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"Dances With Popper":
An Examination of
Dembski's Claims on Testability
By Wesley R. Elsberry
Posted January 2, 2005
Otto: Apes don't read philosophy.
Wanda: Yes, they do, Otto, they just don't understand it.
("A Fish Called Wanda")
In his new book, The Design Revolution, "intelligent design" advocate
William A. Dembski invokes the late philosopher Sir Karl Popper as an authority
on "testability" (ch. 39, pp.281-282). Perhaps Dembski has read Popper,
perhaps he hasn't. It's certain, though, that Dembski does not understand
Popper, and has a long history of not understanding Popper. Which is surprising,
because Popper was an extraordinarily accessible philosopher.
Dembski bases his chapter on "Testability" in The Design Revolution
(ch.39) on an essay he posted to the Internet in 2001. Between these two,
Dembski switches from the term "falsifiability" to "refutability" instead. This
is an odd thing for Dembski to do. It is explainable as a response to criticism
that I made of his use of "falsifiability" in 2001, as I showed then that
Dembski's use of "falsifiability" differed markedly from that of Popper, who
defined its usage in science and philosophy. The new version of Dembski's
argument shows a continuing misunderstanding of Popper and overlooks the
fundamental flaws in Dembski's argument.
Sir Karl Popper is justly famous as a philosopher of science. He proposed a
demarcation criterion that, in his view, made the distinction between
scientific theories and non-scientific conjectures. The basis of this criterion
was what Popper called falsifiability. It should be noted that Popper's
proposal of a demarcation criterion has not been generally accepted in more
recent treatments of philosophy of science. But the issue here is not over
whether Popper's falsifiability properly can be used as a demarcation
between science and non-science. What is at issue here is whether William
Dembski accurately conveys the concepts from Popper that Dembski
Popper's concern with testability focused on distinguishing between theories
that are empirically testable and those that aren't. This is the context
into which Popper introduced the concept of "falsifiability". "Falsifiability"
refers to a deductive method of testing a theory: derive an entailed proposition
from the theory that must be true if the theory is true, and attempt to
determine the truth or falsity of the entailed proposition from empirical data.
If the entailed proposition turns out to be false, one is justified in
considering the theory that generated it false. Popper was explicit that
"testability" and "refutability" meant the same thing as "falsifiability", if
they were to mean anything at all.
In order to be falsifiable, Popper asserted, a claim had to have the form of
a universal statement. Only universal claims are susceptible to the
application of modus tollens that underlies falsifiability. What about
conjectures that come in the form of existential statements instead?
Popper considered such "empirically irrefutable".
Let's examine what Popper said on these topics.
Some twenty five years ago I proposed to distinguish empirical or
scientific theories from non-empirical or non-scientific ones precisely by
defining the empirical theories as the refutable ones and the non-empirical
theories as the irrefutable ones. My reasons for this proposal were as
follows. Every serious test of a theory is an attempt to refute it.
Testability is therefore the same as refutability, or falsifiability. And
since we should call 'empirical' or 'scientific' only such theories as can be
empirically tested, we may conclude that it is the possibility of an empirical
refutation which distinguishes empirical or scientific theories.
If this 'criterion of refutability' is accepted, then we see at once that
philosophical theories, or metaphysical theories, will be
irrefutable by definition.
(Popper, 1985, p.214.)
And, of course, Popper held that strict or pure existential statements were
With empirical irrefutability the situation is a little different. The
simplest examples of empirically irrefutable statements are so-called strict
or pure existential statements. Here is an example of a strict or pure
existential statement: 'There exists a pearl which is ten times larger than
the next largest pearl.' If in this statement we restrict the words 'There
exists' to some finite region in space and time, then it may of course become
a refutable statement. For example, the following statement is obviously
empirically refutable: 'At this moment and in this box here there exist at
least two pearls one of which is ten times larger than the next largest pearl
in this box.' But then this statement is no longer a strict or pure
existential statement: rather it is a restricted existential statement.
A strict or pure existential statement applies to the whole universe, and it
is irrefutable simply because there can be no method by which it could be
refuted. For even if we were able to search our entire universe, the strict or
pure existential statement would not be refuted by our failure to
discover the required pearl, seeing that it might always be hiding in a place
where we are not looking.
(Popper, 1985, pp.212-213.)
Let's examine what Dembski says about "intelligent design" in light of
The fundamental claim of intelligent design is straightforward and easily
intelligible: namely, there exist natural systems that cannot be adequately
explained in terms of undirected natural causes and that exhibit features
which in any other circumstances we would attribute to intelligence.
(Dembski, 2004, p.45.)
Dembski delivers a clear "strict or pure existential statement" here. One
doesn't have to accept Popper's notion of falsifiability as a demarcation
criterion to recognize that Popper's argument for considering pure existential
statements as being empirically irrefutable is still sound. But nowhere within
Dembski's chapter on "testability" does Dembski confront and attempt to rebut
Popper's argument. The chapter reads as if Dembski were completely
unaware or ignorant of Popper's statements in this regard.
It is useful to point out the provenance of Dembski's chapter 39 on
"testability" in The Design Revolution. It is derived mostly from an
earlier essay posted to the Metanexus MetaViews email list and web site on
January 24th, 2001 and entitled, "Is Intelligent Design Testable?" (IIDT
hereafter for short. A copy is available at ARN.) Within this essay, one will note the absence of any
reference to or use of the term "refutability". What one does find is reference
Dembski in IIDT wrote:
In relation to science testability is a very broad notion. It certainly
includes Karl Popper's notion of falsifiability, but it is hardly coextensive
with it and can apply even if falsifiability does not obtain. Testability as
well covers confirmation, predicability, and explanatory power. At the heart
of testability is the idea that our scientific theories must make contact with
and be sensitive to what's happening in nature. What's happening in nature
must be able to affect our scientific theories not only in form and content
but also in the degree of credence we attach to or withhold from them. For a
theory to be immune to evidence from nature is a sure sign that we're not
dealing with a scientific theory.
What then are we to make of the testability of both intelligent design and
Darwinism taken not in a generic abstract sense but concretely? What are the
specific tests for intelligent design? What are the specific tests for
Darwinism? And how do the two theories compare in terms of testability? To
answer these questions, let's run through several aspects of testability,
beginning with falsifiability.
FALSIFIABILITY: Is intelligent design falsifiable? Is Darwinism
falsifiable? Yes to the first question, no to the second. Intelligent design
is eminently falsifiable. Specified complexity in general and irreducible
complexity in biology are within the theory of intelligent design the key
markers of intelligent agency. If it could be shown that biological systems
like the bacterial flagellum that are wonderfully complex, elegant, and
integrated could have been formed by a gradual Darwinian process (which by
definition is non-telic), then intelligent design would be falsified on the
general grounds that one doesn't invoke intelligent causes when purely natural
causes will do. In that case Occam's razor finishes off intelligent design
One will note that Dembski's deployment of "falsifiability" is unrecognizable
as any sort of usage that could be said to be derived from Popper. Demsbki does
not proceed from some "theory of intelligent design" and find a proposition that
is an entailed consequence and test its empirical validity, as Popper required
for his "falsifiability". Dembski asserts that an essentially unrelated
proposition, whether some phenomenon can be explained sufficiently well by
reference to a completely unrelated theory, somehow has implications for the
truth value of the conjecture of interest. This has no corresponding construct
in Popper's framework, perhaps for the simple reason that it is an obviously
invalid approach that Popper wouldn't have touched with a ten foot pole. (See
below for more.) It was this clearly erroneous deployment of "falsifiability"
that I strongly critiqued in my presentation on June 17th, 2001 at
the CTNS/AAAS "Interpreting Evolution" conference at Haverford College with
William Dembski and Michael Behe in attendance (see slides 23-25).
Now, by examination of Dembski's chapter on "testability" in The Design
Revolution, it appears that Dembski did get the message that his deployment
of "falsifiability" was flawed. But rather than fix the underlying problem,
Dembski chose simply to introduce another term with which to replace
"falsifiability" that he could redefine to suit his already existing text.
Unfortunately, Dembski again ties the new term of choice, "refutability",
to Sir Karl Popper. Here is Dembski's justification for invoking Popper on
The main point of Popper's criterion of falsifiability is not so much that
scientific claims must have the possibility of being demonstrably false as
that they must have the possibility of being eliminated as the result of new
evidence. To underscore this point Popper even wrote a book titled
Conjectures and Refutations. That is the point of refutability.
(Dembski, 2004, p.281.)
The aphorism about judging a book by its cover leaps to mind. Examination of
the book in question, though, leads to an understanding that Popper
treated testability and refutability as synonyms for
falsifiability (see pp. 37, 39, 197, 219, 256, and 258. See p. 279 for
discussion of Carnap, who makes a similar error to that of Dembski.). In other
words, the point of refutability is, according to Popper, quite unlike
what Dembski has represented in his book.
So much for invoking the authority of Popper as a prop for Dembski's version
of "refutability". But does Dembski's formulation have any merits of its own?
Let's have a look.
Refutability comes in degrees. Theories become more refutable to the degree
that new evidence could render them unacceptable. Note that refutability asks
to what degree theories could be refuted, not to what degree they actually
have been refuted. Thus refutability gauges how sensitive theories are to
refutation in principle rather than on the basis of any particular evidence.
The more sensitive to evidence generally, the more refutable the theory.
According to Popper, one mark of a good scientific theory is that it is highly
refutable in principle while consistently unrefuted by the evidence in
practice. Better yet are those theories on which scientists have expended
tremendous diligence to refute them, only to have their efforts come to
nothing. Within Popper's scheme of scientific rationality, theories are
corroborated to the degree that they resist refutation.
Let's now ask, Is intelligent design refutable? Is Darwinism refutable? Yes
to the first question, no to the second. Intelligent design could in principle
be readily refuted. Specified complexity in general and irreducible complexity
in biology are, within the theory of intelligent design, key markers of
intelligent agency. If it could be shown that biological systems that are
wonderfully complex, elegant and integrated — such as the bacterial flagellum
— could have been formed by a gradual Darwinian process (and thus that
their specified complexity is an illusion), then intelligent design would be
refuted on the general grounds that one does not invoke intelligent causes
when undirected natural causes will do. In that case Occam's razor would
finish off intelligent design quite nicely.
(Dembski, 2004, pp.281-282.)
Here Dembski's "refutability" runs head-on into Popper's argument concerning
the empirical irrefutability of strict or pure existential statements, such as
the fundamental claim of intelligent design quoted above. The result is
fatal for Dembski's "refutability" and the claims he makes for it. No
matter how many systems ID advocates assert might have specified
complexity or irreducible complexity and later have them overturned
by empirical inquiry finding that directed natural causes, such as natural
selection, are perfectly capable of explaining them, the ID advocates can always
propose yet another system as a candidate. (Dembski's phrasing of "undirected
natural causes" excludes natural selection, since natural selection is
constrained and thus guided by local environmental conditions and
factors like co-evolution. If Dembski wishes to redefine "guided" as "guided by
an intelligent agent", he needs to do so explicitly.) The cycle is endless, as
Popper quite clearly saw with his example of search for the
ten-times-larger-pearl. We already see the beginning of this, as ID advocates
used to be very keen on using the human blood clotting cascade as a model system
showing "intelligent design". Good responses to the ID claims on blood clotting
have made this system less tenable as an illustration, but ID advocates do not
thereby say that the "fundamental claim of intelligent design" is thereby to
that degree refuted. To the contrary, they simply have picked up and
moved on to another system to serve as a poster-type example, in this case the
flagellum of E. coli bacteria. We can already observe that the "sensitivity" of
"intelligent design" conjectures to empirical evidence appears to be "none
whatsoever". On Dembski's own criteria, as well as Popper's, "intelligent
design" is irrefutable.
As criticism of ID arguments about the E. coli flagellum accumulate, one can
see that ID responses are tending to insulate against empirical refutation. One
class of ID responses claims that the information needed to make flagella was
"front-loaded" into some ancestral strain of bacteria. Another is that the
"intelligent designer" acted at the quantum level to produce the flagellum. And
a third class of response requires video-camera certainty concerning every step
of proposed natural pathways to development of bacterial flagella. (It is useful
to note here that "intelligent design" advocates select examples where knowledge
concerning their historical origins is sketchy to non-existent. If "intelligent
design" were more than a "bare possibility", the ID advocates should be able to
use as examples biological systems whose historical origins are well-known, but
which remain unexplained by various evolutionary hypotheses or
mechanisms. Instead, whenever there is sufficient evidence of the origin of a
biological system, it uniformly is explained by some evolutionary
hypothesis or mechanism. By the sort of inductive process invoked by Dembski
elsewhere (e.g., Dembski, 2004, pp.95-96), "intelligent design" advocates should
concede that this will continue to be the case for all future examples.)
On a further note, Dembski's claim that "Darwinism" is irrefutable is clearly
a mistake. If one credits Dembski's formulation of "refutability", it is clear
that he has misapplied it in his haste to say something negative about
"Darwinism". Dembski's entire program of finding "specified complexity" in
biological systems is dependent upon his "generic chance elimination argument"
(GCEA) being able to consider — and eliminate — evolutionary hypotheses for the
origin of some event. If Darwinian hypotheses were actually
irrefutable, as Dembski claims, then his GCEA would get nowhere in considering
biological systems. Dembski cannot "have his cake and eat it, too" in this
instance, since "Polite society frowns on such obvious bad taste." (See also my essay on Huxley and the "typing monkeys" metaphor.)
As I have noted before elsewhere, falsifying tests for natural selection date
back to Darwin.
Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout nature
one species incessantly takes advantage of, and profits by, the structure of
another. But natural selection can and does often produce structures for the
direct injury of other species, as we see in the fang of the adder, and in the
ovipositor of the ichneumon, by which its eggs are deposited in the living
bodies of other insects. If it could be proved that any part of the structure
of any one species had been formed for the exclusive good of another species,
it would annihilate my theory, for such could not have been produced through
(Darwin, 1859, ch. 6)
Famously, Popper himself had a go at an opinion on the status of "Darwinism",
which he originally critiqued as being "almost tautological" and thus relegated
its status to that of a useful "metaphysical research program". Popper recanted
his earlier stance in an article published in Dialectica in 1978, saying
that natural selection could be formulated in a way that was far from
tautological and also testable. Dembski, predictably, also fails to learn this
lesson from reading Popper. Perhaps understanding of Popper will one day come to
Dembski. Until then, we'll know to check the original sources when Dembski makes
a claim about Popper.
Darwin, Charles R. 1859. On The Origin of Species by Means of Natural
Selection, or The Preservation of Favoured Races in the Struggle for Life,
First Edition. (http://www.talkorigins.org/faqs/origin.html, last accessed
Dembski, William A. 2001. "Is Intelligent Design Testable?" MetaViews. (http://www.arn.org/docs/dembski/wd_isidtestable.htm, last
Dembski, William A. 2004. The Design Revolution. Downer's Grove, IL:
Popper, Sir Karl. 1978. "Natural Selection and the Emergence of Mind,"
Popper, Sir Karl. 1985. "Metaphysics and criticizability." In: Popper
Selections, David Miller (ed.). Princeton, NJ: Princeton University Press.
Originally published in 1958.
Popper, Sir Karl. 1992. "Conjectures and Refutations." Routledge; 5th
Originally posted to The Panda's Thumb.