David Boersema
Pacific University
I
want to begin with two stories, as it were, though by “stories” I do not mean
they are fictional. Indeed, they are
not. First story: Stanford philosopher
John Perry tells of a time he once followed a trail of sugar on a supermarket
floor, pushing his cart down the aisle on one side of a tall counter and back
the aisle on the other, seeking the shopper with the torn sack to tell him he
was making a mess. With each trip
around the counter, the trail became thicker, but Perry seemed unable to catch
up with the offending shopper. Finally
it dawned on him that he was the person he was trying to catch.
Second story: In the late 1970s, Berkeley geologist
Walter Alvarez came across an unusual layer of clay near the town of Gubbio,
Italy. It was a layer of clay only a
few inches thick and lay stratigraphically at the boundary between two
recognized geological periods, the earlier Cretaceous and the later Tertiary, a
boundary known to geologists as the K-T boundary, formed approximately 65
million years ago. An analysis of this
clay boundary revealed that it contained a small amount (9.4 parts per billion)
of a rare inert metallic element, iridium.
As small as this amount was, it was 300 times the levels of iridium that
was expected, given the levels found on either side of the K-T boundary. As relatively high levels of iridium (up to
500 ppb) are known to exist in meteorites, an extra-terrestrial origin of the
Gubbio iridium level was hypothesized and, along with other data, this
information led in 1980 to the hypothesis that the mass extinction of life on
earth that occurred 65 million years ago was the result of bolide impact (i.e.,
of a meteorite or other celestial body striking the earth). Many geologists, skeptical of this impact
hypothesis, claimed that such a high iridium spike does not entail bolide
impact and that, in fact, it could be accounted for by terrestrial volcanic
activity. To support this claim,
Dartmouth geologist Charles Officer noted that the 1983 eruption of Mount
Kilauea in Hawaii resulted in an iridium deposit that was 11,500 times the
background concentration. Indeed, many
geologists have pointed to the K-T iridium spike (which is found at numerous
sites around the world) and to iridium spikes or “extended deposits” at other
stratigraphic points in the geologic column to bolster their contention that
the K-T mass extinction and other mass extinctions were the result of
terrestrial vulcanism not bolide impact.
What do these two stories have
to do with each other and what do they have to do with issues relating to
evidence? In particular, what do they
say about what makes a phenomenon evidence for a hypothesis and how does a
phenomenon become evidentiary? In
Perry’s sugar case, the phenomenon of the sugar trail was not taken as evidence
of any specific hypothesis beyond “Some local shopper has a torn bag of
sugar.” It was not taken as evidence
that one hypothesis, say:
H: Noseworthy’s bag of sugar
is torn
was more likely than a second
hypothesis, say:
H*: Snigglebottom’s bag of sugar is torn.
Rather, I take it, Perry saw
the phenomenon of the trail of sugar as evidence that someone’s bag of sugar
was torn and this motivated him to seek the offending shopper. Perry was taking a phenomenon as evidence in
what I take to be an exploratory
fashion, i.e., in a context of discovery.
In the iridium case, the phenomenon of the iridium spike
was offered as evidence to support one hypothesis, say:
H’: Bolide impact caused the K-T extinctions
over a different hypothesis:
H”: Terrestrial volcanic activity caused the K-T
extinctions.
Here, I take it, Alvarez saw
the phenomenon of the iridium spike (or relative iridium levels) as evidence to
justify acceptance of one hypothesis over another and, so, was taking a
phenomenon as evidence in what I take to be an explanatory fashion, i.e., a context of justification.
I raise this point of contexts of discovery and
justification because I believe that any philosophical account of evidence (or
scientific evidence) must be taken against the background of what we want an
account of evidence for. That is, why
do we want or need an account of the nature and function of evidence? What do we hope to accomplish by providing
such an account? These questions
prompted Peter Achinstein’s recent book, The
Book of Evidence (Oxford: Oxford University Press, 2001) and, indeed,
underlay what he notes as “the dean’s challenge.” That is, what problems – even “just conceptual” problems – will
be resolved or clarified by providing a philosophical account of evidence?
What is “the dean’s challenge”
that I just alluded to? According to
Achinstein, a former dean at his college (whom he described as “a scientist
with high intelligence but low boiling point”) chastised Achinstein with the
following remark: “Peter, you have never made a contribution of interest to
scientists.” Nonplussed, Achinstein
took this remark to be directed not at him personally, but at philosophers of
science generally. To meet the dean’s
implicit challenge to come up with something that would be of interest to
scientists, Achinstein wrote his book to provide “an empirical and robust”
account of scientific evidence. In this
paper, I will use concerns I have about Achinstein’s account in order to say
something of my own about evidence.
First, a few summary remarks about Achinstein. He distinguishes four kinds of evidence: (1)
subjective evidence, (2) epistemic situation evidence, (3) veridical evidence,
and (4) potential evidence. Subjective
evidence is relativized to a specific person or group and requires only that
someone believes a given hypothesis h
is true on the basis that some evidence e
is true. Epistemic situation evidence
is relativized to some type of epistemic situation. While this is not simply having certain “background information,”
it does have to do with knowing or having reason to believe that certain
propositions are true. Someone in a
given epistemic situation would be justified, for Achinstein, in believing h on the basis of e (though either h or e could in fact be false). Veridical evidence is non-relativized,
objective (i.e., e is evidence for h whether or not anyone knows or believe
it), and h is true. Finally, potential evidence, like veridical
evidence, is non-relativized to subjects or epistemic situations, and involves
good reason to believe h on the basis
of e, though that good reason to
believe is weaker than for veridical evidence.
Unlike veridical evidence, potential evidence does not require that h be true.
Now, one difficulty I have
with Achinstein’s account is that I believe it takes evidence only in contexts
of justification or explanation and bypasses much scientific practice and
activity, namely that which centers on exploration and discovery. I will not say more about this point right
now, but will come back to it at the end of this paper.
Restricting myself to the context of justification, the
iridium spike case mentioned above points to several questions. First, can the same phenomenon be supporting
evidence for different, and even conflicting, hypotheses? Both Alvarez (i.e., the supporters of the
impact hypothesis) and Officer (i.e., the supporters of the vulcanism
hypothesis) cite the iridium spike (or extended deposits) as supporting evidence
for their respective hypotheses.
Achinstein, I take it, would disagree, at least with respect to what he
calls “veridical evidence.” Since the
hypotheses conflict, they can not both be true, so, at least in terms of
veridical evidence, the iridium spike can not be evidence for both, any more
than, say, a particular solar eclipse could count as veridical evidence for
both Ptolemaic and Copernican planetary models. Yet each of Alvarez and Officer do cite the iridium spike as supporting evidence and this is not
simply to be shunted aside by saying that it counts as “epistemic-situation
evidence” as opposed to “veridical evidence,” since both parties are in the
same epistemic situation. They agree on
what phenomenon has occurred, that is, that there is a specific aberrant
iridium level at the K-T boundary, but they disagree on what explains this
phenomenon and on what it signifies.
In spite of the fact that scientists on both sides of this debate point
to the same phenomena as supporting evidence for their respective hypotheses,
Achinstein’s views appear to disallow this.
For him, phenomena cannot count as evidence, at least veridical
evidence, for a false hypothesis. I
take it that at least some scientific practice does not corroborate his
position.
Given Achinstein’s view, can
some phenomenon count as (veridical) evidence for multiple hypotheses? It cannot, he says, if the hypotheses are
incompatible. But, with respect to scientific practice, what is such
incompatibility? The relation among
multiple hypotheses for Achinstein cannot simply be that the hypotheses are
consistent (that is, that they do not contradict each other); otherwise
evidence for any true hypothesis could be evidence for any other true
hypothesis. What if they are simply
independent? Can the iridium spike at
the Gubbio K-T boundary serve as evidence that Lamarckian evolution is false,
or, for that matter, true? Given
Achinstein’s view, it is not clear (at least, not clear to me!). For Achinstein, a phenomenon can be veridical
evidence for a hypothesis even if it is not recognized as such. The interference patterns displayed by light
were veridical evidence of the wave theory of light independent of whether or
when it was recognized as such. We
certainly discover evidence, for
Achinstein, we do not create it (though we might well create the phenomenon
that provides us with the evidence, e.g., creating short-lived particles in
particle accelerators). Given this
view, again, can the iridium spike at the Gubbio K-T boundary serve as evidence
for the falsehood (or truth) of Lamarckian evolution? I do not see how it can, but I take it that for Achinstein, it
might, since although I see Lamarckian evolution as an independent hypothesis
of the impact hypothesis, both veridical evidence and the independence of
multiple hypotheses are not epistemic issues, but metaphysical ones.
Besides concerns about the possibility of evidence for
false hypotheses and multiple hypotheses, there is also a concern about
unlikely hypotheses, that is, hypotheses that are unlikely to be true given a
background. Unlikely phenomena occur;
they are unlikely, but they happen. If
what counts as evidence is, as Achinstein holds, that it makes the likelihood
of the truth of the hypothesis to be > 1/2, how do we account for hypotheses
that have a likelihood of < 1/2 yet turn out true? (Someone wins the lottery!)
Finally, it is not clear to me how Achinstein’s view
makes sense of the lack of evidence
and its relation to hypotheses. By
“lack of evidence,” I do not mean negative evidence, that is, phenomena that
contradict a given hypothesis or make it less likely to be true. Rather, I mean no supporting evidence being
found. In the Michelson-Morley
experiments, was the failure to detect a change in the interference patterns of
light evidence that there was no aether?
As we know, for some physicists at the time, no; for others, yes. Would the absence of an iridium spike at the
Gubbio K-T boundary be evidence that impact (or, for that matter, volcanic
activity) did not happen?
What has been the point of all of these questions and
concerns I have raised in connection with Achinstein’s claims about the nature
and function of evidence? It has not
been merely to quibble about particular issues. Rather, it is to say that I do not think he has actually
answered, or even adequately addressed, his dean’s challenge. I certainly do not take the job of
philosophers of science merely to be anthropologists of science, only describing
what they see happening. Philosophy, I
hope, is not the handmaiden of science any more than it is, or should be, the
handmaiden of anything else.
Nevertheless, how can a philosophical analysis of the nature and
function of evidence help scientists?
(This is, after all, what the dean wanted to know.) And this gets back to the question of what
do we want or need a philosophical account of evidence for? And this, for me at least, gets back to the
issues of discovery and justification.
The sort of analysis that philosophers can provide that might please the
dean would be relative to the varied cognitive interests that scientists have
in the various aspects of their investigations, e.g., in terms of process
(discovery) or results (justification).
But I do not see Achinstein’s analysis localizing itself to these varied
and different cognitive concerns. One
of those concerns, of course, is to justify, or explain, hypotheses. Related to the justification of hypotheses
are concerns about prediction and (presumably, via prediction and explanation)
control of phenomena. Clearly, we want
to know why things happen as they do and often we want to be able to manipulate
phenomena in order to make things happen.
But in addition, we take observations and measurements; we construct and
perform experiments, we frame models and test them; we propose theories and
refine or replace them. My point here
is the obvious one that scientists have many cognitive concerns other than
justifying hypotheses. One convenient,
and I believe useful, way of categorizing these varied concerns is to lump them
into the aforementioned explanatory concerns and exploratory concerns. How does the concept of evidence fit into
exploratory concerns? What role does
evidence have in, say, constructing models whose purpose is to generate rather
than test observations or measurements of experimental designs, etc.? I do not have a definitive answer or even a
spectacular one or, for that matter, probably not even a particularly fruitful
one. I suspect a good answer lies along
the lines of rekindling the old “logic of discovery” issue that Hanson and
Goodman engaged in decades ago (not that I am at all advocating their views on
this). Perhaps another way of
formulating the question, and this is why I related the John Perry sugar incident
at the beginning, is: what counts as evidence, or good reason, to pursue some
line of scientific inquiry (or, say, good reason to propose some model)? Again, I have no definitive answer, but I am
inclined to say that it has to do with the informativeness or surprise of this
phenomenon relative to background expectations, not that it made the likelihood
of a given hypothesis higher. That is
to say, we do not normally expect to see trails of sugar on supermarket floors,
so when we do encounter such a phenomenon, we have good reason to
investigate. Likewise, scientists had
not expected to find an iridium spike at the K-T boundary and, so, had good
reason to investigate. In fact, the
Alvarez team was looking for fossils to see if there had been “microplate
rotation” in the Apennine Mountains in Italy (that is, the rotation of a very
small tectonic plate beneath Italy, where the larger Eurasian and African
Plates converge). Surprising phenomena,
then, it strikes me, are the salient feature of exploratory evidence. The two cases just mentioned (Perry’s sugar
trail and the iridium spike) are cases in which the surprise has to do with
encountering a phenomenon, or perhaps a pattern of phenomena, where it was not
expected. Other surprising cases can be
where a phenomenon, or pattern of phenomena, are expected and are not
encountered (say, when we go to a grocery store and find, not trails of sugar
on the floor, but no bags of sugar for sale, or when we find light acting like
a particle when we fully expect it to behave like a wave). Rather than frame “good reason to
investigate” in terms of surprise, it is probably more fruitful to frame it in
terms of information (or informativeness) relative to background knowledge,
much as some philosophers of science have done vis-à-vis models of
explanation. This approach will, I
think, also be fruitful in elucidating when and how a phenomenon becomes
evidence or evidentiary. That is, we do
not take every phenomenon to be evidentiary, either for exploratory purposes or
explanatory purposes. There are, after
all, an indeterminate number of phenomena (and varied ways of categorizing
them), only a select subset of which we take either as evidence for some
hypothesis or as evidence that some hypothesis is worth pursuing. Those that we do take as evidentiary (i.e.,
those that we do have good reason to believe or good reason to investigate) are
ones that are informative, at least potentially. Accounting for the concept of evidence in terms of
informativeness might not meet Achinstein’s dean’s challenge, but I think it is
a fruitful approach.
Notes
Earlier versions of this paper were
presented at the Oregon Academy of Sciences, February 2004 and at the 56th
annual Northwest Philosophy Conference, October 2004.
John Perry, The Problem of the Essential Indexical, Oxford: Oxford University
Press, 1993.
Walter Alvarez, T-Rex and the Crater of Doom, Princeton: Princeton University
Press, 1997.
Charles Officer, The Great Dinosaur Extinction Controversy, New York: Addison-Wesley,
1996.
Norwood Russell Hanson, Patterns of Discovery, Cambridge:
Harvard University Press, 1958.
Nelson
Goodman, Fact, Fiction, and Forecast,
Indianapolis: Bobbs-Merrill, 1955.