LoveisaFallacy谬论.ppt

Love is a Fallacy,What is a fallacy?,A fallacy is the use of poor,or invalid reasoning for the construction of an argument.It is also used to refer to an argument which appears to be correct but is not.,An argument in logic presents evidence in support of some thesis or conclusion.An argument has two components:a conclusion and certain premises.The conclusion is said to be drawn or inferred from the premises.An argument is deductively valid when its premises provide evidence for the conclusion.An argument that fails to be conclusively deduced is invalid,it is said to be fallacious.,Argument,Logic Fallacies,1.Dicto Simpliciter（绝对判断）2.Hasty Generalization（轻率归纳）3.Post Hoc（错误因果）4.Contradictory Premises（矛盾前提）,1.Dicto Simpliciter,Polly,I said gently,the argument is a fallacy.Exercise is good is an unqualified generalization.For instance,if you have heart disease,exercise is bad,not good.Many people are ordered by their doctors not to exercise.You must qualify the generalization.You must say exercise is usually good,or exercise is good for most people.Otherwise you have committed a Dicto Simpliciter.Do you see?,1.Dicto Simpliciter（绝对判断）Dicto Simpliciter is an argument based on an unqualified generalization.The argument applies a general rule to a particular case in which some special circumstances make the rule inapplicable.,Examples,Women are on average not as strong as men and less able to perform well politically.Therefore,women cant pull their weight in government work.虽然女性确实在政界担任较少工作，但这不意味着女性群体中所有的人都是这样。Japanese waged a war to Chinese,which had nothing to do with humanity but everything to do with aggression.Therefore every Japanese is guilty and should be condemned.虽然日本侵华的邪恶战争任历历在目，但并不是所有的日本人都有罪，都应该受到谴责。,2.Hasty Generalization,Next we take up a fallacy called Hasty Generalization.Listen carefully:You cant speak French.Petey Burch cant speak French.I must therefore conclude that nobody at the University of Minnesota can speak French.Really?said Polly,amazed.Nobody?I hid my exasperation.Polly,its a fallacy.The generalization is reached too hastily.There are too few instance to support such a conclusion.,2.Hasty Generalization（轻率归纳）Hasty generalization is a fallacy in which a conclusion is not logically justified by sufficient or unbiased evidence.A hasty generalization always proceeds from the particular to the general.,Hasty Generalization,Its pattern:X is true for A.X is true for B.X is true for C.X is true for D.Therefore,X is true for E,F,G,etc.,Example,Mike failed biology exam.Jane failed biology exam.Maria failed biology exam.Therefore,I conclude that most students who take biology exam will fail it.,Analysis,In understanding and characterizing general situations,a logician cannot normally examine every single example.However,the examples used in inductive reasoning should be typical of the problem or situation at hand.Maybe Mike,Jane,and Maria are exceptionally poor students.Maybe they were sick and missed too many lectures.If a logician wants to make the case that most students will fail biology exam,she should(a)get a very large sample-at least one larger than three or(b)if that isnt possible,she will need to go out of his way to prove to the reader that her three samples are somehow representative of the norm.,3.Post Hoc,Listen to this:Lets not take Bill on our picnic.Every time we take it out with us,it rains.I know somebody just like that,she exclaimed.A girl back home-Eula Becker,her name is.It never fails.Every single time we take her on a picnic.Polly,I said sharply,its a fallacy.Eula Becker doesnt cause the rain.She has no connection with the rain.You are guilty of Post Hoc if you blame Eula Becker.,3.Post Hoc（错误因果）Post Hoc is a fallacy in which one event is said to be the cause of a later event simply because it occurred earlier.Post Hoc is a fallacy with the following form:1.A occurs before B.2.Therefore A is the cause of B.,Examples,Joan is scratched by a cat while visiting her friend.Two days later she comes down with a fever.Joan concludes that the cats scratch must be the cause of her illness.Most young criminals watch violent movies before they commit their crimes;obviously,violent movies lead to juvenile delinquency.甲事情发生在乙事情之前，这并不代表着先发生的甲事情就是后发生的乙事情的原因。,4.Contradictory Premises,If God can do anything,can He make a stone so heavy that He wont be able to lift it?Of course,she replied promptly.But if He can do anything,He can lift the stone,I pointed out.Yeah,she said thoughtfully.Well,then I guess He cant make the stone.But He can do anything,I reminded her.She scratched her pretty,empty head.Im all confused,she admitted.Of course you are.Because when the premises of an argument contradict each other,there can be no argument.If there is an irresistible force,there can be no immovable object.If there is an immovable object,there can be no irresistible force.Get it?,4.Contradictory Premises（矛盾前提）Contradictory Premises is an argument that draws a conclusion from inconsistent or incompatible premises.Essentially,a proposition is contradictory when it asserts and denies the same thing.,Sometimes it is difficult to distinguish between real and apparent incompatible premises.Lets look at the example below.A father said to his son:you can trust no one.,Analysis,The father who is trying to convince his child that no one should be trusted is obviously making an exception of himself.If he really were making incompatible claims(since you should trust no one,and you should trust me),no rational conclusion could or should be drawn by the child.However,the incompatible premises are only apparent;the father has carelessly overstated the first premise.If he had said,Dont trust most people or Trust very few people,or Dont trust anyone except me,he would have had no trouble avoiding the contradiction.,THANKS,