5 question

Forms Of Deduction & Sherlock Holmes' Reasoning

CSN, Phil 102

Applying Forms of Deduction to Sherlock Holmes' Reasoning

Make sure not to switch your clue and deduction when you analyze Holmes' reasoning. Remember, the clue is the premise or reason or the observation and the deduction is the conclusion Holmes draws from the premise, reason, or observation.

If you do modus ponens for Holmes' deductions, then the clue must go into the "if..." part and the deduction or conclusion must go into the "then..." part. You then affirm the clue in the second premise and then affirm the deduction in the conclusion.

For example, "If Helen Stoner had a railway return ticket in her hand (clue), then she arrived by train (deduction or conlusion)." Helen Stoner had a railway return ticket in her hand. So, she arrived by train.

If you do modus tollens for Holmes' deductions, then the deduction or conclusion which is disproved goes into the "if..." part and the clue which would support that deduction or conclusion goes into the "then...." part. You then deny the clue in the second premise and then deny the deduction in the conclusion.

For example, "If the gypsies had killed Julia Stoner (deduction or conlcusion to be disproved), then they would have been able to enter her room through the window or the door (clue which would support the deduction or conclusion)." They could not have entered her room through the window or the door. So, the gypsies had not killed Julia Stoner.

The clue must be stated first and identified as "clue". You then draw a line and then state the deduction below the line and identify it as "deduction". Use one simple and clear sentence to state the clue and one simple and clear sentence to state the deduction. First state the clue and deduction and then give the form of deduction.

I. Example :

Clue : The murderer had a long stride (as shown by footprints on the clay outside and the dust within the house).

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Deduction: The murderer must be more than six feet tall.

Modus Ponens format:

1) If the murderer's footprints show a very long stride, then he must be over six feet tall.

2) The murderer's footprints show a very long stride.

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C: The murderer must be more than six feet tall.

II. Example :

Clue: The letter "A" in "RACHE" on the wall was written in German fashion.

Assumption : A German would have written the letter "A" in "RACHE " in Latin character.

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Deduction: The person who wrote "RACHE" on the wall was not a German.

Modus Tollens Format:

1) If a German had written "RACHE" on the wall, then he would have written the "A" in Latin Character.

2) The "A" was not written in Latin character.

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C: A German did not write "RACHE" on the wall.

III. Example :

Clue : The expression on Drebber's face suggests horror and hatred of something/someone he was looking at.

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Deduction : Drebber did not voluntarily take his life.

Disjunctive syllogism format:

1) Either Drebber died voluntarily or he was murdered.

2) Drebber did not die voluntarily.

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C: Drebber was murdered.