Quotations and Facts from Carl Hempel’s
Philosophy of Natural Science
“A case
history as an example”
“As a
simple illustration of some important aspects of scientific inquiry let us
consider Semmelweis' work on childbed fever. Ignaz Semmelweis, a physician of
Hungarian birth, did this work during the years from 1844 to 1848 at the Vienna
General Hospital. As a member of the medical staff of the First Maternity
Division in the hospital, Semmelweis was distressed
to find that a large proportion of the women who were delivered of their babies
in that division contracted a serious and often fatal illness known as
puerperal fever or childbed fever. In 1844, as many as 260 out of 3,157 mothers
in the First Division, or 8.2 per cent, died of the disease; for 1845, the
death rate was 6.8 per cent, and for 1846, it was 11.4 per cent. These figures
were all the more alarming because in the adjacent Second Maternity Division of
the same hospital, which accommodated almost as many women as the First, the
death toll from childbed fever was much lower: 2.3, 2.0, and 2.7 per cent for
the same years.”
Semmelweis notes
that there was no city-wide epidemic; it was selective. Also, the mortality rate for
cases of street birth were lower than those for regular admission.
Question: How
would you go about attempting to solve the puzzle? Why was the mortality rate in the First
Division so much higher than in the Second Division?
Differences:
1) Overcrowding?
(No, actually worse in the 2nd Division)
2) Diet?
(No, the same)
3) Rough handling by medical students? (No. Semmelweis notes in
refutation of this view that (a) the injuries resulting naturally from the
process of birth are much more extensive than those that might be caused by
rough examination; (b) the midwives who received their training in the Second
Division examined their patients in much the same manner but without the same
ill effects; (c) when, in response to the commission's report, the number of
medical students was halved and their examinations of the women were reduced to
a minimum, the mortality, after a brief decline, rose to higher levels than
ever before.)
4) Psychological
explanation? There were different paths for priest administering last rites –
direct access where
mortality was lower. (No. They changed path to allow direct access in
both cases, mortality rate stayed the same.)
5) Birthing position? First Division, on backs;
Second Division, on side. (No. Changed First Division to lateral position, no change in mortality
rate).
Finally, a Clue:
“At last, early in 1847,
an accident gave Semmelweis the decisive clue for his
solution of the problem. A colleague of his, Kolletschka,
received a puncture wound in the finger, from the scalpel of a student with
whom he was performing an autopsy, and died after an agonizing illness during
which he displayed the same symptoms that Semmelweis
had observed in the victims of childbed fever. Although the role of
micro-organisms in such infections had not yet been recognized at the time, Semmelweis realized that "cadaveric matter" which
the student's scalpel had introduced into Kolletschka's
blood stream had caused his colleague's fatal illness. And the similarities
between the course of Kolletschka's disease and that
of the women in his clinic led Semmelweis to the
conclusion that his patients had died of the same kind of blood poisoning: he,
his colleagues, and the medical students had been the carriers of the
infectious material, for he and his associates used to come to the wards
directly from performing dissections in the autopsy room, and examine the women
in labor after only superficially washing their hands, which often retained a
characteristic foul odor.”
Then, a Hypothesis:
“Again, Semmelweis put his idea to a test. He reasoned that if he
were right, then childbed fever could be prevented by chemically destroying the
infectious material adhering to the hands. He therefore issued an order
requiring all medical students to wash their hands in a solution of chlorinated
lime before making an examination. The mortality from childbed fever promptly
began to decrease, and for the year 1848 it fell to 1.27 per cent in the First
Division, compared to 1.33 in the Second.”
Coherence with other Evidence:
- In the First Division,
patients attended by medical students.
In the Second Division, patients attended by midwives who were not
trained on cadavers.
- Among newborn victims,
all were from infected mothers, who transmitted the infection through a common
bloodstream.
Refinement of Hypothesis:
“Further clinical
experiences soon led Semmelweis to broaden his
hypothesis. On one occasion, for example, he and his associates, having
carefully disinfected their hands, examined first a woman in labor who was
suffering from a festering cervical cancer; then they proceeded to examine
twelve other women in the same room, after only routine washing without renewed
disinfection. Eleven of the twelve patients died of puerperal fever. Semmelweis concluded that childbed fever can be caused not
only by cadaveric material, but also by ‘putrid matter derived from living
organisms.’”
Testing the Hypothesis:
“The
test is based on an argument to the effect that if the contemplated
hypothesis, say H, is true, then certain observable events (e.g.,
decline in mortality) should occur under specified circumstances (e.g., if the
priest refrains from walking through the wards, or if the women are delivered
in lateral position); or briefly, if H is true, then so is I, where I is
a statement describing the observable occurrences to be expected…”
A
Hypothesis Disconfirmed:
If H is true, then so is I
But (as the evidence shows) I is not true.
------------------------------------------------------
H is not true.
“Any argument of this
form, called modus tollens in logic, is
deductively valid; that is, if its premisses (the
sentences above the horizontal line) are true, then its conclusion (the
sentence below the horizontal line) is unfailingly true as well. Hence, if the premisses of (2a) are properly established, the hypothesis
H that is being tested must indeed be rejected.”
How
Does Confirmation Work?
From
his hypothesis that childbed fever is blood poisoning produced by cadaveric
matter, Semmelweis infers that suitable antiseptic
measures will reduce fatalities from the disease. This time, experiment shows
the test implication to be true. But this favorable outcome does not
conclusively prove the hypothesis true, for the underlying argument would have
the form
If H is true, then so is I.
(As the evidence shows) I is true.
--------------------------------------------
H is true.
And this mode of reasoning, which is
referred to as the fallacy of affirming the consequent, is deductively
invalid, that is, its conclusion may be false even if its premisses
are true.
This is in fact illustrated by Semmelweis' own experience. The initial version of his
account of childbed fever as a form of blood poisoning presented infection with
cadaveric matter essentially as the one and only source of the disease; and he
was right in reasoning that if this hypothesis should be true, then destruction
of cadaveric particles by antiseptic washing should reduce the mortality.
Furthermore, his experiment did show the test implication to be true. Hence, in
this case, the premisses of were both true. Yet, his
hypothesis was false, for as he later discovered, putrid material from living
organisms, too, could produce childbed fever.
* * * * * EXTREMELY IMPORTANT POINT * * * * *
“[T]he observation that a favorable
outcome of however many tests does not afford conclusive proof for a hypothesis
should not lead us to think that if we have subjected a hypothesis to a number
of tests and all of them have had a favorable outcome, we are no better off
than if we had not tested the hypothesis at all. For each of our tests might
conceivably have had an unfavorable outcome and might have led to the rejection
of the hypothesis. A set of favorable results obtained by testing different
test implications…of a hypothesis, shows that as far as these particular
implications are concerned, the hypothesis has been borne out; and while this
result does not afford a complete proof of the hypothesis, it provides at least
some support, some partial corroboration or confirmation for it.”