognise a valid from an invalid deductive argument
Deductive arguments
&
Inductive Arguments
Lecture 3 Deductive Arguments
Propositions are either true or false.
Arguments are NOT true or false. Arguments are either good or bad, valid or invalid, strong or weak.
A good deductive argument has two characteristics:
(1)The
conclusion follows (必然发生) from the premises (i.e. the argument is valid/truth-preserving).
(2)The
Premise : The sun has risen every morning during the entire history of the earth. Conclusion : The sun will rise tomorrow morning.
Argument 1
All Americans are capitalists. Roger is an American. Therefore Roger is a capitalist.
Premise 1 : It is Tuesday.
Premise 2 : Glenn always wears a grey waistcoat on Tuesday.
Conclusion : Glenn will be wearing a grey waistcoat today.
Premise 1 : All men are mortal.
true
A valid deductive argument is a truthpreserving (维护事实) argument.
In other words, in a valid deductive argument the truth of the premises is preserved (维护) in the truth of the conclusion. If a deductive argument is valid (and the premises are true), we can be 100% certain of the conclusion. If it is invalid, however, it proves nothing and is completely useless!
Argument 2
All Americans are capitalists. Roger is a capitalist.
Therefore Roger is an American.
The sun is coming out so the rain should stop soon. If Tom goes to the party, Dana will also go. Tom is at the party. Therefore Dana is at the party. Either Lu Meng is in class or she is in her dormitory. She is not in class, therefore she is in her dormitory. I once ate a 糖葫芦 and got sick. Therefore, if we want good health, we should avoid eating 糖葫芦. The dog would have barked if it had seen a robber. The dog didn’t bark so it didn’t see a robber. No one in Paris understands me, so either my French is poor or Parisians are idiots.
Premise 2 : Socrates is a man.
Conclusion : Therefore Socrates is mortal.
A deductive argument guarantees its conclusion
i.e. if the premises are true, the conclusion MUST be
premises are all true.
Since Chengdu is west of Wuhan and Urumqi is west of Chengdu, Urumqi must be west of Wuhan. Witches (女巫们) float because witches are made of wood and wood floats.
Know the difference between deductive and inductive arguments Be able to recognise a valid from an invalid deductive argument
Deductive arguments
&
Inductive Arguments
Lecture 3 Deductive Arguments
Propositions are either true or false.
Arguments are NOT true or false. Arguments are either good or bad, valid or invalid, strong or weak.
A good deductive argument has two characteristics:
(1)The
conclusion follows (必然发生) from the premises (i.e. the argument is valid/truth-preserving).
(2)The