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

Grades of Possibility? 105
CHAPTER FIVE
The Possible and the
Actual
1. GRADES OF POSSIBILITY?
However many ways there may be of being alive, it is certain that there
are vastly more ways of being dead, or rather not alive.
—RICHARD DAWKINS 1986A, P. 9
Any particular non-existent form of life may owe its absence to one of
two reasons. One is negative selection. The other is that the necessary
mutations have never appeared.
—MARK RIDLEY 1985, P. 56
Take, for instance, the possible fat man in that doorway; and, again, the
possible bald man in diat doorway. Are they the same possible man, or
two possible men? How do we decide? How many possible men are
there in mat doorway? Are there more possible thin ones than fat ones?
How many of them are alike? Or would their being alike make them
one? Are no two possible things alike? Is this the same as saying that it
is impossible for two things to be alike? Or, finally, is the concept of
identity simply inapplicable to unactualized possibles?
—WILLARD VAN ORMAN QLINE 1953, P. 4
There seem to be at least four different kinds or grades of possibility:
logical, physical, biological, and historical, nested in that order. The most
lenient is mere logical possibility, which according to philosophical tradition
is simply a matter of being describable without contradiction. Super-
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,
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
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
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
horse with suitable wingspan could presumably stay aloft. It might have to be
a tiny horse, something aeronautical engineers might calculate from
considerations of weight-strength ratios, the density of air, and so forth. But
now we are descending into the third grade of possibility, biological possibility,
for once we begin considering the strength of bones, and the payload
requirements for keeping the flapping machinery going, we concern
ourselves with development and growth, metabolism, and other clearly
biological phenomena. Still, the verdict may appear to be that of course
flying horses are biologically possible, since bats are actual. Maybe even
full-sized flying horses are possible, since there once were pteranodons and
other flying creatures approaching that size. There is nothing to beat actuality,
present or past, for clinching possibility. Whatever is or has been actual
is obviously possible. Or is it?
The lessons of actuality are hard to read. Could such flying horses really
be viable? Would they perhaps need to be carnivorous to store enough
energy and carry it aloft? Perhaps—in spite of fruit-eating bats—only a
carnivorous horse could get off the ground. Is a carnivorous horse possible?
Perhaps a carnivorous horse would be biologically possible if it could evolve,
but would such a diet shift be accessible from where horses would have to
start? And, short of radical constructive surgery, could a horse-descendant
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
six-limbed mammal?
This brings us to our fourth grade of possibility, historical possibility.
There might have been a time, in the very distant past, when the possibility
of six-limbed mammals on Earth had not yet been foreclosed, but it might