Professor Norman Swartz
Simon Fraser University

The Presuppositions of Empirical Research

Example #1
A carpet vendor has to measure her customer's living room for some new broadloom. She has forgotten her tape measure, but does have a meterstick. She lays the meterstick on the floor, snug up against the wall, with the left edge of the stick in one corner of the room. She then makes a pencil mark at the right edge. Next she shifts the stick right until the left edge of the stick is at her mark, and again marks the right edge. She does this three times, but then finds that less than a full length remains to be measured. She turns the stick around so that the zero is now at the right side and lays it in the right corner and notes that her last pencil mark is at 70 cm. She concludes that the wall she is measuring is 3.7 meters in length. She then measures the opposite wall and concludes that it, too, is 3.7 meters in length. She then repeats the process for the remaining two opposite walls, and finds that they each measure 4.1 meters. To see if the room is rectangular, she runs a piece of twine from one corner to its opposite, pulls the twine taut and then cuts it to fit exactly. She then uses the same piece of twine on the other two corners to see if they are the same distance apart as the first two. The twine fits exactly. On the basis of these measurements, she concludes that the area of the room is 15.17 m2.
Think about this: What are some of the presuppositions in the foregoing scenario which would have to be true in order that the carpet vendor's conclusion be true? Here is a (partial) list:

  • The meterstick does not change length as it is moved about in space.
  • The meterstick she is using is in fact well made, and is (very nearly) 1 meter in length. (Remember, wooden metersticks – the cheap ones which you are likely to find in lumberyard sales – often shrink with age.)
  • Pencil marks do not move about on walls.
  • A string pulled taut twice will be the same length on both occasions.
  • The area of a four-sided room, whose opposite sides are equal and whose diagonals are equal, may be determined by multiplying its width by its length.
  • Lengths add like numbers.
  • The order in which the measurements are taken is irrelevant, i.e. would be the same if they had been taken in the reverse order.
  • The walls are not bowed.
  • Etc., etc., etc.

Example #2
     It was common knowledge in Jenner's time that a person could catch smallpox only once. Many people tried to inoculate themselves with matter from smallpox sores. They hoped to catch a light case of the disease, and then be immune to it for the rest of their lives. Lady Mary Wortley Montagu, an English author, had introduced this method in England in 1718. But the method was dangerous.

     Jenner began experimenting in his home town, Berkeley, Gloucestershire. Many people there believed that diarymaids who had caught cowpox could not catch smallpox. Cowpox is a minor disease that causes a few sores on the hands but carries little danger of disfigurement or death. In 1796, Jenner took matter from the hand of Sarah Nelmes, a local dairymaid. She had become infected with cowpox while milking the cows. Jenner then made two cuts on the arm of James Phipps, a healthy eight-year old boy, and inserted the matter from one of Sarah's cowpox sores. The boy then caught cowpox. Six weeks later, Jenner introduced smallpox matter into the boy's arm. Ordinarily fatal, the smallpox matter had no effect. (World Book Encyclopedia, 1979, "Edward Jenner", vol. 11, p. 73)
Here is a (partial) list of presuppositions that would have to be true for Jenner's conclusion to be true:
  • Nelmes and others who appeared to have cowpox, really did, i.e. did not have some other disease having symptoms similar to cowpox. (If Nelmes and others in Berkeley had had some other disease, then other doctors, in other towns, using the matter from other persons who did in fact have cowpox, might not have succeeded in protecting their patients from smallpox.)
  • Phipps and other subjects whom Jenner inoculated had not had mild, undetected, cases of smallpox earlier.
  • There was nothing in the water or atmosphere in Berkeley that conferred immunity against smallpox on everyone living in that town.
  • Jenner's instruments were sterile or, at the very least, not the source of some unknown material that conferred immunity against smallpox.
  • The material, in being transferred from the donor to the recipient, was not subjected to anything in the process of transferral that would have created the immunity-conferring agent.
  • Jenner himself did not have (mystical? magical?) curative powers (perhaps even unknown to himself). (There was nothing on his skin, or in his exhalations, that could confer immunity against cowpox.)
  • There are not different strains of cowpox such that one strain (such as might be found in Berkeley) would confer immunity against smallpox, but that other strains (in other locales) might not.
  • There was nothing else in the history of the subjects whom Jenner inoculated that was responsible for their immunity to smallpox (e.g. they did not all attend the same school or church, did not all work in the same factory, etc.).
  • The subjects who were inoculated with cowpox were genuinely immune to smallpox: they were not asymptomatically incubating the disease, only to come down with a fatal case some time later (e.g. several years).
  • The immunity to smallpox was relatively long lasting. Persons who were (or seemed to be) immune to smallpox six weeks after having being inoculated with material from cowpox sores would also be immune to smallpox for a longer period. (What we now call "booster inoculations" would not be needed frequently, or even worse, at shorter and shorter intervals.)
  • The time of day – of the taking of the material from a cowpox lesion, of the transferring that matter to another person, and of the subsequent inoculating the latter person with smallpox material – was irrelevant to the outcome (i.e. the time of day was not a causal factor). Ditto for temperature, atmospheric pressure, season, and weather conditions.
  • Persons' diets did not affect the outcome of the experiment.
  • Persons' age did not affect the outcome of the experiment.
  • Persons' race did not affect the outcome of the experiment.
  • If a donor were pregnant, that would not affect the outcome of the experiment; more exactly, pregnancy is not a causal factor, either positively or negatively. (Note: one would want to be very cautious [and have done some experimenting on animals] before inoculating pregnant women with either cowpox or smallpox material – it might have a deleterious affect on the fetus.)
  • The particular time of a woman's menstrual cycle, whether she was the donor or the recipient of the cowpox material, did not affect the outcome of the experiment.
  • Persons' prior medical history – e.g. whether they had had pneumonia, dysentery, gout, whooping cough, etc. – would not affect the outcome of the experiment.
  • Transferring matter from a smallpox sore to a cut in the skin of another person would induce smallpox in that second person (provided that the second person had not previously had either cowpox or smallpox). (Note: this particular presupposition was, presumably, already fairly well established by Montague.)
  • It did not matter from where on one's body – whether from the arm, the leg, the neck, etc. – the cowpox and smallpox matter was taken; nor did it matter where, on the body of another person, this material was inserted.
  • Smallpox immunity was not being conferred by something attendant upon the inoculation, e.g. by parallel incisions in one's arm.
  • The immunity was not being conferred by a chemical reaction between the skin (or blood) of the person being inoculated and the particular metal (iron?) of Jenner's knife (scalpel).
  • There was nothing done to, or by, the persons (who had been inoculated with smallpox matter) after their inoculations that would prevent their contracting smallpox, i.e. whatever immunity to smallpox they exhibited was the result of what had happened to them before their inoculation with smallpox matter.
  • Immunity to smallpox cannot be induced/conferred by the subject's belief that he/she has been given an effective immunization. (In vocabulary which has become current since the late 1940s, we would say that the immunity is not a 'placebo effect'.)
  • The lifestyles of the donors and recipients, e.g. whether they were physically active or whether they were fairly sedentary, would not affect the outcome of the experiment.
  • It was not the treatment for cowpox (e.g. applying salves) that conferred the immunity to smallpox.
  • The mere transferring of matter (flesh and/or bodily fluids) from the body of a person infected with cowpox to the body of another person does not confer on that latter person immunity to smallpox; the transferred matter must be from festering sores.
Here are a few presuppositions which have to be made in some other medical experiments, but not(!!!) in Jenner's (in 1796).
  • The agents of smallpox and cowpox are germs (bacteria, viruses, etc.). (The germ theory of disease was not to become generally accepted for another two to four decades.)
  • Human beings have an immune system which attacks foreign organisms in the body and which, in some instances, produces lifelong antibodies.
  • The immune system can attack foreign organisms anywhere in the body.
  • The agents of infection are carried throughout the body by the blood stream.