7.3 Non-deductive reasoning 265
B and C are not cases of ABE. ABE proceeds
from an observed fact to a hypothesis which
would explain the fact, and explain it better
than other hypotheses would. In B the
conclusion is a prediction based on an
observation. C is a recommendation based on
a claim about cost, and supported by a
comparison between cycling and driving. In
neither case is the conclusion justified by what
it supposedly explains.
D is a clear case of arguing from observed
facts to an explanatory hypothesis. There are
three observations: (1) that humans have less
hair than most land mammals; (2) that they
have more fat; (3) that they walk upright. D is
giving support to what is known as the
‘Aquatic Ape Theory’: a claim advanced by
some anthropologists that for a considerable
time during their evolution humans took to
the water as their natural environment. D
claims that this theory is the obvious
conclusion because if it were true it would
explain all three of the observed facts at a
stroke. The Aquatic Ape Theory is a nice one,
and it certainly does offer a plausible
explanation for many differences that are
found between humans and other primates
or mammals generally. However, D itself is a
one-sided argument. It does not acknowledge
that there may be other explanations that are
just as persuasive. It is overstating the case
therefore to say that it is the obvious
conclusion.Argument from analogy
A third line of non-deductive reasoning that is
very frequently used is argument from analogy
(AfA). An analogy is a comparison, an
observed similarity. In C, above, an analogy is
drawn between cycling helmets and seatbelts.
The comparison is an obvious one: both
devices are designed to reduce injury in the
event of an accident. The assumption in the
argument is that they do. Its conclusion is that
the same rule should apply to both cycling
and driving.Commentary
The two examples of ABE are A and D. Having
observed that my coffee is barely warm, it
would then be no surprise to learn that it had
been made some time before. That would be a
plausible explanation, and therefore a
plausible hypothesis. However, that is by no
means the only possible, or even plausible,
explanation for lukewarm coffee. The water
may not have been allowed to boil. It may
have been made in the belief that I don’t
drink my coffee hot. And so on.
Which of the following are examples of
argument to the best explanation, and why?
How safe are their conclusions? (The second
question applies only to the examples of ABE.)
A My coffee is barely warm so it must
have been made some time ago.
B These bushes won’t survive because
they’re not getting enough sun.
C Head injuries from cycling are very
common. Not only do they cost lives,
they cost the health service millions of
dollars of taxpayers’ money every year.
The wearing of cycling helmets on public
highways should therefore be
compulsory. It’s illegal in most countries
to drive without a seatbelt on. Cycling
should not be treated differently.
D Most land mammals have a dense
coating of fur. Humans, by contrast,
have little hair and a thicker than
normal layer of fat, more like aquatic
mammals than the ancestral apes.
Humans are also unusual in habitually
walking upright. The obvious conclusion
is that a large part of human evolution
took place in a watery environment,
where fat would provide insulation and
wading on two legs would be the
natural way to move.Activity