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Evaluating Principles: Reliability vs. Truth
Principles are statements, and as
such they may reasonably evaluated as either true or false. The
problem is that most principles we use are false. Consider a fairly
straightforward piece of reasoning:
- Monty is addicted to
cocaine because cocaine is a highly addictive drug and he's
been taking it every few days for over a month now.
This rationale, properly reconstructed,
would result in a principle something like this:
- If person x uses highly
addictive drug y on a regular basis, then x becomes addicted
to y.
It's hard to find a better principle than
this one, but it is literally false. Why? Well, because the degree
of susceptibility to an addictive drug will vary from individual to individual.
Also, people can develop immunities to drug, or even be taking other chemicals
that prevent the addictive drug from exerting its effect.
The point here is that almost all
principles have exceptions, and in order to show that a principle is false,
strictly speaking all you have to do is point out one exception to it.
There are couple of ways to deal with this
problem. One way is to say something like this: Most principles have
exceptions, so to avoid concluding that most of the principles we use are false,
we should just be careful to state them in terms of probability. For
example, the principle above should actually be stated as:
- If person x uses highly
addictive drug y on a regular basis, then x is very likely
to become addicted to y.
Now that the principle
acknowledges that these exceptions exist, the exceptions no
longer make the principle false. Something like this
approach can work, but it is very cumbersome to have to write
the degree of likelihood into the principle itself.
A different approach is
just to say that the truth of a principle isn't really
it's most important property. Even a literally false
principle will be very useful if it doesn't have too many
exceptions, and we basically know what those exceptions are.
What really concerns us is the strength or reliability
of the principle.
To make this clear,
consider the following reasoning:
- Skippy:
Well, I am definitely not going to be getting the flu this
winter. I got a flu shot.
It's hard to say what the
proper degree of generality here should be, but let's say this
is meant as an expression of confidence in the effectiveness of
flu shots in particular, not just any old shot. Hence, the principle
would be:
- If person x gets a flu
shot developed to be effective during period y, then person x
does not get the flu during period y.
Of course, this principle
is literally false, since flu shots are not perfectly reliable.
But the real question is: How reliable is a flu shot in
preventing the flu? This figure is hard to pin down,
because it varies with the year and with the age of the
individual. However, for most people in most years the
shot is about 70% effective. This means that the group of
people who get flu shots reduce their risk of getting the flu by
about 70%. In the U.S., about 1 in 3 unvaccinated
people get the flu per year. This means that Skippy has
reduced the likelihood of getting the flu from about .33% to
.3(.33) @ 10%
So what should we say about Skippy's
reasoning? Basically we should say this:
- By getting a flu shot you have
significantly reduced
the likelihood that you are going to get the flu, but there is
still about a 10% chance that you will get it, so you
shouldn't be quite that confident.
You'll notice that the
criticism we are making here is that Skippy's principle is too
weak to support a very high degree of confidence in the
conclusion. If Skippy had said only that he thinks it is
very unlikely
that he will get the flu this year, we would have no criticism.
This leads us to our next
logical error.
Weak Principle
- Definition: Using a principle that is not
sufficiently reliable to justify the expressed degree of confidence in the
conclusion it supports.
- MOI: Identify
the principle in question and its apparent level of reliability. Show
why you think the author of the reasoning has too much confidence in the
conclusion given the reliability of the principle.
Clearly, to do a weak principle analysis properly you will need to provide
the rationale, and be sure that your attribution of this principle is
charitable.
Example 1
- District Attorney: Mr Brumley, I'm going to ask you again
to look directly at the defendant. Are you absolutely certain that he
is the man who robbed your drugstore that night 2 years ago?
- Brumley: Yes sir, there is not a doubt in my mind. I
recognize his face as clearly as if he had robbed me just this morning.
Analysis: This is a weak
principle because it can not justify the absolute confidence with which the
conclusion is expressed. Even someone who is very good at facial
recognition can not rule out the possibility that the witness simply looks very
much like the individual who robbed the drugstore. Also, the reliability
of this principle depends a great deal on how good ones memory is, and given the
2 year period that has elapsed, it is reasonable to doubt the reliability of Mr.
Brumley's memory.
Example 2
- Seth: Oh, man,
this is great weed, dude. You want a hit?
- Roger: You know
I don't smoke marijuana. You do that much longer and
pretty soon, you're going to be using really addictive stuff
like meth or cocaine.
Here, Roger is expressing a great deal of
confidence in the popular idea that marijuana is a gateway drug. It's hard
to say what level of generality Roger is thinking at, but let's suppose the
principle concerns marijuana itself:
- If person x is a
regular user of marijuana, then x will eventually use more
addictive drugs.
The truth is that we don't really know how
reliable this principle is. Quite a few studies do show that people who
use marijuana are considerably more likely to use harder drugs than people who
don't use marijuana. But few studies suggest that the use of marijuana
itself causes one to use harder drugs. The following
analysis would be quite appropriate here:
Analysis
This is a weak principle because it's
degree of reliability has not been established scientifically. Hence, no
particular degree of confidence in the conclusion is warranted.
Of course, it would still be open to Roger to argue
something like this: Since nobody knows whether marijuana is a gateway
drug, it is better to err on the side of caution and not use it at all.
That argument is based on what has become known as The Precautionary Principle:
- If x does not have
strong evidence regarding the safety of action y, then x
should not do y.
Some form of this principle is widely
accepted, though its reliability is still debated.
Example 3
- Uncle Jed died because he choked on a
handful of cinnamon jelly beans.
This is an interesting example to think about. Although choking is the fourth leading cause of accidental death
in the United States (there were 4300 deaths attributed to accidental choking in
the U.S. in 2003) it is actually a very poor predictor of death. So in fact this
principle is very weak. What's interesting is that we often will not
criticize a weak principle as weak if it occurs in an explanation that is
appropriate to the context at hand. Many explanations actually have this
property. For example, we might explain the fact that a person was cured
of a particular disease by reference to the treatment he received, even though
we know that this treatment is only effective in a small number of cases.
It's true that we would prefer to have a better explanation than this,
but that doesn't mean that we can reasonably insist on it.
Example 4
- I don't think Aunt Myrtle is going to
miss Uncle Jed much. He was getting to be pretty hard to live with.
Analysis
This is a weak principle because the difficulty of
living with a person does not reliably predict whether those related to him will
miss him when he is dead. We would need to know many other things, like
whether the deceased had other redeeming qualities, or whether the grumpiness
itself was particularly troublesome to the spouse or likely to be remembered.
Abusing Principles
A preference for evaluating principles in terms of
their truth value rather than their reliability is at the heart of a common error in reasoning.
It is known by several different names, but we will call it Abuse of
Principle. Abuse of Principle is attempting to refute a principle by
focusing on a known or irrelevant exception.
Abuse of Principle
Definition: Attempting to refute a principle
(or the reasoning that it supports) by showing
that it is has exceptions, but giving no reason to believe that the principle is
generally unreliable or that the exceptions apply to the particular case at hand.
MOI: Identify the principle and the alleged exceptions in the
absence of reasons for thinking the exceptions cast doubt on the reliability of
the principle as stated.
Abuse of Principle is a mistake because showing
that a principle has exceptions (and is therefore literally false) is not the
same as showing that it is unreliable. Here are some examples of principle
abuse.
Example 1
- Phyllis: It says here that college athletes are admitted
with much lower average high school GPA's and SAT scores than other
students. To me that isn't fair.
- Rachel: That's bull. My roommate Delilah is on a volleyball
scholarship. I don't know what her SAT scores were, but she told me she was
almost the valedictorian at her high school.
Analysis
The principle here is "If x is a college
athlete, then x was subject to lower academic admissions criteria than other
students." Rachel's reaction actually demonstrates a failure to grasp the
principle, since it doesn't imply anything about the actual qualifications of
particular students. However, it constitutes principle abuse in that Rachel is
trying to refute the principle by claiming that Delilah is an exception.
(This example could also be well analyzed as Straw Man.)
Example 2
Gizmo: Look, I've clicked on all those automotive consumer links you sent me and
I know they say that Hyundai's have one of the highest customer
satisfaction ratings. But I notice you still aren't driving one, and I personally know two
people who hate their Hyundai's: my dad and my wife's boss.
Analysis
In this example the relevant principle is: If
a product has the strongest consumer ratings, then it is the best product. The speaker identifies two exceptions to this
principle apparently believing that this constitutes a basis for rejecting the
principle itself. The fact that the speaker personally knows two people who are
unhappy with their cars is not a rational basis for believing the principle is
unreliable.
Example 3
Cole: Quit giving me that "Airplanes are safer than cars," crap!
People have been saying that to me all my life. Well, I'm sorry, but I was
in a friggin' 747 when it crash landed! So unless you've been
through something like that, you can keep your travel advice to yourself.
Analysis
In this example, the relevant
principle is "If x is a form of airline travel and y is a form of automobile
travel, then x is safer than y." Cole cites an example of unsafe airline
travel as an exception as an attempt to refute the principle. However, the
original principle does not imply that there are no airline crashes. The
fact that Cole was personally almost involved in such an accident does nothing
to elevate the likelihood of an airplane accident relative to a car accident.
Example 4
Angus: Americans participate in the illusion that citizens of the United
States enjoy a Constitutional right to free speech. But there is no such right
and never has been. If you think otherwise, just try yelling "I've got a bomb!"
on your next vacation. If you don't have the stomach for that, try something
less exciting, like visiting your kids' elementary school class and doing a show
and tell on your favorite pornographic websites. Either way you'll find out how
free this country really is.
Analysis
Here, Angus assumes that the relevant principle is
something like: If x is speech, then x is protected by the U.S. Constitution.
Although many people do seem to think the
right to free speech is absolute, in reality it, like all legal principles, has
exceptions. Speech intended to incite violence, or which is judged to pose a
clear and present danger to others is not protected. This does not mean there
is no right to free speech, it just means the right is not absolute.
Example 5
Barb: You know, in the United States you don't have a chance of
being elected president if people think you don't believe in God. I heard on
NPR this morning that 7 out of 10 voters says they won't vote for a candidate
who doesn't.
Butch: Jeez, I really wish you wouldn't repeat that kind of crap.
I mean do you, yourself, know lots of people who say that? I sure don't. A
person's religious views are a purely private matter. They have nothing to do
with whether someone is competent to be elected president.
Identification: Here the generalization in question is: If person
x is a U.S. presidential candidate widely believed not to believe in God, then x
will not be elected. Butch questions this
generalization on the basis that neither he, nor anyone he knows holds that
view. But Butch may simply be one of the 30% of the voting public who does not
hold that view, and it may be that he is acquainted mainly with people who share
his views.
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