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← Lecture 8-9 - A Student Example of Causal Reasoning about Chocolate

From the "Think Again: How to Reason and Argue" course on Coursera

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Afficher la révision 3 créée 07/05/2014 par Claude Almansi.

  1. We received a wonderful example of -- for
    the kinds of bargains we studied
  2. in the weeks on inductive reasoning.
  3. We studied generalization, and we studied
    applications of those generalizations.
  4. And using that for causal reasoning that
    we tested by manipulations
  5. and by the sufficient condition tests,
  6. and all of those are brought together
    in a wonderful little video.
  7. So, take a look.
  8. >> Hello.
  9. My name's Catriona, and I live in Sweden.
  10. I've spent many years, eating a little bit
    of chocolate everyday.
  11. And I've always strongly believed that
    eating chocolate makes you happier.
  12. In other words, most people will be
    in a better mood
  13. after they've eaten chocolate
    than before.
  14. And I'll give you four pieces of evidence
  15. in support of my conclusion.
  16. Evidence one: I've met a lot of people
    over the years
  17. and when I give them some of my chocolate,
    they always seem happy about it.
  18. I can't think of a single case
    of someone eating chocolate
  19. who hasn't looked satisfied afterwards.
  20. Evidence two: It could be that
  21. eating chocolate isn't really causing
    their happiness.
  22. They're just happy people eating
    chocolate.
  23. So, I asked unhappy people whether they
    ever eat chocolate.
  24. And they confirmed that they do,
    precisely because it cheers them up.
  25. Evidence three:
  26. Now, I went to a chocolate factory where
    the workers get free chocolate every day.
  27. And these people could theoretically be
    unhappy like the rest of us could.
  28. but they all had smiles on their faces,
    and some smears of chocolate.
  29. Evidence four: You may ask whether these
    people are also in love
  30. and it's love that's causing
    the happiness,
  31. not actually the chocolate.
  32. So I had to find people who eat chocolate,
    but are not in love.
  33. I did a mini survey at my corner shop.
  34. And 80% of the people who responded that
    they were not in love,
  35. reported that they definitely felt
    their mood improve
  36. when they ate chocolate.
  37. So, it does seem to be the case that
  38. eating chocolate is probably sufficient
    to improve your mood.
  39. Obviously there are people who never eat
    chocolate and are very happy.
  40. For example a successful dieter
    or your dentist.
  41. You can find happiness by falling in love
    or getting promoted
  42. or finally reaching the top of Mt.
    Everest, whatever's your biggest dream.
  43. But for an instant helping of happiness,
  44. chocolate will cheer you up, mm.
  45. >> Wasn't that great?
  46. I already knew that I loved chocolate
  47. and that it improved my mood,
  48. so maybe I was convinced
    by this argument
  49. because, I already believed
    the conclusion.
  50. And that's when you ought to be careful.
  51. When you look at an argument, and it's for
    a conclusion you already believed,
  52. then there's going to be a tendency
    to think it's a great argument.
  53. so if you really want to know
  54. whether it is or is not a good argument.
  55. You gotta look carefully at the ones that
    are trying convince you
  56. of things that already had a tendency to
    believe in the first place.
  57. So let's look carefully at her conclusion.
  58. Catriona's conclusion was very carefully
    qualified.
  59. On the one hand, she said that chocolate
    is one of the things that makes you happy.
  60. She doesn't claim that it's the only thing
  61. that makes you happy: you know,
  62. Love might make you happy, climbing
    Mount Everest might make you happy,
  63. all kinds of things can make you happy.
  64. She's only claiming that
    chocolate's one of them.
  65. Another qualification, is that she says
  66. chocolate will probably
    improve your mood.
  67. She don't say it's definitely,
  68. don't say it works always,
    or for all people.
  69. But it'll probably improve your mood:
    so it's an inductive argument,
  70. and she doesn't have to prove it
    beyond a shadow of a doubt.
  71. That's a good move in an argument,
    because it's very hard to prove things
  72. beyond a shadow of a doubt.
  73. But there is one part of her conclusion
    that makes the argument difficult.
  74. And that's that she says chocolate
    improves your mood.
  75. Now that's a causal claim.
  76. That's not just that you eat chocolate
    and you also happen to be happy.
  77. That's that eating chocolate causes you
  78. to be happier than you were before,
  79. that is, to be in a better mood
    than you were before.
  80. So let's see if the different bits of
    evidence that she presents
  81. really do support that conclusion.
  82. So the first bit of evidence
    that Catriona provides is this.
  83. "Now I've met a lot of people, over the
    years, and whenever
  84. I give them some of my chocolate they're
    clearly happy about it."
  85. What kind of argument is that?
  86. That is a statistical generalization
    from a sample,
  87. because she hadn't met
    everybody in the world,
  88. she's only met some people.
  89. But the sample that she's been able to
    check, they've all liked chocolate.
  90. She adds one more claim:
  91. "I can't think of a single case of
    someone eating chocolate
  92. and not looking satisfied afterwards."
  93. You know, this is kind of repetitious,
  94. because she says
    she can't think of a case.
  95. She just told you that whenever she gives
    them chocolate, then they're happy
  96. and if they're happy they're satisfied,
    and so maybe that doesn't add too much.
  97. It's still a generalization.
  98. What are we going to think about this
    generalization?
  99. Well, let's assume that
    she's got a lot of friends.
  100. So, she's checked a lot of people, she's
    given a lot of people chocolate
  101. and -- and they're all happy.
  102. Well, is the premise true?
  103. Well, do we believe
    that every single person
  104. that she gave chocolate to
    was satisfied and happy?
  105. I kind of tend to doubt that, Catriona,
    sorry, but --
  106. it's just, such uniformity is unusual
    among human beings.
  107. And so my bet is that a couple of them
    didn't.
  108. But notice that she didn't really need
    that, because
  109. she's only arguing that giving chocolate
    will probably improve your mood,
  110. not that it always will for everybody.
  111. So she doesn't need that strong claim
    about everybody.
  112. So she's still on pretty firm ground with
    regard to claiming that almost everybody,
  113. or most of the people, have improved their
    mood when she gave them chocolate.
  114. We still might worry about a bias sample.
  115. There might be some cultures out there,
    or some types of people out there,
  116. that don't like chocolate.
  117. Maybe they're allergic to chocolate,
  118. or something like that,
    that would be really sad,
  119. I'm sorry to even think about that,
    it makes me sad.
  120. But, there probably are, and they're
    people that don't like chocolate.
  121. And so, it's not going to work for them.
  122. So maybe she wants to qualify her
    conclusion a little bit, you know,
  123. "among the groups that I've met:
  124. different cultures might be different,
  125. different biological conditions might make
    people allergic to chocolate."
  126. But, with regard to most of the people
    she's met,
  127. chocolate seems to work pretty well.
  128. So we've got now a statistical
    generalization:
  129. "Most people, if you give them chocolate,
    it improves their mood."
  130. Great.
  131. What does that show you about you, because
    remember her conclusion is
  132. that if you have chocolate, then that
    chocolate will probably improve your mood.
  133. So now she needs to take that
    generalization
  134. and apply it back down
    to you as an individual,
  135. to make a prediction about what'll happen
    to you if you eat some chocolate.
  136. So in this particular bit of evidence, she
    needs to generalize up
  137. to "most people will have their mood
    improved
  138. if they eat chocolate," and then back down
    again.
  139. And since that's true with most people,
  140. and you're not special in any particular
    way
  141. -- or it would be a conflicting
    reference class fallacy --
  142. since you're not special
    in any particular way,
  143. the chocolate will also improve
    your mood.
  144. That's the way this first bit of evidence
    seems to go.
  145. So now we're ready for her second bit of
    evidence.
  146. Let's get all adjusted here
  147. so that we can think about this new
    bit of evidence that Catriona provides.
  148. Okay, what she says is:
  149. it could be that chocolate isn't really
    causing the happiness.
  150. Maybe they were just happy
    before eating the chocolate.
  151. Now, that's a problem.
  152. If you're going to say that it improves
    people's moods
  153. when they eat chocolate,
  154. it's not good enough
    that people are already happy,
  155. and then that makes' em want to eat
    chocolate.
  156. And so they eat chocolate to celebrate how
    happy they are.
  157. Well you gotta prove that it actually does
    cause the improvement in the mood.
  158. Okay, how are you going to do that?
  159. Well, we looked, in the lectures on
    induction, at causal reasoning.
  160. What you need to do is manipulate.
  161. You want to look at people who are not
    happy
  162. and see if the chocolate makes them
    go up in mood
  163. and manipulate the amount of chocolate,
    manipulate how happy they are,
  164. and see how those have effects
    on each other.
  165. And that's how you decide among the
    hypotheses.
  166. And that's exactly what
  167. Catriona does.
  168. "So I asked some unhappy people
    whether they ever eat chocolate,
  169. and they confirmed that they do,
    precisely because it cheers them up."
  170. Ah, so now we have a sample of people,
    where we've manipulated their happiness.
  171. We've looked at happy people and unhappy
    people.
  172. We've looked at people at times
    when they're happy
  173. and times when they're unhappy.
  174. And then we can look at their chocolate
    consumption
  175. and we can check to make sure
    that they're happier afterwards.
  176. It's just like
    checking whether smoking causes cancer.
  177. We talked about that in the lectures on
    induction.
  178. If you want to know
    whether smoking causes cancer,
  179. what you do is you induce smoking
    in creatures
  180. -- in this case, lab monkeys --
    that did not previously smoke
  181. and see whether that increases
    their cancer rate.
  182. And what she's doing is she's saying:
  183. "let's take people that
    don't have happiness,
  184. give them chocolate, and see
    if that increases their happiness."
  185. So she's using exactly the kind of
    manipulation test
  186. that we talked about
    in the lectures on induction.
  187. Good job, Catriona.
  188. Now let's switch to the fourth bit of
    evidence,
  189. and I want to get ready for that one too.
  190. Now, what Catriona says for her fourth
    bit of evidence is:
  191. "you may ask, what if all these people
    are also in love,
  192. and it's love that's making them happy,
    not really chocolate?"
  193. Because remember,
    she doesn't want to claim
  194. chocolate's the only thing
    that makes you happy.
  195. Love might be sufficient for happiness,
  196. just like chocolate
    also causes happiness.
  197. How are we going to tell which one it is?
  198. Well, in the lectures on causal
    reasoning,
  199. when we were looking at the tables,
    and the diners who died
  200. from the poisoned food,
    one of the points that we made
  201. was that if you have two candidates
    for a sufficient condition,
  202. you've gotta look at situations
    where one of them is present
  203. without the other and the other present
    without the first one.
  204. And that's exactly what Catriona does.
  205. She says, I had to find people who eat
    chocolate, but are not in love.
  206. Whoa, well that's what
    you're looking for,
  207. because if you have some people
    who eat chocolate,
  208. but are not in love,
  209. and some people who are not in love
    but eat chocolate, you can test it.
  210. And what did she find?
  211. Well: "I did a mini-survey
    outside my nearest corner shop,
  212. and 80% if the people who confessed
  213. that they were not in love
  214. said they definitely felt their mood
    improve when they ate chocolate.
  215. Notice she's not claiming it's so
    universal, 100% anymore,
  216. she's down to 80%:
    good move.
  217. But in addition, she's now ruled out that
    it's really the love
  218. that's the only sufficient condition:
    even when the love's not present,
  219. you still get happiness
    with eating chocolate.
  220. So that's applying one of the tests that
    we talked about
  221. in the section on causal reasoning to test
    for sufficient conditions.
  222. Okay, we've done our bits of evidence one
    and two and four.
  223. We skipped over number three.
  224. So what's your third bit of evidence?
    Now, we've gotta look at that one.
  225. Okay, what Catriona says is:
    "I went to a chocolate factory
  226. where all the workers
    eat chocolate every day."
  227. Not only that, it's free.
  228. Free chocolate! Yeah, mm!
  229. I love it.
  230. "Like everyone else, these people could
    theoretically be unhappy,
  231. but they have smiles on their faces."
  232. Wouldn't you?
  233. Free chocolate all day long.
  234. But wait a minute, wait a minute:
    is this a good argument?
  235. Like I said, it's a conclusion I like.
  236. But we still gotta look at it critically.
  237. Sorry, Catriona,
    these people are happy,
  238. but how do you know that they're happy
    because of the chocolate, right?
  239. They've got a job in the factory,
    It's good having a job.
  240. Chocolate's very popular, so maybe the
    factory's doing really well,
  241. which means they can pay them
    more money, better benefits, good job:
  242. maybe that's what's making them happy?
  243. Sure, all these people in the factory
    are happy.
  244. But it might not be the chocolate.
  245. It might be the fact
    that they got a good job.
  246. So I think this fourth bit of evidence is
    not really all that strong.
  247. Her number three, by the way:
    the fourth one we considered.
  248. It's not all that strong:
    now, what does that show you?
  249. You could say:
    "Wait a minute, she's got four arguments,
  250. one of them's no good,
    so it all is back to nothing.
  251. But of course, that's not
    the right way to think about it.
  252. She gave us four bits of evidence.
  253. If one of them doesn't work,
    and we've only got three left,
  254. we've still got three left.
  255. So if those first three bits of evidence
    are good, and if they show at least
  256. that it'll probably improve your mood,
    and they establish
  257. the cause of conclusion that she claimed,
    then it seems to me
  258. she's given us pretty good evidence for
    the conclusion she was claiming.
  259. And in doing so, she's exemplified a lot
    of the different forms of reasoning
  260. that we studied in the weeks
    on inductive reasoning.
  261. So, thank you very much, Catriona,
  262. for submitting this wonderful video,
    this wonderful argument.
  263. We all learned from it
    and we appreciate it.