Matt, I think what you have here is violation of formal logical rules called modus tollens (MT) and modus ponens (MP) as they are used to formulate chain of inference. MT can be stated as "If A, then B" , and "B is not the case", we may conclude that "A is not the case". For example,
If Mr. Smith has AIDS, then HIV test will be positive.
Mr. Smith's HIV test is normal.
Therefore, he does NOT have AIDS.
and MP can be stated as "if A, then B", and "A", we may conclude "B". For example,
If Mr. Smith is HIV positive, he is at risk for AIDS.
Mr. Smith is HIV positive.
Therefore, he is at risk for AIDS.
Violation of MT occurs due to fallacy of denying the antecedent (In "if....then..." statement, the sentence after "if" is called "antecedent" and the one after "then" is called "consequent")
That is, if antecendent is false, then it is asserted that the consequent is also false. Thus instead of formulating valid MT argument, one argues invalidly "if A, then B", "A is not the case", therefore "B is not the case". For example, in the case of HIV example, fallacy may be constructed as:
"If Mr. Smith has viral prodrome, then his HIV test will be abnormal.
Mr. Smith does not have viral syndrome.
Therefore, his HIV test should be normal.
The premises in this argument may all be true. Nevertheless,it is invalid. Mr. Smith may still be HIV positive even he did not have viral symptoms.
Violation of MP is known as the fallacy of affirming consequent. Instead of formulating valid MP argument, we argue "if A, then B" and "B", therefore "A". For example,
If Mr. Smith has adnopathy he will have lymphoma.
He has adnopathy.
Therefore, Mr. Smith has lymphoma.
We recognize the fallacy here because we know that there are many causes of adenopathy.
In your case, the fallacy occured by asserting that if antecedent is true (false), then consequent is also true (false). As I illustrated above, medical inference is full of errors in application of rules of formal logic.
ben
Benjamin Djulbegovic, MD,PhD
Professor of Oncology and Medicine
H. Lee Moffitt Cancer Center & Research Institute
at the University of South Florida
Department of Interdisciplinary Oncology
SRB #4, Floor 4, Rm #24031 (Rm# West 31)
12902 Magnolia Drive
Tampa, FL 33612
Editor: Cancer Treatment Reviews (Evidence-based Oncology Section)
http://www.harcourt-international.com/journals/ctrv/
e-mail:[log in to unmask]
http://www.hsc.usf.edu/~bdjulbeg/
phone:(813)979-7202
fax:(813)979-3071
-----Original Message-----
From: Matt Williams [mailto:[log in to unmask]]
Sent: Saturday, June 12, 2004 10:37 AM
To: [log in to unmask]
Subject: An odd question
Dear List,
I have an odd question which I haven't been able to answer.
Someone I work with has asked if there is a medical example of the following
set of rules:
1:If it is raining, I won't run
2: If it is hot, I won't run
3: If it is raining AND hot, then I will run
I can't obviously think of an example, so I thought I would enlist your
brains.....
Thanks for any answers,
Matt Williams
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