Illustrating Duhem A case of planetary misbehavior

1
Illustrating DuhemA case of planetary
misbehavior

• A physicist of the Newtonian era takes Newtons
  mechanics, T, and his law of gravitation, N, the
  accepted initial conditions, I, and calculates
  the path of a newly discovered planet, p. But the
  planet deviates from the calculated path.
• Does our Newtonian physicist consider, that once
  established, the deviation refutes the law, N?

2
Illustrating DuhemA case of planetary
misbehavior

• No. He suggests that there must be some hitherto
  unknown planet, p, that perturbs the path of p.
  He calculates the mass, orbit, etc. of this
  hypothetical planet and then asks an experimental
  astronomer to test his hypothesis.
• The planet p is so small that even the biggest
  available telescopes cannot possibly observe it
  the experimental astronomer applies for a
  research grant to get an even bigger one.

3
Illustrating DuhemA case of planetary
misbehavior

• In 3 years time, the new telescope is ready. Were
  the hypothetical planet p to be discovered, it
  would be hailed as a new victory of Newtonian
  science. But it is not.
• Does our scientist abandon Newtons theory and
  his idea of the perturbing planet? No. He
  suggests that a cloud of cosmic dust hides the
  planet from us. He calculates the location and
  properties of this cloud and asks for a research
  grant to send up a satellite to test his
  calculations. Were the satellites instruments
  (possibly new ones, based on a little-tested
  theory) to record the existence of the
  conjectured cloud, the result would be hailed as
  an outstanding victory for Newtonian science.
• Both Uranus and Neptune were discovered this way.

4
Holism

• Duhem/Quine thesis It is bodies of theory,
  rather than individual hypotheses that entail
  predictions.
• If T then (if H, then I)
• Not I
• ---------------------------------------------
• Not some or more statements of T or not H

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Holism

• If T, then (if H, then I)
• not I
• ------------------------------
• Not some part of T or not H
• To decide that not H follows, rather than some
  part or whole of T, is to choose to hold one or
  the other firm and the other infirm.
• Neither logic nor the experimental results
  dictates that one choice is necessary, the
  right one, etc.

6
Holism

• Crucial tests
• Can assume the logic of reductio ad absurdum
• Either H or not H
• If H, then I
• If not H, then not I
• not I
• ----------
• Not H

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Holism

• But I does not follow from H alone (nor does not
  I follow from H alone)
• I if H, and not I if not H, follow from a whole
  group of theories that are taken by the scientist
  to be beyond dispute.
• If not H occurs, it does not pinpoint that H is
  the problem but only that something in T and H
  (including but not limited to that H follows from
  T) is the problem.
• It does not pinpoint where the error is.
• The scientist may declare H is falsified, but
  this is not entailed by logic or the experimental
  results.

8
Holism

• In the late 1980s, researchers developed a drug
  predicted to be capable of thwarting the
  replication of the HIV virus implicated in AIDS.
• H All things being equal the drug will be
  effective
• In experiments, one group was given the drug and
  the other a placebo.
• The initial trials (running over 2 years)
  confirmed the drugs predicted success. But in
  the 3rd year, it stopped being effective.
• Initial results (showing effectiveness) were
  untrustworthy
• So not H

9
Holism

• So not H
• Then current knowledge indicated that it didnt
  matter if the drug was stored and delivered in
  glass or plastic containers.
• After the initial 2 year period, the drug was
  stored and delivered in plastic rather than glass
  containers.
• Thus the conclusion that not H presumed the truth
  of this background knowledge and it was wrong.

10
Holism

• Another problem for crucial tests
• Either H1 or H2
• Not H2
• -------------------------------
• H1
• Can we be sure these two hypotheses are the only
  possibilities? Only then, does H1 follow (sort
  of, given that both hypotheses are not tested in
  isolation)
• But we cant be sure they are the only
  possibilities.