(For other posts in this series, see here.)

In the previous post, I discussed Karl Popper’s idea of using falsification as a demarcation criterion to distinguish science from non-science. The basic idea is that for a theory to be considered scientific, it has to make risky predictions that have the potential that a negative result would require us to abandon the theory. i.e., declare it to be false. If you cannot specify a test with the potential that a negative result would be fatal to your theory, then according to Popper’s criterion, that theory is not scientific.

Of course, I showed that falsification cannot be used to identify true theories by eliminating all false alternatives, because there is no limit to the theories can be invented to explain any set of phenomena. But steadily eliminating more and more false theories surely has to be a good thing in its own right. This is why falsificationism is highly popular among working scientists because it enables them to claim that science progresses by closing down blind alleys.

But there is a deeper problem with the whole methodology of falsificationism and that it is that even if prediction and data disagree, we cannot infer with absolute certainty that the theory is false because of the interconnectedness of scientific knowledge. Pierre Duhem pointed out over a century ago that in science one is never comparing the predictions of a single theory with experimental data, because the theories of science are all inextricably tangled up with one another. As Duhem said (The Aim and Structure of Physical Theory, Pierre Duhem, 1906, translated by Philip P. Wiener, 1954, p. 199, italics in original):

To seek to separate each of the hypotheses of theoretical physics from the other assumptions on which this science rests is to pursue a chimera; for the realization and interpretation of no matter what experiment in physics imply adherence to a whole set of theoretical propositions.

The only experimental check on a physical theory which is not illogical consists in comparing the entire system of the physical theory with the whole group of experimental laws, and in judging whether the latter is represented by the former in a satisfactory manner.

In other words, since every scientific theory is always part of an interconnected web of theories, when something goes wrong and data does not agree with the prediction, one can never pinpoint with certainty exactly which theory is the culprit. Is it the one that is ostensibly being tested or another one that is indirectly connected to the prediction? One cannot say definitively. All one knows is that something has gone wrong somewhere. Duhem provides an illuminating analogy of the difficulty facing a scientist by saying that the work of a scientist is more similar to that of a physician than a watchmaker.

People generally think that each one of the hypotheses employed in physics can be taken in isolation, checked by experiment, and then, when many varied tests have established its validity, given a definitive place in the system of physics. In reality, this is not the case. Physics is not a machine which lets itself be taken apart; we cannot try each piece in isolation and, in order to adjust it, wait until its solidity has been carefully checked. Physical science is a system that must be taken as a whole; it is an organism in which one part cannot be made to function except when the parts that are most remote from it are called into play, some more so than others, but all to some degree. If something goes wrong, if some discomfort is felt in the functioning of the organism, the physicist will have to ferret out through its effect on the entire system which organ needs to be remedied or modified without the possibility of isolating this organ and examining it apart. The watchmaker to whom you give a watch that has stopped separates all the wheelworks and examines them one by one until he finds the part that is defective or broken. The doctor to whom a patient appears cannot dissect him in order to establish his diagnosis; he has to guess the seat and cause of the ailment solely by inspecting disorders affecting the whole body. Now, the physicist concerned with remedying a limping theory resembles the doctor and not the watchmaker. (p. 187)

Duhem is arguing that one can never deduce whether any individual scientific theory is false, even in principle. This seems to be fly in the face of direct human experience. Anyone with even a cursory knowledge of scientific history knows that individual scientific theories have routinely been pronounced wrong and been replaced by new ones. How could this happen if we cannot isolate a single theory for comparison with data? How can scientists decide which of two competing theories is better at explaining data if a whole slew of other theories are also involved in the process? Is Duhem saying that we can never arrive at any conclusion about the truth or falsity of any scientific theory?

Not quite. What he goes on to say is that, like a physician, a scientist has to exercise a certain amount of discerning judgment in identifying the source of the problem, all the while being aware that one does not know for certain. Duhem argues that this is where the reasoned judgment of the scientific community as a whole plays a role in determining the outcome, overcoming the limitations imposed by strict logic. While there may be a temporary period in which scientists argue over the merits of competing theories,

In any event this state of indecision does not last forever. The day arrives when good sense comes out so clearly in favor of one of the two sides that the other side gives up the struggle even though pure logic would not forbid its continuation… Since logic does not determine with strict precision the time when an inadequate hypothesis should give way to a more fruitful assumption, and since recognizing this moment belongs to good sense, physicists may hasten this judgment and increase the rapidity of scientific progress by trying consciously to make good sense within themselves more lucid and more vigilant. (Duhem, p. 218, my italics.)

In the next post, I will discuss the importance that the consensus judgment of expert communities plays in science.