The Scientific Status of Design Inferences
by Bruce L. Gordon, Ph.D.
History and Philosophy of Physics
Baylor University
Scientific practice assumes that the universe, in both its origin and
function, is a closed system of undirected physical processes. While many
scientists reject this assumption as the ultimate truth, they still think that
it is essential for science to function as if it were true.
This means that they have accepted methodological naturalism as a
necessary constraint on their practice as scientists. Methodological naturalism
is the doctrine that in order to be scientific, an explanation must be
naturalistic, that is, it must only appeal to entities, causes,
events, and processes contained within the material universe. Even if we grant
that this restriction on permissible explanations has been a fruitful strategy
for science, we must still ask whether it is methodologically required by
science. Arbitrarily rejecting methodological naturalism may be unwise as an
explanatory strategy within science. But perhaps there is a perfectly rigorous
method for ascertaining when such restrictions cannot be
applied if a correct explanation for something is to be given. Would such a
principled decision, subject to a strict and objective methodology, not also
conform to the canons of scientific explanation?
A number of philosophers of science have attempted to give an account of
what it means to offer a scientific explanation for a phenomenon. We briefly
consider three such accounts: the deductive-nomological model, the
causal-statistical (statistical-relevance) model, and the
pragmatic model.
The deductive-nomological (D-N) model was the earliest
model of scientific explanation and has been very influential. It postulates
four criteria for scientific explanations:
1. The explanation must be able to
be put in the form of a valid deductive argument, with the thing to be
explained as its conclusion.
2. The explanation must contain at least one general law that is required
for the derivation of this conclusion.
3. The explanation must have empirical content that can be tested.
4. The premises of the argument constituting the explanation must be
true.
Subsequently, it became clear that the D-N model had irremediable
shortcomings falling into two categories: (a) there are arguments meeting the
criteria of the D-N model that fail to be genuine scientific explanations; and
(b) there are genuine scientific explanations that fail to meet the criteria of
the D-N model. In short, these four criteria are neither sufficient nor
necessary to guarantee that an explanation is scientific. To see this, I offer
two standard counterexamples: the man and the pill, and the explanation of
paresis.
That the D-N model is insufficient as an account of scientific
explanation can be illustrated by this humorous counterexample. A man explains
his failure to become pregnant over the last year, despite an amorous
relationship with his wife, on the ground that he has regularly consumed her
birth control pills. He appeals to the law-like generalization that every man
who regularly takes oral contraceptives will not get pregnant. This example
conforms to the D-N pattern of explanation. The problem is that his use of
birth control is irrelevant because men do not get pregnant. So it is possible
to construct valid arguments with true premises in which some fact asserted by
the premises is irrelevant to the real explanation of the phenomenon
in question.
To see that the model does not provide conditions that are
necessary for a proper scientific explanation, consider the
explanation for the development of paresis (a form of tertiary syphilis
characterized by progressive physical paralysis and loss of mental function).
In order to develop paresis, it is necessary to have untreated latent syphilis,
but only about twenty-five percent of the people in this situation ever develop
it. So we have a necessary condition for the development of the disease, but we
cannot use this to derive the conclusion that paresis will develop in an
individual case, or even to predict that it will. In fact, we're better off
predicting that it will not develop, since it doesn't in seventy-five percent
of the cases. Even so, the proper scientific explanation for paresis is that it
results from untreated latent syphilis. This is just one example of a genuinely
scientific explanation that does not conform to the D-N model.
To remedy the defects of the D-N model of scientific explanation, the
causal-statistical or statistical-relevance model was
proposed. Advocates of this model stress the role of causal components
in scientific explanations and generally deny that explaining something
scientifically must involve rigorous deductive or inductive arguments.
Because they recognize that there are rational explanations for
unexpected events (like the onset of paresis after untreated latent
syphilis), they reject the idea that universal or statistical laws and
empirical facts must provide conditions of adequacy for a scientific
explanation of the occurrence of events.
The positive idea behind the causal-statistical model is that a scientific
explanation presents two things: (1) the set of factors statistically relevant
to the occurrence of that event; and (2) the causal framework or link
connecting those factors with the event to be explained. Statistical
relevance may be defined as follows: factor B is statistically
relevant to factor A if and only if the probability of A, given that B has
already occurred, is different from the probability of A occurring on its own,
that is, P(A|B) ? P(A). The causal network or link connecting the factors with
an event is simply an account of the underlying causal processes and
interactions that bring it about. A causal process is a continuous
spatio-temporal process; a causal interaction is a relatively brief
event in which two or more causal processes intersect. The causal-statistical
theory arose from the conviction that legitimate scientific explanations have
to explain events in terms of the things that actually caused them to
happen.
While the causal-statistical model seems fairly solid, it nonetheless finds
a counter-example in quantum mechanics, the theory describing the
behavior of atomic and subatomic particles. The details of why the model fails
for quantum mechanics are complicated. Roughly, the causal-statistical account
appeals to processes that are deterministic and continuous in space-time, while
it is generally accepted that quantum mechanics is not consistent with this
view of the world. Since quantum mechanics is regarded as one of the triumphs
of twentieth-century science, we have a reason for thinking that this model of
explanation is too narrow. Of course, we also have the option of
saying that quantum mechanics provides us with a mathematical
description of quantum phenomena that enables amazingly accurate
predictions, but it does not explain these phenomena at all - a
complete explanation would get to the root cause of experimental
outcomes, not just predict them.
The shortcomings of the D-N and causal-statistical models led to a third
proposal for scientific explanations, the pragmatic model. This
approach not only denies that scientific explanations have a characteristic
form (as in the D-N model), but also denies that they supply distinctive
information (as in the causal-statistical model) outside of that provided by
the theories, facts, and procedures of science itself. Calling an explanation
"scientific" means nothing more than saying it draws on what gets recognized as
science to provide an explanation, and whether this criterion is satisfied is
something determined by the community of scientists themselves. Beyond this,
the pragmatic theory is highly contextual.
Bas van Fraassen, the originator of the pragmatic theory, maintains that a
scientific explanation is a telling response to a why-question that is
identifiable by its topics of concern, contrast classes, and
explanatory relevance conditions. An explanation is a telling
response simply if it favors the occurrence of the state of affairs to be
explained. The topic of concern is the thing to be explained. The
contrast class is the set of alternative possibilities, of which the
topic of concern is a member, for which an explanation might be requested in a
particular context. The explanatory relevance conditions are the
respects in which an answer might be given. For example, to borrow one
of van Fraassen's illustrations, our topic of concern might be why an
electrical conductor is warped. In this case, the contrast class might consist
of other nearby conductors that are not warped, the warping of the conductor as
opposed to its retaining its original shape, etc. The explanatory relevance
conditions might be the presence of a particularly strong magnetic field, the
presence of moisture on the conductor, and so on. All of these things are
highly dependent on context.
The pragmatic theory is relatively simple and direct in comparison with the
other two models. It also is capable of accommodating the special aspects of
the other two theories of explanation, and it has a very broad range of
application. Critics of the pragmatic model have questioned whether every
why-question asked by a scientist requires a contrast class, whether scientific
questions might sometimes involve explanations of how as well as why (for
example, the question of how genes replicate), whether a telling response must
always favor the topic of concern, and whether the theory is too broad and
would legitimize as scientific those explanations that the community of
scientists might wish to exclude (though actual acceptance by the scientific
community seems to be built into the criteria of legitimacy in this case).
Notice that none of the foregoing theories of scientific explanation make
mention of methodological naturalism as a constraint (though it is perhaps
implicit in the definition of a causal process used by the causal-statistical
approach). Some philosophers of science would say this absence points to its
status as a grounding assumption for any theory of scientific explanation;
others would maintain that this absence shows it is not an essential part of
scientific explanation and its relevance as a condition is context-dependent. A
brief consideration of the role that a rigorous theory of design inferences can
play in science reveals the latter attitude to be most reasonable.
As William Dembski points out, drawing design inferences is already an
essential and uncontroversial part of various scientific activities ranging
from the detection of fabricated experimental data, to forensic science,
cryptography, and even the search for extra-terrestrial intelligence (SETI). He
identifies two criteria as necessary and sufficient for inferring intelligence
or design: complexity and specification. Complexity ensures that the
event in question is not so simple that it can readily be explained by chance.
It is an essentially probabilistic concept. Specification ensures that
the event in question exhibits the trademarks of intelligence. The notion of
specification amounts to this: if, independently of the small probability of
the event in question, we are somehow able to circumscribe and define it so as
to render its reconstruction tractable, then we are justified in eliminating
chance as the proper explanation for the event. Dembski calls such an event one
of specified small probability.
If an event of small probability fails to satisfy the specification
criterion, it is still attributable to chance, as is the case, for example,
with any sequence of heads and tails produced by 1,000 tosses of a fair coin.
But if an event is genuinely one of specified small probability, then
the proper conclusion is that the cause of that event is intelligent agency. A
brief example will suffice to clarify the notion. Suppose that a bank vault
lock has a quadrillion possible combinations. Each of the quadrillion possible
combinations is equally improbable, yet one of them in fact opens the lock. The
actual combination that opens the vault is an event of specified small
probability. If a person given one chance to open the vault succeeds in doing
so, then the proper conclusion is that he opened the vault by design, namely by
having prior knowledge of the right combination. One of Dembski's important
contributions has been to render the notion of specification mathematically
rigorous in a way that places design inferences on a solid foundation.
The mathematical analysis used to determine whether an event is one of
specified small probability rests on empirical observations set in the context
of the theoretical models used to study the domain (quantum-theoretic,
molecular biological, developmental biological, cosmological, etc.) under
investigation, but the design inference itself can be formulated as a valid
deductive argument. One of its premises is a mathematical result that Dembski
calls the law of small probability. That the design inference lends
itself to this precision of expression is significant because it enables us to
see that a rigorous approach to design inferences conforms to even the most
restrictive theory of scientific explanation, the D-N model. In fact, even
though the accounts of scientific explanation we considered were inadequate as
universal theories, all three of them captured important intuitions.
Furthermore, it is short work to see that rigorous design inferences satisfy
the conditions imposed by all of them.
Design inferences conform to the requirements of a deductive-nomological
explanation because it satisfies all four criteria of this explanatory
model.
1) The explanation it offers can be
put in the form of a deductive argument.
2) It contains at least one general law (the law of small probability),
and this law is required for the derivation of the thing to be explained (in
this case the nature of the cause of the event in question).
3) It has empirical content because it depends on both the observation of
the event and the empirical facts relevant to determining the objective
probability of its occurrence.
4) The sentences constituting the explanation are true to the best of our
knowledge, because they take into account all of the relevant factors in
principle available to us prior to the event we are seeking to explain.
Design inferences also satisfy the requirements of the causal-statistical
model of explanation by isolating the factors statistically relevant to the
explanation of the event under investigation. This is accomplished by
determining that the event in question is one of small probability and by
ensuring that the criterion of specifiability is satisfied, thereby eliminating
natural law and chance as possible explanations. It also makes manifest the
causal network undergirding this statistical regularity, since it causally
connects the relevant explanatory factor (intelligent agency) to the occurrence
of the event (though not necessarily by means of mechanism).
Finally, design inferences satisfy the pragmatic model of explanation
because they provide telling answers to why questions, where those questions
are identifiable by their topics of concern, their contrast classes, and their
explanatory relevance conditions. The topic of concern in a design inference is
the observed occurrence of an improbable event that bears prima facie
evidence of specification. The contrast class is constituted by the set of
alternatives of which the topic of concern is a member. For example, the
contrast class might include the occurrence of other more probable events in
the causal context under consideration, or equally improbable events in that
context that bear no evidence of specification, etc. The explanatory relevance
conditions might be the presence of highly particular initial conditions in the
physical system, indications of thermodynamic counter-flow, the presence of
apparently intelligent informational content, and so on. All of these things
are dependent upon the context, but what is sought is a correct account of the
cause of the event in question. The response provided by the design inference
is therefore a telling one by the standards of the pragmatic model, because
when an event of demonstrably specified small probability occurs, this state of
affairs is favored by design-theoretic explanations.
Since design inferences satisfy all three models of scientific explanation,
there seems little reason to bar their legitimacy as a mode of scientific
explanation. Indeed, when generating scientific conclusions in cryptography or
forensics, the design inference is not controversial. The sticking point
centers on the issue of methodological naturalism. What happens if
design-theoretic analysis, when applied to certain natural phenomena, yields
the conclusion that these phenomena are the result of intelligent design? And
what if this state of affairs implies that there is an intelligent cause that
transcends our universe? Nothing but the unacknowledged operation of a
questionable double standard would bar using design-theoretic tools in this
context when their employment would be uncontroversial if no such implications
were in view. So can design inferences, when applied to nature, be a
form of scientific explanation? Considered without prejudice, this question
requires an affirmative answer.