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Necessary & Sufficient conditions: A Medical Example March 22, 2014

Posted by larry (Hobbes) in Logic, Medicine, Science.
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A sufficient condition A for B is one where A being the case is sufficient for bringing about B.

A necessary condition B for A is one where if B is not the case, then A won’t be either.

Logically it looks like this. A is sufficient for B: if A, then B.

B is necessary for A: if not-B, then not-A.

Ex. of a necessary condition: oxygen (B) is necessary for being able to breathe (A). Therefore, if not-B, then not-A.

It is easier to come up with necessary conditions than it is sufficient conditions. For instance, what is sufficient for being able to breathe?

A set of necessary and sufficient conditions for A is often considered to be equivalent to, or for, A.

A slightly more realistic and complicated way of expressing this set of relationships is this. A is sufficient for D and B is necessary for D. This translates to (if A, then D) and (if D, then B). The contrapositive of each yields (if not-D, then not A) and (if not-B, then not-D). It is relatively clear, I think, that A and B each have a distinct relationship to D (I am ignoring the issue of transitivity illustrated in the example.). The potential complexity of this relationship is borne out is the following medical example from research into the dementias.

Here is a quote from a medical investigation of causes of Alzheimer’s and other dementias (from Daily Kos).

“Researchers have found that a protein active during fetal brain development, called REST, switches back on later in life to protect aging neurons from various stresses, including the toxic effects of abnormal proteins. But in those with Alzheimer’s and mild cognitive impairment, the protein — RE1-Silencing Transcription factor — is absent from key brain regions.”

“Our work raises the possibility that the abnormal protein aggregates associated with Alzheimer’s and other neurodegenerative diseases may not be sufficient to cause dementia; you may also need a failure of the brain’s stress response system,” said Bruce Yankner, Harvard Medical School professor of genetics and leader of the study, in a release.”

While the situation is more complicated than the simple example I gave initially, the logic is the same.

From the quote, we have: protein aggregate A; failure brain stress response system B; absence of RE1 (=R); dementia D.

Second paragraph of the above quote may be saying that A & B is sufficient for D.

But from the first paragraph, we also have absence of R (RE1) as a necessary condition for D. I.e., if D, then not-R (absence of RE1).

So, A&B is sufficient for D, hence (if A&B, then D). But, not-R is necessary for D. Or, equivalently, (if R, then not-D). I.e., R is sufficient for not-D.

Similarly as in the simple example: A, B, and R are related in a complex way to D, a relationship that is not entirely spelled out in the quote.

We are, therefore, left with an important question: what is status of A & B with respect to R? Is R a component of either A or B? The quote doesn’t link all these factors together. While this may be obvious from the quote, the logical situation underlying this hiatus may not be clear. Hopefully, it now is and also clearer what additional relationships need to be explored in order to lead to better understanding of the dementias, particularly Alzheimer’s, and thereby better control of their onset and progression if not complete defeat.

Quackery discussion about Chiros via Quackometer June 11, 2009

Posted by larry (Hobbes) in Medicine.
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A fascinating discussion has been taking place on Quackometer, specifically about McTimoney Chiropractors who have advised those in the profession to take down their web sites entirely or revise them to ensure that no claims are made that can not be supported by chiropractic research. The letter is itself bizarre. The comments are lively and in good spirits – no personal vitriol.

Those who mentioned the Singh case and the judgment of justice Eady may be interested to know that, November of last year, Eady was attacked for trying to bring in a privacy law “by the back door”.

QCs defend Mr Justice Eady as newspapers accuse him of privacy law rulings” – Francis Gibb, legal editor, Times Online.

Some of the commenters have referred to various logical fallacies, so I thought it might be helpful if I defined a few of those for you. Logical fallacies are committed all the time, and recently by Mervyn King, governor of the Bank of England. He committed the fallacy, in an interview, of affirming the consequent, which is high on the ‘oh dear’ list.

Fallacies & related matters:

a) Affirming the consequent: (King) [simplified discussion]

Hypothesis under test (ceteris paribus): Policy P will lead to desired results R;  in short, if P then R.

¡  Policy implemented & desired results are not observed:

  • Can argue from not observing desirable results following implementation of the policy that the policy failed the test;
  • This is a valid inference – [(if P then R) & not-R, then not-P], justified by appeal to the rule of modus tollens.

¡  Policy implemented & desired results are observed:

  • Can NOT argue from simply having observed desirable results following implementation of the policy that it passed the test;
  • This is a fallacious inference – [(if P then R) & R, then P] = the fallacy of affirming the consequent.

b) ceteris paribus: all else being equal.

c) tu quoque: you too. Considered to be a comment ad hominem where an attack is made on the person rather than on the position or argument.

allopathic: other suffering. An allopathic treatment is a traditional medical treatment that is used to oppose or suppress the symptoms rather than directly treat the cause. There does not appear to be a generally agreed consistent usage of this term.

iatrogenic: doctor induced. A cult reference to this is Ivan Illich’s Limits to Medicine, Medical Nemesis: The Expropriation of Health (1976).

Godwin’s law (Mike Godwin, 1990): the longer an issue is discussed, the more likely it is that a comparison will be made to the Nazis or Adolf Hitler. (Meme, Counter-meme)

scientism: used pejoratively to indicate an improper use of science or scientific claim.

consilience: the unity of knowledge. The most recent salient advocate of this doctrine is Ed Wilson in his book of the same name. But this position goes back in the modern era to the unity of science movement of the first half of the 20th century advocated by many logicians and philosophers of science.

post hoc ergo propter hoc: after this, therefore because of this; sometimes referred to as the ‘correlation, not causation’ fallacy. No one mentioned this fallacy, but it crops up in arguments in such discussions all the time – either committed or rejected as fallacious. (See the hypothetical test above.)

Some fallacies, while logically fallacious, may not be rhetorically fallacious. And the Hellenic Greeks, who first systematized this approach, often thought rhetoric was superior to logic.  Certainly rhetoric was viewed as being more influential in changing minds than logic.


Singh (in the THE) is mistaken that the only key issue in the libel law debate is cost. Also at issue is the character of the libel laws themselves. The libel laws here are so bad that the US Congress is intending to pass more legislation protecting US citizens from UK libel laws. They would not be considering this did they not think that the situation was unreasonable and serious – they view such laws as inconsistent with freedom of speech, guaranteed by the Constitution. Brown should have put forward a written constitution by now. The libel laws here are dangerous and need to be substantially revised.

Some think that McTimoney taking down information on their web site is “proof” that they know that what they are providing is bogus. This is logically incorrect. All it shows is that they feel threatened or are paranoid. It is not an indication of guilt. This kind of inference is not justified. A claim that their treatments are bogus must be justified on other grounds. Having said this, I agree that secrecy is not the best policy and that it would be better for them and their associates to reconsider this action at the earliest opportunity.

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