THOMSON VERSUS HERTZ
Finally, let me invoke an example more recognizably scientific. It involves a dispute between Heinrich Hertz and J. J. Thomson over the nature of cathode rays.[89] In experiments conducted in 1883 Hertz observed that the cathode rays in his experiments were not deflected by an electrical field.
He took this to be strong evidence that cathode rays are not charged particles (as the English physicist William Crookes had concluded), but some type of ether waves. In 1897 J. J. Thomson repeated Hertz's experiments but with a much higher evacuation of gas in the cathode tube than Hertz had been able to obtain. Thomson believed that when cathode rays pass though a gas they make it a conductor, which screens off the electric force from the charged particles comprising the cathode rays.[90] This screening off effect will be reduced if the gas in the tube is more thoroughly evacuated. In Thomson's 1897 experiments electrical deflection of the cathode rays was detected, which Thomson took to be strong evidence that cathode rays are charged particles.Here, however, I want to consider the evidential report of Hertz in 1883, not of Thomson in 1897. Let
e = In Hertz's cathode ray experiments of 1883 no electrical deflection of cathode rays was detected.
h = Cathode rays are not electrically charged.
Hertz took e to be strong evidence for h. In 1897 Thomson claimed, in effect, that Hertz's results as reported in e did not provide strong evidence for h, since Hertz's experimental setup was flawed: He was employing insufficiently evacuated tubes. To use my previous terminology, Thomson was claiming that Hertz's selection procedure for testing h was inadequate.[91]
Here we can pick up on a point emphasized by Brush. Hertz, we might say, failed to use a selection procedure calling for considering a competitor to h to explain his results (that is, that cathode rays are charged particles, but that the tubes Hertz was using were not sufficiently evacuated to allow an electrical force to act on these particles).
But—and this is the point I want to emphasize—in determining whether, or to what extent, Hertz's putative evidence e supports his hypothesis h, it seems to be irrelevant whether Hertz's e was a novel prediction from an already formulated hypothesis h or an already known fact to be explained by h. Hertz writes that in performing the relevant experiments he was trying to answer two questions:Firstly: Do the cathode rays give rise to electrostatic forces in their neighbourhood? Secondly: In their course are they affected by external electrostatic forces? (Hertz 1896, p. 249)
In his paper he did not predict what his experiments would show. Nor were the results of his experiments treated by him as facts known before he had formulated his hypothesis h. Once he obtained his experimental result he then claimed that they supported his theory:
As far as the accuracy of the experiment allows, we can conclude with certainty that no electrostatic effect due to the cathode rays can be perceived. (p.251)
To be sure, we might say that Hertz's theory itself predicted some such results, even if Hertz himself did not (i.e., even if Hertz did not himself draw this conclusion before getting his experimental results). But even if we speak this way, Hertz did not claim or imply that his experimental results provide better (or weaker) support for his theory because the theory predicted them before they were obtained. Nor did Thomson in his criticism of Hertz allude to one or the other possibility. Whichever it was—whether a prediction or an explanation or neither—Hertz (Thomson was claiming) should have used a better selection procedure. This is what is criticizable in Hertz, not whether he was predicting a novel fact or explaining a known one.
I end with a quote from John Maynard Keynes (1921, p. 305), whose book on probability contains lots of insights. Here is one:
The peculiar virtue of prediction or predesignation is altogether imaginary. The number of instances examined and the analogy between them are the essential points, and the question as to whether a particular hypothesis happens to be propounded before or after their examination is quite irrelevant.