Darwin's Dangerous Idea - Evolution and the Meaning of Life

120 THE POSSIBLE AND THE ACTUAL Possibility Naturalized 121
bility, whatever it is. Some other philosophers have defended views of
comparative possibility (see especially Lewis 1986, pp. 10ff.), but I don't
want to fight over it. If this is not an account of possibility, so be it. It is, then,
a proposed replacement for a definition of possibility. Perhaps after all we
don't need the concept of biological possibility (with its required all-ornothing
application) for any serious investigative purpose. Perhaps degree of
accessibility in the space of the Library of Mendel is all we need, and is in
fact a better concept than any all-or-nothing version could be. It would be
nice, for instance, to have some way of ranking the following in terms of
biological possibility: ten-pound tomatoes, aquatic dogs, flying horses, flying
trees.
That will not be enough to satisfy many philosophers, and their objections
are serious. Briefly considering them will at least make it clearer what I am
claiming and what I am not claiming. First of all, isn't there something
viciously circular about defining possibility in terms of accessibility?
(Doesn't the latter term just reintroduce the former in its suffix, and still
undefined?) Well, not quite. It does leave some definitely unfinished business,
which I will simply acknowledge before moving on. We have supposed
that we are holding some concept or other of physical possibility clamped for
the time being; our idea of accessibility presupposes that this physical
possibility, whatever it is, leaves us some elbow room—some openness of
pathways (not just a single pathway) in the space. In other words, we are
taking on the assumption that nothing stops us from going down any of the
pathways that are open so far as physics is concerned. 11
Quine's questions (at the head of this chapter) invited us to worry about
11. This idea of elbow room is something we need to presuppose in any case, for it is the
minimal denial of actualism, the doctrine that only the actual is possible. David Hume, in
A Treatise of Human Nature (1739), spoke of "a certain looseness" we want to exist in
our world. This is the looseness that prevents the possible from shrinking tightly around
the actual. This looseness is presupposed by any use of the word "can"—a word we can
hardly do without! Some people have thought that, if determinism were true, actualism
would be true—or, to turn it around, if actualism is false, indeterminism must be true—
but this is highly dubious. The implied argument against determinism would be disconcertingly
simple: this oxygen atom has valence 2; therefore, it can unite with two
hydrogen atoms to form a molecule of water (it can right now, whether or not it does);
therefore, something is possible that isn't actual, so determinism is false. There are
impressive arguments from physics that lead to the conclusion that determinism is false—
but this isn't one of them. I am prepared to assume that actualism is false (and that this
assumption is independent of the determinism/indeterminism question), even if I can't
claim to prove it, if only because the alternative would be to give up and go play golf or
something. But for a somewhat fuller discussion of actualism, see my book Elbow Room
( 1984 ), especially ch. 6, "Could Have Done Otherwise," from which material in this note
is drawn. See also David Lewis' (1986, ch. 17 ) concurring opinion, about the related issue
of the irrelevance of the issue of indeterminism to our sense that the future is "open."
whether we could count nonactual possible objects. One of the virtues of the
proposed treatment of biological possibility is that, thanks to its "arbitrary"
formal system—the system arbitrarily imposed on us by nature, at least in
our neck of the woods—we can count the different nonactual possible
genomes; they are Vast but finite in number, and no two are exactly alike.
(By definition, genomes are distinct if they fail to share a nucleotide at any
one of several billion loci.) In what sense are the nonactual genomes really
possible? Only in this sense: if they were formed, they'd be stable. But
whether or not any conspiracy of events could lead to their being formed is
another matter, to be addressed in terms of accessibility from one location or
another. Most of the genomes in this set of stable possibilities will never be
formed, we can be sure, since the heat death of the universe will overtake the
building process before it has made a sizable dent in the space.
Two other objections to this proposal about biological possibility cry out to
be heard. First, isn't it outrageously "gene-centered," in anchoring all
considerations of biological possibility to the accessibility of one genome or
another in the Library of Mendel? Our proposed treatment of biological
possibility flatly ignores (and hence implicitly rules impossible) "creatures"
that are not end points of some branch of the Tree of Life that has already
taken us as far as we are today. But that just is the grand unification of
biology that Darwin discovered! Unless you harbor fantasies about spontaneous
creation of new life forms by "Special Creation" or (the philosophers'
secular version) "Cosmic Coincidence," you accept that every feature of the
biosphere is one fruit or another of the Tree of Life (or, if not our Tree of
Life, some other Tree of Life, with its own accessibility relations). No man is
an island, John Donne proclaims, and Charles Darwin adds that neither is any
clam or tulip—every possible living thing is connected by isthmuses of
descent to all other living things. Notice that this doctrine rules in whatever
marvels technology can produce in the future, provided—as we have already
noted—that technologists themselves, and their tools and methods, are firmly
located on the Tree of Life. It is a small further step to rule in life forms from
outer space, provided they, too, are the products of a Tree of Life rooted, as
ours is, in some nonmiraculous physical ground. (This topic will be explored
in chapter 7.)
Second, why should we treat biological possibility so differently from
physical possibility? If we assume that "laws of physics" fix the limits of
physical possibility, why shouldn't we attempt to define biological possibility
in terms of "laws of biology"? ( We will turn to an examination of physical
laws and physical necessity in chapter 7, but in the meantime, the difference
appears large.) Many biologists and philosophers of science have maintained
that there are biological laws. Doesn't the proposed definition rule them out?
Or does it declare them superfluous? It doesn't rule them out. It permits
someone to argue for the dominion of some law of biology over the