Perhaps we need to try to define terms here. I have always
taken tautology to mean something that is obviously or
trivially true or is true by definition. The talk.origins
jargonfile seems to support this:
Tautology
(n) See truism.
Truism
(n) See tautology.
<congratulations to Wesley for this clever definition of
tautology by using a tautology>
I think I would rewrite Steve's comment something like this:
Even though statement X might be a tautology, it is not
necessarily meaningless, or, even though the theory of
evolution might contain tautological statements, the theory
of evolution is not itself a tautology.
Eliott Sober has a good discussion of the "tautology problem"
in his book <Philosophy of Biology> (p. 69-73). It seems that
tautology, as used in logic, is much more restrictive than
what I've indicated above. Perhaps this is best illustrated
by some examples, the following are tautologies:
a) pigs either exist or they don't exist
b) all tables are tables
note that both statements are true from the logical structure
alone (the truth of the statements has nothing to do with
pigs or tables).
Sober goes on to say that a statement like the following:
c) all bachelors are unmarried
is often included under the heading "tautology" even though
technically it is an analytic proposition since truth
or falsity is determined by the meanings of the terms used and
not solely from logic.
Likewise, mathematical truths like:
d) y = mx + b
are sometimes considered as tautologies.
With respect to our discussions here I don't think it's
particularly useful to make distinctions about the
various classifications above. For example, "survival
of the fittest" is clearly not a tautology in the
sense of (a) or (b), however statements related to
"survival of the fittest" are usually expressed as
some sort of analytic statement similar to (c).
The important point is that all of the categories above
(tautology, analytic proposition, mathematical truth)
have, in and of themselves, absolutely zero explanatory
content. Another important point is that all theories
are going to contain tautologies, definitions and
mathematical truths. Thus, merely pointing out that
statement X is a tautology is not necessarily an argument
against theory Y in which statement X appears.
What one has to look out for are situations where a tautology
is presented as if it is an explanation, for example:
# A body tends to move in a straight line with constant velocity
# (unless acted on by a force) because of inertia.
The problem with this is that inertia is just a name for the
observation that a body tends to move in a straight line with
constant velocity unless acted on by a force. In view of this,
shall we say that Newton's first law is a meaningless tautology?
Bite your tongue, infidel!! ;-)
At the risk of rambling on too long (too late!:) it might be
useful to give an example that is somewhat less trivial than
the above. Consider Newton's second law:
F = ma
One can define both mass and acceleration in a relatively
straightforward manner and it is very tempting to take
these definitions combined with Newton's second law as
the definition of force, i.e. to define force as the
quantity mass*acceleration. In fact, this is so tempting
that even the great Mechanician (in my parable "Manuel and
the Mechanician" which I posted some time ago) fell for
it. Of course, *I* would never make such a mistake ;-).
What's the mistake? Defining force in this manner reduces
Newton's law to a "tautology" of the type (d) above and
thus renders it completely useless in terms of making any
predictions or in saying anything physically meaningful
about "forces". It takes some effort to explain exactly
why this is, so I'm going to skip over that. For Newton's
second law to have any significance it is necessary for
all three quantities (force, mass and acceleration) to
be defined independently of the law itself. Only then
does Newton's law have any force ;-).
Brian Harper
Associate Professor
Applied Mechanics
The Ohio State University
"Quantum physicist and Jungian analyst, when dropped from
a great height, fall at the same rate of speed, their
descent unaffected by speech or creed" -- David Berlinski