Two major problems have been identified regarding Popper's Falsificationism (see Chalmers, Chapter 7). The first problem is referred to as the Duhem/Quine thesis: when a falsifing observation is made, it is impossible to determine whether the theory is false or whether the observation is false.
The second problem is discarding a falsified theory is too stringent. Chalmers gives several historical examples where a falsifying observation should not be construed as disproof of a promising scientific theory. See p.17, for example, for a pertinent discussion of the apparent sizes of Venus and Mars that appeared to falisify Copernicus's theory.
"An embarrassing historical fact for falsificationists is that if their methodology had been strictly adhered to by scientists then those theories generally regarded as being among the best examples of scientific theories would never have been developed because they would have been rejected in their infancy." (p.91)The assumption here is that the history of science provides a valuable test of methodological principles. If a modern methodology cannot account for past "successes" then the methodology must be false.
Popper takes issue with this view. He counters that the progress of science might have been faster if historical figures had been falsificationists. Methodology doesn't necessarily have to account for history.
Regarding the Duhem/Quine thesis, one possible rejoinder is to note that the logic of discovery is contingent on several possibilities or constraints. Often these problems can be solved via so-called systems logic. For example, in high school mathematics, you might have encountered algebra problems such as the following:
2x = 3y + zThe solution would go something like the following:
z = 0 - 2y
y - x = 2
Solve for x, y and z.
2x = 3y - 2y
2x = y
y = x + 2
2x = x + 2
2x - x = 2
x = 2
2(2) = y
y = 4
z = 0 - 2(4)
z = -8
Therefore x =2, y = 4, and z = -8.
Similarly, systems logic is the logic one applies to complex intertwined propositions.E.g.
Either Andy or Betty is a horn player.
Only horn players dislike rap.
Betty plays bridge.
Those who like rap never play bridge.
Now consider the problem raised by Duhem and Quine. Suppose that an observer observes a black swan. Duhem and Quine would note that this observation is consistent with falsifying any one of the following statements:
Theory: "All swans are white." Observation conditions: "The lighting was appropriate for accurate color observation." Observer disposition: "The observer is reliable." Observer language: "The observer understands the word `black'." Observer state: "The observer is not white/black color blind." Observer character: "The observer is not prone to make jokes." Definitional: "This animal is a swan." Definitional: "This color is black." Situational: "The feathers have not been painted/dyed black." Methodology: "Falsificationism is a good methodology."
Although it is indeterminate which statement is false, the observation is nevertheless valuable in constraining the possibilities.
A falsificationist might point out that, in principle, one can resolve which hypothesis is incorrect by carrying out further falsifying experiments. For example, the above issues can be addressed by experimentally testing various supplementary hypotheses: E.g.
Hypothesis: "The observer understands the word `black'." Experiment: Show different color chips to the experimenter and observe descriptive language. Hypothesis: "The observer is reliable." Experiment: Send another observer to make observations. Hypothesis: "The feathers have not been painted/dyed black." Experiment: Pluck out some feathers and observe whether they grow back as black in color. Hypothesis: "This animal is not a swan." Experiment: Try to breed this swan with another swan. If there are offspring, then the statement "this animal is not a swan." is false.
In this last case, notice the "reversing" of the original hypothesis -- "This animal is a swan." Biologists define a species is a breeding population that cannot breed with other populations. Since an animal can fail to breed for other reasons (infertile, etc.), successful breed of the swan falsifies the reverse statement: "This animal is not a swan."
By following this process, one creates a network of hypotheses. By systematically falsifying various hypotheses, it is possible for the "matrix" to "resolve" in an analogous fashion to solving systems of equations in algebra or resolving complex intertwined propositions.
Recall that Popper's original motivation was to distinguish a scientific theory from a non-scientific theory. Popper wanted to include physics and biology, and exclude such things as Freudian psychotherapy and Astrology.
The problem with a systems solution is that it can be used to account for the failure of any hypothesis. For example, if a test of an astrological claim ("You will meet a tall dark stranger.") proves false, then the astrologer could claim various ad hoc hypotheses that would plausibly account for the failure.
In this case, Popper's theory fails to distinguish "science" from "non-science" as he originally planned.