Un seul problème : Les questions sont plutôt faciles et j'ai décelé une erreur dans le "corrigé". Venant de quelqu'un qui a écrit plusieurs livres sur l'art de la logique, c'est plutôt... ironique. Je vous laisse faire le test par vous-même voir si on arrivera à la même conclusion sur ladite question.
Voici le e-mail que j'ai envoyé à l'auteur et la réponse automatique que j'ai reçu. Je camoufle le tout, question de ne pas vous influencer avant le test.
Hi,
I couldn't resist to send you a message after having read the answers of your "Logic test".
Not only I think that my answer to the question 15 ("valid") was good, I think that the one presented as the "correct one" (therefore, "invalid") is clearly wrong.
From the beginning, you give us premises. Some are accurate, some are not but it doesn't change anything for the test (example, question 10: Paris is in New Zealand). Question 10 could also has been : "A lives in X" ; "X is in Y" ; "Therefore A lives in Y".
The first premise of question 15 is : "Water is a molecule composed of two hydrogen atoms and one oxygen atom.". You give us a definition : Water is THIS. I know it may not be completly accurate (as it was for Paris in New Zealand) : water could be defined otherwise, but in this test, we've gotta think only in function of your premises. We can use some variables : "Z is a molecule composed of two hydrogen atoms and one oxygen atom", or if you prefer : "Z is D".
The conclusion is : "Therefore we can predict that every future examination of water will reveal the same chemical composition.". My answer is : of course, yes. Because water is D, anytime we examine water it will always be D. If it isn't, that would mean that the substance examined wasn't water, because the premise define it by his chemical composition. See it that way :
a) Z is D
b) Every observations or examination as confirmed that Z is D (superfluous, from the beginning, premises don't have to be confirmed, we're only evaluating their link to the conclusion)
Conclusion : Therefore we can predict that every future examination of Z will reveal that it is D. (of course, conclusion is premise a).
In the explication of your answer, you write : "However, it is possible also that there may exist a substance that looks like water, boils as water, freezes as water, nourishes plants and life as water, and yet has a different chemical composition to what we know as water." Then, it would simply not be water as defined by the premise a). Therefore, it wouldn't be an examination of water (or Z). You use the expression "what we know as water". As I said, we only know what is written in the premise (same thing as Paris in New Zealand).
I strongly suggest that you review your position.
Philippe
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If you have contacted me concerning the Logic Test, then thank you for your comments and participation. Unfortunately owing to a massive amount of inquiries I will be unable to address issues in the near future. However you may be interested to discuss the issues raised at neatorama.com/2009/01/06/logic-test/ . Please find below some information that may be relevant.
Concerning number 15-
I receive more emails concerning this question than all the others together.
The answer given on the web site is, as you may have said, is incorrect and needs adjusting, but then it would not be controversial and I would probably not get any feedback.
The syllogism itself is invalid and can only be considered as a strong inductive argument and not deductive. It is the 'we can predict' part of the conclusion which comes from nowhere, so the argument is not strictly formal. Not everyone agrees with this so I will leave you to draw your own conclusions. I discuss similar points in my book 'The Red Herring and the Power of Logic', if I may put in a plug!
However, just as an aside there are other types of water. Heavy water for example is water in which the hydrogen is replaced by its heavier isotope, deuteruim (D2O). It is chemically almost identical to normal water but looks, feels and tastes exactly the same. But don't drink it. This says nothing about the validity but only about the truth of the conclusion.
Concerning number 10
a) Jenny lives in Paris.
b) Paris is in New Zealand.
Conclusion
Therefore Jenny lives in New Zealand.
If you cannot understand why this syllogism is valid and argue that there is more than one Paris in the world therefore should be valid then one has to appreciate certain rules concerning the validity of deductive arguments. One important rule is that one must only consider the factors or information given in the premises and conclusion regardless of whether they are considered true, false or in this case limited. One must not import additional information that is not already present. The reason why Paris was chosen was precisely because there are several places called Paris in the world. Hope this helps, but if you still disagree I can recommend 'An Introduction to Logic' by Irving Copi which discusses these issues in a manner that is not too formidable.
Thanks also to those that pointed out that it was John Venn and not Charles Venn who developed the Venn diagrams.
Hope you enjoyed the test, and thanks again for your comments and any recommendations.
Colin Beckley
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