of the victim caused her blood to end up on John’s shirt. The so-called Koplik spots on a person’s gums are evidence for the hypothesis (often called a diagnosis in this context) that the person has the measles, because having the measles causes those spots to appear. In fact, there are so many cases in which we have observable indicators (i.e., evidence) for features of the world we might not directly observe (deer, measles, etc.) that one might be tempted to simply define evidence in terms of a causal indicator relation, maybe along the following lines:
An observable feature O of the world is evidence for the existence of an unobserved or unobservable feature U of the world if and only if U caused O, and O thereby indicates the existence of U.
A brief remark on the distinction between unobserved and unobservable features, invoked above, is in order. In our first example, the deer was simply unobserved, but is of course observable. But there also seem to be clear cases where we can’t observe that for which we might have observable evidence. Newtonian gravitation is a good example. We can observe its effects when we see a football drop into the outstretched arms of the receiver. However, we can’t observe the gravitational force directly. In fact, it seems we can’t observe any of the fundamental forces of nature directly. For this reason, many have argued that forces are prime examples of entities that are in principle unobservable, and that the only reason we have for believing that they exist at all is the evidence manifest in their actions. We can see neither the strong nor the weak nuclear force, but we can see their effects, the most prominent of which are of course all those familiar physical objects that surround us.1 This is the reason that the proposed analysis of the notion of evidence refers to both unobserved and unobservable features.
Let’s go back now to the idea that evidence can be defined in terms of causal relations between different features of the world. If this idea is correct, it would also explain nicely why scientific disputes can be resolved by appealing to a neutral, objective standard, as we claimed earlier. At first glance, it seems that causal relations are objective features of the world. Their existence is independent from human observers – the extinction of the dinosaurs was caused by certain features of the world (most likely by a meteor that hit the earth), even though there were no humans to observe it. Thus, what humans believe has no influence on the causal relations that exist in the world. This explains how evidence can help us decide between competing theories: Since two genuinely differen theories will not agree on all possible evidence, then, if we find evidence that can be better accounted for by Jane’s theory than by Bob’s, we have objective reasons for deciding the competition in favor of Jane. The reasons are objective, because causal relations that ground evidential relations are objective. Thus, if Bob holds on to his theory despite the evidence favoring Jane’s, he acts unreasonably.
This last observation leads to a further important fact concerning our ordinary concept of evidence. We think that our beliefs about the world have to be properly responsive to empirical evidence, and if they are not, those beliefs appear irrational. Thus, there is an important connection between being rational and being responsive to evidence. This connection will be the topic of later chapters. For now, our next task is to discuss what items in the “world” are appropriate candidates for evidence.
2.2 Basic Evidence and Derived Evidence
2.2.1 What We See
One question that many philosophers, as well as some scientists, have asked (at least implicitly) is this: What should be regarded as ultimate or basic evidence? In other words, what is our evidential bedrock? This question might seem strange at first. Clearly, it is information about the world and what is happening in it that should serve as the ultimate evidence to be used in the construction and confirmation of our empirical theories. For example, a physician’s observation that her patient exhibits Koplik spots is (a piece of) the ultimate evidence for her diagnosis that the patient has the measles. Or, to take another example, my gas gauge’s needle position is my evidence for the belief that I have enough gas in the tank to get home easily. In light of these examples, it would seem that ordinary observations of the world, along with measurements, constitute our basic evidence.
However, things are not altogether that easy. Consider the expression “information about the world” more closely. In what form do we access this information, and how do we process it? One might think that the observation of, say, Koplik spots is unproblematic. Looking at her patient’s gums, the physician simply observes the presence of those spots. But is that really the best way to describe the situation? That it might not be becomes clear when we imagine a person who has never heard of Koplik spots, or of the measles, and who has never seen those “spots” before. Clearly, to him, what he observes are some lesions or blisters on the gums, but not Koplik spots. A lot of theoretical knowledge goes into observing Koplik spots as opposed to observing mere discolored tissue. Pushing this line of thought even further, an extraterrestrial life form, for example, one who hasn’t seen gums, or humans, before, won’t even see spots of tissue discoloration. It might describe its observation as that of a reddish surface with small whitish regions, if indeed it has the concepts of “whitish” and “reddish.” What this seems to show is that what we directly observe are not things of a certain kind (such as human gums) carrying indicators of a disease (such as Koplik spots), but rather, what we directly observe are distributions of colors and edges in our visual field.
The same point is perhaps even more obvious in the example involving my gas gauge. Clearly, I need to know what the instrument is supposed to show before I can use the needle position as evidence for any hypotheses about the amount of gas left in my car, and thus as evidence for being able to get home safely. (Many of us have probably experienced confusion about a brand-new car’s instrumentation.) Furthermore, I already need to know what roles needles play in an instrument, and even what instruments in general are, viz., measuring devices of various kinds. None of this information can be gleaned directly from what we actually observe: distributions of colors, surfaces, and the like.
Considerations such as these have led many to propose that the ultimate, or basic, evidence for all of our hypotheses about the world are various distributions of colors, edges, sounds, and the like, which result from impacts of the world on our sense organs. As the American philosopher Willard van Orman Quine famously put it:
“[…] the only information that can reach our sensory surfaces from external objects must be limited to two-dimensional optical projections and various impacts of air waves and some gaseous reactions in the nasal passages and a few kindred odds and ends. How […] could one hope to find out about that external world from such meager traces?”
(Roots of Reference, 1974, 2)
Quine raises two issues here. First, he claims that our contact with the world is constituted by the impact of various forms of information, embodied in various physical systems, on our sense organs. Second, he rightly notices that it is somewhat surprising that we can have knowledge of the world on the basis of such “meager traces.” We won’t discuss the second issue now, which is related to the question of skepticism and realism, but relegate it to Chapter 12. Instead, we will briefly discuss the view that the ultimate evidence is to be found in distributions of various physical properties and/or magnitudes in one’s sensory field. The result of our discussion will be to dismiss this view as a fruitful account of scientific evidence. We will also see that the view possibly arises from equating, rightly or wrongly, evidential with causal relations.
2.2.2 Causes and Evidence
The main issue with the view that the ultimate evidence consists in features within a person’s sensory field is that it seems highly unnatural. Clearly, I don’t seem to observe edges and colors and only then form, by some unknown process, beliefs about my environment. I don’t seem to first see rectangularly shaped red patches supported by a shiny, gray surface
