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Philosophy 352: Philosophy of Science
Dr. Jackie Kegley
The Hypo-Deductive Method of Reasoning
A Case History
PROBLEM: Ignaz Semmelweis, a physician of Hungarian birth, was head of the
medical staff of the Vienna General Hospital from 1844-1846. He was concerned with a
particular problem: a larger proportion of women delivered of their babies in the First
Section of the Maternity Ward died of childbed fever than those delivered in the Second
Section. How was one to account for this much higher death rate, e.g., 11.45 percent
compared with 2.7 percent?
Possible solutions: Some hypotheses ruled out as incompatible with well-established
facts, e.g., Childbed fever was caused by atmospheric changes. But surely all of Vienna
would be affected by such and not just the women of the First Division Maternity Ward.
Some hypotheses tested and found null, e.g. delivery of women lying on their backs
rather than sides was the cause and the rough examination by interns was the cause.
Solution found by accident, e.g. a colleague, Kolletschka, received a puncture wound in
the finger from the scalpel of a student with whom he was performing an autopsy; he fell
ill and died of a condition strikingly similar to childbed fever. Semmelweis found that
doctors and students were coming to examine the women in the First Division directly
from autopsy and thus they were carrying the infectious material to the women. (They
only superficially washed their hands.)
Hypothesis to be tested: Childbed fever was caused by infectious material adhering
to the hands.
Test: If childbed fever caused by infectious material adhering to the hands. (H)
Then childbed fever could be prevented by chemically destroying the
infectious material adhering to the hands. (I)
Ordered doctors and students to wash their hands in a solution of
chlorinated lime before making examinations.
The mortality rate from childbed fever promptly began to decrease. (H is
true)
If H is true, then so is I.
As the evidence shows, I is true.
Therefore H is true.
But confirmation has problems. This logical form is the invalid logical
form confirming the consequent. The invalidity of this form can be
shown through this example.
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If it rains, the picnic will be cancelled.
The picnic was cancelled.
Therefore it rained.
There is also the problem of auxiliary hypotheses.
Semmelweis assumes the chlorinated lime will kill the infectious material. So
the real form of his argument is:
If H and A (B and C, etc.) then I.
I
Then Probably H and A (B,andC)
Let’s Look at Disconfirmation
Semmelweis did test out other hypotheses and found them false.
If delivery on the back causes childbed fever, then delivery on the side will
reduce the morality rate.
Delivery on the side did not reduce the mortality rate.
Delivery on the back causes childbed fever is not true.
If H is true, then so is I.
But as the evidence shows I is not true.
Therefore H is not true.
Disconfirmation is a valid logical form called Modus Tollens. We can see this by an
everyday example.
If it is true that Mary is three months pregnant, then the doctor should be
able to detect a heart beat or note other effects of the baby’s presence.
The doctor cannot detect a heart beat or any effects of the baby’s presence.
Therefore, Mary is not pregnant.
One can also see the force of disconfirmation by noting the concepts of necessary and
sufficient conditions.
Sufficient condition= if present, it can bring about the effect, but other things
can also bring it about. A rock can break a window, but so can a chair.
Necessary condition= if it is absent, the effect cannot occur. Without oxygen,
combustion cannot occur.
In the conditional, the antecedent is a sufficient condition for the consequent
and the consequent is a necessary condition for the antecedent. Thus, if the
consequent is false, then p, the antecedent must be false.
However, need also to note that auxiliary hypotheses also affect disconfirmation.
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Also, Semmelweis’s case and his conclusions have more substance because there is
additional support.
His hypothesis found further support because it could account for other
facts: (1) the mortality rate in the Second Division was lower because they were
attended by midwives who had nothing to do with autopsies. (2) The mortality rates
were lower for women who arrived with babies in arm. They were rarely examined.
(3) Victims of childbed fever among newborn babies were those of mothers who had
contracted the disease during labor, when the infection could be transmitted to the
baby through the common bloodstream of mother and baby.
Rules and Criteria for Judging Hypohteses
These rules are no guarantee of the truth of a hypothesis, but they do give us
guidance to distinguish good, probably true hypotheses from bad probably false
ones.
1. A hypothesis should be physically testable. Suggested explanations of natural
events that go counter to the laws of nature and disregard common
knowledge about the natural world are not useful For example, one can
hypothesize that childbed fever was caused by evil spirits, but how can one
prove or disprove this?
2. A hypothesis should not be contrary to already available evidence. Thus the
hypothesis that atmospheric changes caused childbed fever was countered by
the fact that all people in Vienna were not affected by this disease.
3. A hypothesis should be conceived so that it entails consequences that can be
tested. Thus the hand washing and the decrease in childbed fever.
4. A hypothesis should be simple and clear. This does not mean “easy to
understand,” but rather “involving a minimum of presuppositions or
extraneous additions. It should be workable and systematically coherent,
drawing together the facts in a clear-cut pattern. (as did the hypothesis about
infectious matter) How would you deal, for example, with the hypothesis that
“class warfare” causes economic decline.
5. A hypothesis should be based on quantity, variety, and precision of evidence.