Darwin's Dangerous Idea (19 page)

man, who flies faster than the speed of light, is
logically
possible, but Duperman, who flies faster than the speed of light
without moving anywhere,
CHAPTER FIVE

is not even logically possible. Superman, however, is not
physically
possible, since a law of physics proclaims that nothing can move faster than the speed
The Possible and the

of light. There is no dearth of difficulties with this superficially straightforward distinction. How do we distinguish fundamental physical laws from logical laws? Is it physically or logically impossible to travel
Actual

backwards in time, for instance? How could we tell for sure whether a description that is
apparently
coherent—such as the story in the film
Back to
the Future
—is subtly self-contradictory or merely denies a very fundamental (but not logically necessary ) assumption of physics? There is also no dearth of philosophy dealing with these difficulties, so we will just acknowledge them and pass on to the next grade.

Superman flies by simply leaping into the air and striking a gallant midair pose, a talent which is certainly physically impossible. Is a flying horse physically possible? The standard model from mythology would never get off the ground—a fact from physics (aerodynamics), not biology—but a 1. GRADES OF POSSIBILITY?

horse with suitable wingspan could presumably stay aloft. It might have to be a tiny horse, something aeronautical engineers might calculate from
However many ways there may be of being alive, it is certain that there
considerations of weight-strength ratios, the density of air, and so forth. But
are vastly more ways of being dead, or rather not alive.

now we are descending into the third grade of possibility,
biological possibility,
for once we begin considering the strength of bones, and the pay-

—RICHARD DAWKINS 1986A, P. 9

load requirements for keeping the flapping machinery going, we concern
Any particular non-existent form of life may owe its absence to one of
ourselves with development and growth, metabolism, and other clearly
two reasons. One is negative selection. The other is that the necessary
biological phenomena. Still, the verdict may appear to be that of course
mutations have never appeared.

flying horses are biologically possible, since bats are actual. Maybe even full-sized flying horses are possible, since there once were pteranodons and

—MARK RIDLEY 1985, P. 56

other flying creatures approaching that size. There is nothing to beat actuality, present or past, for clinching possibility. Whatever is or has been actual
Take, for instance, the possible fat man in that doorway; and, again, the
is obviously possible. Or is it?

possible bald man in diat doorway. Are they the same possible man, or
The lessons of actuality are hard to read. Could such flying horses really
two possible men? How do we decide? How many possible men are
be viable? Would they perhaps need to be carnivorous to store enough
there in mat doorway? Are there more possible thin ones than fat ones?

energy and carry it aloft? Perhaps—in spite of fruit-eating bats—only a
How many of them are alike? Or would their being alike make them
carnivorous horse could get off the ground. Is a carnivorous horse possible?

one? Are no
two
possible things alike? Is this the same as saying that it
Perhaps a carnivorous horse would be biologically possible if
it could evolve,
is impossible for
two
things to be alike? Or, finally, is the concept of
but would such a diet shift be accessible from where horses would have to
identity simply inapplicable to unactualized possibles?

start? And, short of radical constructive surgery, could a horse-descendant

—WILLARD VAN ORMAN QLINE 1953, P. 4

have both front legs and wings? Bats, after all, make wings of their arms. Is there any possible evolutionary history of skeletal revision that would yield a There seem to be at least four different kinds or grades of possibility: six-limbed mammal?

logical, physical, biological, and historical, nested in that order. The most This brings us to our fourth grade of possibility,
historical possibility.

lenient is mere logical possibility, which according to philosophical tradition There might have been a time, in the very distant past, when the possibility is simply a matter of being describable without contradiction. Super-of six-limbed mammals on Earth had not yet been foreclosed, but it might

106 THE POSSIBLE AND THE ACTUAL

The Library of Mendel
107

also be true that once our four-finned fishy ancestors got selected for moving onto the land, the basic four-limbed architecture was so deeply anchored in our developmental routines that alteration at this time is
no longer possible.

But even that distinction may not be sharp-edged. Is such an alteration in fundamental building-plan flat impossible, or just highly unlikely, so resistant to change that only an astronomically improbable sequence of selective blows could drive it into existence? It seems there might be two kinds or grades of biological impossibility: violation of a biological
law of
nature
(if there are any), and "mere" biohistorical consignment to oblivion.

Historical impossibility is simply a matter of opportunities passed up.

There was a time when many of us worried about the possibility of President Barry Goldwater, but it didn't happen, and after 1964, the odds against such a thing's ever happening lengthened reassuringly. When lottery tickets are put on sale, this creates an opportunity for you: you may choose to buy one, provided you act by a certain date. If you buy one, this creates a further opportunity for you—the opportunity to win—but soon it slides into the past, and it is no longer possible for you to win
those
millions of dollars. Is this everyday vision we have of opportunities—
real
opportunities—an illusion?

FIGURE 5.1

In what sense
could
you have won? Does it make a difference if the winning lottery number is chosen
after
you buy your ticket, or do you still have an biological possibility, and then perhaps it will suggest some payoffs for how opportunity to win, a real opportunity, if the winning number is sealed in a to make sense of the grander varieties.1

vault before the tickets are put on sale (Dennett 1984)? Is there
ever
really any opportunity at all? Could anything happen other than what actually happens? This dread hypothesis, the idea that
only
the actual is possible, has 2. T

been called
actualism
(Ayers 1968). It is generally ignored, for good reasons, HE LIBRARY OF MENDEL

but these reasons are seldom discussed. (Dennett 1984, and Lewis 1986, pp.

The Argentine poet Jorge Luis Borges is not typically classified as a philos-36-38, offer good reasons for dismissing actualism.) opher, but in his short stories he has given philosophy some of its most These familiar and
prima facie
reliable ideas about possibility can be valuable thought experiments, most of them gathered in the stunning col-summed up in a diagram, but every boundary in it is embattled. As Quine's lection
Labyrinths
(1962). Among the best is the fantasy—actually, it is questions suggest, there is something fishy about casual catalogues of merely more a philosophical reflection than a narrative—that describes the Library possible objects, but since science cannot even express—let alone confirm—

of Babel. For us, the Library of Babel will be an anchoring vision for helping the sorts of explanations we crave without drawing such a distinction, there is to answer very difficult questions about the scope of biological possibility, so little chance that we can simply renounce all such talk. When biologists we will pause to explore it at some length. Borges tells of the forlorn wonder whether a horned bird—or even a giraffe with stripes instead of explorations and speculations of some people who find themselves living in blotches—is possible, the questions they are addressing epitomize what we want biology to discover for us. Alerted by Quine, we can be struck by the dubious metaphysical implications of Richard Dawkins' vivid claim that there are many more ways of being dead than of being alive, but manifestly 1. Back in 1982, Francois Jacob, the Nobel laureate biologist, published a book entitled
The Possible and the Actual,
and I rushed to read it, expecting it to be an eye-opening he is getting at something important. We should try to find a way of recasting essay on how biologists should think about some of these conundrums about possibility.

such claims in a metaphysically more modest and less contentious To my disappointment, the book had very little to say on this topic. It is a fine book, and framework—and Darwin's starting in the middle gives us just the foothold has a great title, but the two don't go together, in my humble opinion. The book I was we need.
First we
can deal with the relation between historical and eager to read hasn't yet been written, apparently, so I'll have to try to write part of it myself, in this chapter.

108 THE POSSIBLE AND THE ACTUAL

The Library of Mendel
109

a vast storehouse of books, structured like a honeycomb, composed of beginning in one volume and continuing without a break in some other thousands (or millions or billions) of hexagonal air shafts surrounded by volume or volumes.

balconies lined with shelves. Standing at a railing and looking up or down, It is amusing to think about some of the volumes that must be in the one sees no top or bottom to these shafts. Nobody has ever found a shaft that Library of Babel somewhere. One of them is the best, most accurate 500-isn't surrounded by six neighboring shafts. They wonder: is the warehouse page biography of you, from the moment of your birth until the moment of infinite? Eventually, they decide that it is not, but it might as well be, for it your death. Locating it, however, would be all but impossible (that slippery seems that on its shelves—in no order, alas—lie
all the possible books.

word), since the Library also contains kazillions of volumes that are mag-Suppose that each book is 500 pages long, and each page consists of 40

nificently accurate biographies of you up till your tenth, twentieth, thirtieth, lines of 50 spaces, so there are two thousand character-spaces per page. Each fortieth ... birthday, and completely false about subsequent events of your space either is blank, or has a character printed on it, chosen from a set of life—in a kazillion different and diverting ways. But even finding one read-100 (the upper- and lowercase letters of English and other European able volume in this huge storehouse is unlikely in the extreme.

languages, plus the blank and punctuation marks).2 Somewhere in the Li-We need some terms for the quantities involved. The Library of Babel is brary of Babel is a volume consisting entirely of blank pages, and another not infinite, so the chance of finding anything interesting in it is not literally volume is all question marks, but the vast majority consist of typographical infinitesimal.4 These words exaggerate in a familiar way—we caught Darwin gibberish; no rules of spelling or grammar, to say nothing of sense, prohibit doing it in his summary, where he helped himself to an illicit "infinitely"—

the inclusion of a volume. Five hundred pages times 2,000 characters per but we should avoid them. Unfortunately, all the standard metaphors—

page gives 1,000,000 character-spaces per book, so there are 1001,000,000 books

"astronomically large," "a needle in a haystack," "a drop in the ocean"—fall in the Library of Babel. Since it is estimated3 that there are only 10040 (give or comically short. No
actual
astronomical quantity (such as the number of take a
few) particles
(protons, neutrons, and electrons) in the region of the elementary particles in the universe, or the time since the Big Bang measured universe we can observe, the Library of Babel is not remotely a physically in nanoseconds) is even visible against the backdrop of these huge but finite possible object, but, thanks to the strict rules with which Borges constructed numbers. If a readable volume in the Library were as easy to find as a it in his imagination, we can think about it clearly.

particular drop in the ocean, we'd be in business! If you were dropped at Is this truly the set of
all
possible books? Obviously not—since they are random into the Library, your chance of ever encountering a volume with so restricted to being printed from "only" 100 different characters, excluding, much as a grammatical sentence in it would be so vanishingly small that we we may suppose, the characters of Greek, Russian, Chinese, Japanese, and might do well to capitalize the term—"Vanishingly" small—and give it a Arabic, thereby overlooking many of the most important
actual
books. Of mate, "Vastly," short for "Very-much-more-than-astronomically."5