When certain consequences of a theory are struck by experimental contradictions, we learn that this theory should be modified but we are not told by the experiment what must be changed. It leaves to the physicist the task of finding out the weak spot that impairs the whole system. No absolute principle directs this inquiry, which different physicists may conduct in very different ways without having the right to accuse the one another of illogicality. For instance, one may be obliged to safeguard certain fundamental hypotheses while he tried to reestablish the harmony between the consequences of a theory and the facts complicating the schematism in which these hypotheses are applied, by invoking various causes of error and by multiplying correction. The next physicist, disdainful of these complicated artificial procedures, may decide to change some one of the essential assumptions supporting the entire system. The first physicist does not have the right to condemn in advance the boldness of the second one, nor does the latter have the right to treat the timidity of the first physicist as absurd. The method they follow are justifiable only be experiment, and if they both succeed in satisfying the requirements of experiment each is logically permitted to declare himself content with the work that he has accomplished.
That does not mean that we cannot very properly prefer the work of one of the two to that of the other. Pure logic is not the only rule for our judgements, certain opinions which do not fall under the hammer of the principle of contradiction are in any case perfectly unreasonable. These motives which do not proceed from logic and yet direct our choices, these “reasons which reason does not know” and which speak the ample “mind of finesse” but not to the “geometric mind,” constitute what it appropriately called good sense.
Now, it may be good sense that permits us to decide between two physicists. It may be that we do not approve the haste with which the second one upsets the principles of a vast and harmoniously constructed theory whereas a modification of detail, a slight correction, would have sufficed to put these theories in accord with the facts. On the other hand, it may be that we may find it childish and unreasonable for the first physicist to maintain obstinately at any cost, at the price of continual repairs and many tangled-up stays, the worm-eaten columns of a building tottering in every part, when by razing these columns it would be possible to construct a simple, elegant, and solid system.
But these reasons of good sense do not impose themselves with the same implacable rigor that the prescriptions of logic do. There is something vague and uncertain about them; they do not reveal themselves at the same time with the same degree of clarity to all minds. Hence, the possibility of lengthy quarrels between the adherents of an old system and the partisans of a new doctrine, each camp claiming to have good sense on its side, each party finding the reasons of the adversary inadequate. The history of physics would furnish us with innumerable illustrations of these quarrels at all times and in all domains.
1. Duhem, Pierre Maurice Marie. The Aim and Structure of Physical Theory. Princeton: Princeton UP, 1954. Print.