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Logic : Syllogistic Fallacies and ‘Determine the Best Conclusion’ Questions

The Civil Service Exam – Professional Level is divided into four categories/competency areas – Verbal, Analytical, Numerical, and General Information. Some of the CSE’s Analytical Reasoning Questions will require you to determine the best conclusion which one can logically make based on a given set of statements/premises which are all assumed to be TRUE (This is different from the ‘Determine the Assumption’ and the Reading Comprehension ‘Best Supports the Statement’ questions).

To answer the ‘Determine the Best Conclusion’ type of questions properly, you have to understand the syllogistic fallacies which usually show up in civil service exams. If you are familiar with the fallacies, you can quickly eliminate the wrong conclusions from the answer options provided in the exam.

So, first things first – a syllogism is an argument that has a major premise, a minor premise and a conclusion. In its basic structure, each of the premises must have a common term in the conclusion. Also, both premises will have something in common – which is the middle term. The middle term does not appear in the conclusion. See the example below.

All dogs are mammals. (Major Premise)
All terriers are dogs. (Minor Premise)
Therefore, all terriers are mammals. (Conclusion)
All A are B
All C are A
Therefore, all C are B.

The Middle term is ‘dogs’, and it is common in both the major and minor premises but it is Not in the Conclusion. It is A.
The Major term is ‘mammals’. It can be found in the Major Premise and it serves as the predicate of the Conclusion. It is B.
The Minor term is ‘terriers’. It can be found in the Minor Premise and it serves as the subject of the Conclusion. It is C.

Now that you know the basic structure of a valid argument, let me tell you that it is usually DIFFICULT to spot the best conclusion from the given options. It won’t be as obvious as that in the example above. Sometimes, questions in the CSE will offer a mix of two seemingly related arguments to confuse you. It will be easier if you can:
(1) Sort out the premises of an argument;
(2) Pick the Major, Minor, and Middle Terms of each argument.
(3) Represent the argument in code form (letters)
(4) and then eliminate all the invalid conclusions based on the following Syllogistic Fallacies.

What are syllogistic fallacies? Syllogistic Fallacies are patterns of reasoning which are considered invalid because of a flaw in its logical structure. So, if a conclusion follows the structure of any of the following fallacies I’m going to mention below, it is INCORRECT.

FALLACY 1: Affirming the Consequent >>> If A then B. B is true, so A is true.
Example: If are hungry, you will eat now. You are eating now, so you must be hungry.

FALLACY 2: Denying the Antecedent >>> If A then B. A is false, so B is false.
Example: If you feed large amounts of chocolate to your dog, it will die. If you don’t feed large amounts of chocolates to your dog, it will never die.

FALLACY 3: Four Terms >>> All A is B. All C is D. So all A is D.
Example: All sharks are fish. All doves are birds. So all sharks are birds.
* This can be tricky though. Sometimes, they use one term TWICE to seemingly create a middle term but if you study the premise closely, you will realize that the term conveys two different meanings.
All birds can fly. No fly has only two legs. Therefore all birds have only two legs. (‘Fly’ has two different meanings)
Nothing is better than eternal happiness. A ham sandwich is better than nothing. Therefore, a ham sandwich is better than eternal happiness. (‘Nothing’ is not the same as ‘Better than nothing’)

FALLACY 4: Illicit Major >>> All X is Y. No P (which is a subset of Y) is X. Therefore no P is Y.
Example: All Filipinos are Asians. No Koreans are Filipinos. Therefore no Koreans are Asians.

FALLACY 5: Illicit Minor >>> All X are Y. All X are P. Therefore all P are Y.
Example: All lions are wild. All lions are social animals. Therefore all social animals are wild.

FALLACY 6: Undistributed Middle >>> All A is B. All C is B. Therefore all C is A.
Examples: All kids must to go to school. You must go to school. Therefore you are a kid.
All elephants are big. All airplanes are big. Therefore, all airplanes are elephants.

AGAIN, if the conclusion FOLLOWS the structure of any of the FALLACIES I mentioned above, it is INCORRECT.

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About the Author

I'm a Journalism graduate of the University of the Philippines - Diliman and I'm currently taking up units in Professional Education courses. I have written for various blogs, websites, and magazines in the past. Right now, I work as a freelance eBook writer and a mobile app developer.

While I was managing a fledgling online business and being a full-time mom to my four kids - Yana, Yshi, Yza and Vincent Jr., I took the Career Service Examination - Pencil and Paper Test - Professional Level (CSE-PPT- Prof) for the first time in December 2015. I passed and I got lucky enough to be among the Top 10 examinees in my batch, 3rd in the National Capital Region.