DISAGREEMENTS ABOUT EVIDENCE

Scientists frequently disagree with one another about whether some fact is evidence that a certain hypothesis is true or, if it is, about how strong that evidence is. I have in mind cases in which they agree that some fact has been observed or established, or some experimental result obtained.

Let me mention two cases, one from the nineteenth century, the other quite recent. In 1883 Heinrich Hertz conducted a series of carefully designed experiments with cathode rays to determine whether they are electrically charged (Hertz 1896). He separated the cathode rays from the ordinary electric current that flows from the cathode to the anode, so that the pure cathode rays would enter an electrometer, the deflection of which would determine the presence of electricity. In his experiments, however, the needle of the electrometer remained at rest when cathode rays were produced. In a second experiment Hertz introduced oppo­sitely electrified plates into the cathode tube. If cathode rays were elec­trically charged they should be deflected by these plates, as indicated in a changed position of the phosphorescence produced by the rays. But no such change occurred. Hertz took these two experimental results to be evidence, indeed decisive evidence, that cathode rays are not electrically charged.

Fourteen years later J. J. Thomson (1897) repeated the second of Hertz's experiments and got the same results as Hertz, no deflection of the cathode rays.

Thomson did not dispute that Hertz obtained the results he did, namely, no electrical deflection of the cathode rays. Indeed, Thomson ob­tained the same results in initial experiments. What he challenged was the claim that these results were evidence that cathode rays are electrically neutral.

My second example involves an archaeological hypothesis about the earliest campfires used for cooking, for light, and as a protection against animals. For 60 years it had been hypothesized that the first campfires were built by Peking Man in caves in Zhoukoudian, China, between 200,000 and 500,000 years ago. The evidence for this hypothesis was the existence of burned animal bones in the same layer of soil as stone tools and the sediment there that looks like wood ash. On July 10, 1998, a group of scientists from Israel, the United States, and China rejected the claim that the existence of the burned animal bones and the existence of sediment together provided strong evidence that campfires existed there (Weiner et al. 1998). This claim was based on the discovery that the sediment in question is not wood ash but fine minerals and clay deposited by water. These scientists claimed that the burned animal bones by themselves do not constitute very good evidence that a fire was started by humans.

In both of these cases there is an agreement between disputants over what has or has not been observed—at least at some important level of description.

I mention cases of this sort because philosophers of science have developed theories or definitions of evidence that are designed to do at least two things for scientists: first, to clarify what it means to speak of confirming evidence; and second, and relatedly, to help scientists deter­mine whether, and to what extent, putative evidence supports a hypo­thesis. These goals are championed by a range of philosophers who have developed theories of evidence. A corollary—one emphasized by Carnap (1962)—is to develop a theory of evidence that will enable scientists to settle disputes, such as the ones I mentioned, over whether, or to what extent, putative evidence confirms a hypothesis.

By and large, however, philosophical theories of evidence are ignored by scientists. You don't find scientists with disagreements of the sort in question turning to philosophers for help. Why not? Is this just a matter of people in very different fields ignoring one another's work? That may be part of the answer, but I don't think that is the main problem or the most interesting one. I think the problem is deeper and stems from very basic, but questionable, assumptions philosophers usually make about evidence.

Before indicating what these offending assumptions are, let me say that the notion of evidence I am concerned with is an objective, not a subjec­tive, one: whether e is evidence that h, and how strong that evidence is, does not depend on what anyone believes about e, h, or their relationship. Not all philosophers who talk about evidence recognize, or are interested in, an objective sense of evidence. Subjective Bayesians, for example, re­ject such a notion. But a range of philosophers accept the idea, including objective Bayesians such as Carnap (1962), hypothetico-deductivists, satisfaction theorists such as Hempel (1965), bootstrappers such as Glymour (1980), and others as well.

The first of the two fundamental assumptions philosophers of the sort I have in mind usually make is that evidence is a very weak notion. You don't need very much to have evidence that something is the case. The second is that the evidential relation is a priori not empirical. It is a logical, or semantical, or mathematical relation that can be established by “calculation.” These assumptions need to be explained, illustrated, and, I think, rejected.

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