Logic 1000, Second Assignment Answers
November 28,
2006, due December 5, 2006.
I. Argument reconstruction and evaluation.
Reconstruct the following arguments, identifying premises and conclusions, and
supplying any necessary tacit premises. Then evaluate the arguments, considering all the
virtues, and decide how strong or weak each is.
General Grading Scheme: 20 points per argument—10 for reconstruction (4 for
identifying premises, 3 for conclusion, 3 for a good TP if needed; -1.5 if the UTP is used
where a more focused TP is available; -1.5 if the TPs added don’t really make the
argument valid). 10 for evaluation—4 for plausibility of premises (2 for addressing it; 2
for quality of remarks), 3 for independence (1.5 for addressing it, 1.5 for quality of
remarks), 2 for interest (1 for raising it, 1 for quality); 1 for summary.
1. Free markets allocate resources efficiently because buyers, knowing what they
want and what is available, force sellers to compete by offering low prices and high
quality. (A. Smith)
Reconstruction:
P1. Buyers (in a free market) know what they want and what is available.
P2. If buyers (in a free market) know what they want and what is available, they will
force sellers to compete by offering low prices and high quality.
Tacit premise. If sellers (in a free market) are forced to compete by offering low prices
and high quality, then free markets allocate resources efficiently.
C: Free markets allocate resources efficiently.
Comments: i. P1 and P2 are combined in the original; they can be left together. ii. The
tacit premise needs to connect sellers in a free market being forced to compete… to the
claim that free markets allocate resources efficiently. Using the UTP instead is a small
deduction (1/2 the TP points). The qualification, ‘in a free market’ in the premises is
important, but will mostly be missed I fear. A small deduction should be assigned for
missing it.
Evaluation:
Plausibility: P1: This may be plausible in particular cases, but it certainly isn’t always
the case (consider medical care, education, etc.). Very often, in practice, we may be
unsure of what we want and poorly informed about what is available.
P2: This is plausible, since the antecedent puts buyers in a position where, unless they
have no real choice, they can choose between different sellers on these criteria. But it
could be criticized as too general, since the antecedent could hold in a monopoly
situation, where the buyers really have no choice. On the other hand, someone might
reply that this would not be a ‘free market.’
TP: This is again broadly plausible, but too general to stand. If the contract between
buyer and seller imposes costs on others (society at large perhaps), then free-market
contracts will be reached that don’t pay attention to these costs. Such externalities
prevent the free market from achieving efficient resource allocation, which applies not
just to buyers’ and sellers’ resources, but to the resources of all…
Independence: P1: This is independent, since the evidence for it would come from
actual examination of the knowledge of buyers participating in free markets, which
doesn’t require assuming the efficiency of the markets.
P2: This is independent, since it turns on the psychological facts and social interactions
between buyers and sellers that take place when buyers do know what they want and
what’s available.
TP: There can be an issue here—would you really buy this if you didn’t accept the
conclusion? But I think it is actually independent—given a definition of efficiency, we
may be able to show that resources will tend to be efficiently used when this kind of
competition occurs between sellers.
Interest: This is of some interest to economists, and to those interested in public policy,
and maybe event to people in general. (Efficient use of resources to achieve our ends is
no doubt something we all care about…)
Summary: Put so generally, the conclusion claims too much—even with the condition
on buyers’ knowledge which is often not met (P1 is often false), P2 is vulnerable to
monopolies and other situations where what’s available doesn’t provide enough choice to
pressure suppliers to compete (and competition is often over other things than quality and
price—consider advertising), and TP is vulnerable to externalities. If we add these
conditions (no monopolies, no externalities, etc.) then it might go for the remaining free
markets—but it no longer looks like a general argument for using free markets
everywhere… Not a good argument.
2. “(I)n case we...successively assent to both (contradictory positions), as is usual
among philosophers, with what confidence can we afterwards usurp that glorious
title, when we thus knowingly embrace a manifest contradiction?” (Hume)
P1: If someone knowingly embraces a manifest contradiction, they cannot confidently
(or ‘with confidence’) usurp (take or claim) the glorious title of ‘philosopher.’
P2: It is usual among philosophers to successively assent to two (particular)
contradictory positions.
C: Many philosophers (or these philosophers or such philosophers) do not deserve that
glorious title.
Comments: This one can be read in a range of ways, so it’s important to be charitable
here, so long as the reading is broadly ‘on track’—the key point is that there is a conflict
between assenting to contradictions/ contradictory positions and being (properly) called a
philosopher. Most philosophers, according to Hume, actually do this, and so should not /
should not confidently regard themselves as philosophers. Grading should be fairly
generous here—the key point, as the answer (roughly) to the question Hume poses, is just
the tension between calling yourself a philosopher and accepting contradictory
conclusions.
Evaluation:
Plausibility: P1 is plausible, if we think a philosopher should be reasonable, care about
logic and so avoid believing in contradictions or contradictory claims. P2 may be
plausible on Hume’s say so, but could be challenged (perhaps Hume is being
uncharitable, perhaps he’s wrong to claim the two really are contradictory—this would
take looking into to settle).
Independence: We could settle the first by studying a little about philosophy and the
attitude philosophers take towards those who accept contradictions; the second could be
settled by examining the views of various philosophers and seeing if they are guilty as
charged. Neither process requires assuming the conclusion, so both pass the test.
Interest: A bit weak, given the lack of context—but perhaps not so bad, if philosophers
really do care about consistency and are generally guilty of the inconsistency Hume is
talking about. In that case, philosophers should be very interested in this conclusion…
Summary: Could be a good argument, but room for doubt re. P2, and we might even
want more backup on P1.
3. Even China has safer air travel than the US. Proof? The 15 August 1989
accident that killed 34 people was the first in China since August 1988. During that
time, there were several major accidents in the US killing over 300 people.
Reconstruction;
P1. The 15 August 1989 accident that killed 34 people was the first in China since
August 1988.
P2. During that time, there were several major accidents in the US killing over 300
people.
C: Air travel in China is safer than air travel in the U.S.
Tacit Premise: If P1 and P2, then C. (It’s hard to see what else to do with this one…)
Evaluation:
Plausibility: Both explicit premises are fairly plausible, since they are easily checked
facts (although the first could be questioned on grounds that Chinese statistics might be
manipulated/ unreliable). The tacit premise is very weak—comparisons of fatalities
would only work if everything else (levels of air travel, in particular) were equal in the
two countries…
Independence: All three premises could be established as plausible without assuming
the conclusion—statistics could underwrite the first two, and the last could be established
by detailed comparison showing that the circumstances of air travel are so similar in both
countries that the comparison of fatality numbers would be a fair test of comparative
safety levels
Interest: This is probably interesting for (and directed towards) people traveling in the
U.S.; also of interest to people traveling in China.
Summary: A terrible argument. The comparison is interesting, but far from enough to
support the conclusion.
4. Smoking marijuana definitely leads to heroin use. A report by the US
commissioner of Narcotics on a study of 2,213 hard-core narcotic addicts in the
Lexington, Kentucky, Federal Hospital shows that 70.4 percent smoked marijuana
before taking heroin.
P1: A report… 70.4 percent smoked marijuana before taking heroin.
TP: If P1, then C.
C: Smoking marijuana definitely leads to heroin use.
Evaluation:
Plausibility: P1: This is reasonably plausible—nothing I know rules it out or makes it
seem unreasonable, and it’s the sort of straightforward figure that’s easily grasped and
publically checkable, so I would expect the author to get it right.
TP: This is wildly implausible. It’s an egregious case of ‘post hoc ergo propter hoc’
reasoning. 70.4 percent of the general population might have used marijuana, in which
case P1 wouldn’t show a correlation between marijuana use and heroin use, let alone a
causal connection. Even if there is a correlation, there are many hypotheses other than
the simple causal one that would explain it. For example, marijuana smokers are already
using an illegal drug—perhaps very few people who would later choose to take heroin
would refuse marijuana, and most would have the opportunity to use marijuana—which
is far more commonly available—before having a chance to use heroin.
Independence: P1: Obviously. Simple observation of the patients in question can settle
this with no need to assume C.
TP: Suspect. It’s hard to see how one could justify this statistical nonsense without
having assumed the conclusion. It might be claimed that the report’s method could be
such as to have tested the causal hypothesis (through a retrospective study here). But that
still wouldn’t justify this premise, which links a very weak statistical claim to a strong
causal one…
Interest: Obviously of interest to public policy, justification for the legal status of
marijuana, of concern to anyone thinking about smoking marijuana…
Summary: A lousy argument. There’s no reason here to accept the conclusion at all.
5. From these several reasons, namely, - the improbability of man having formerly
made seven or eight supposed species of pigeons to breed freely under
domestication; - these supposed species being quite unknown in a wild state, and
their not having become anywhere feral;- these species presenting certain very
abnormal characters, as compared with all other Columbidae (species of pigeon),
though so like the rock-pigeon in most respects;- the occasional re-appearance of the
blue colour and various black marks in all the breeds, both when kept pure and
when crossed;- and lastly, the mongrel offspring being perfectly fertile;- from these
several reasons taken together, we may safely conclude that all our domestic breeds
are descended from the rock-pigeon or Coumba livia with its geographical subspecies. C. Darwin, On The Origin of Species, (Heritage Press, NY, 1963) p. 16.
P1: It is improbable that man formerly made seven or eight species of pigeons breed
freely under domestication.
P2: If there were such species, then they are now unknown in the wild state and have
nowhere become feral.
P3: These species would have very abnormal features compared with all the other
Columbidae (other species of pigeon), though they would be very like the rock pigeon in
most respects.
P4: The blue colour and various black marks (of the rock pigeon) occasionally reappear
in all breeds, both when kept pure and when crossed.
P5: The mongrel offspring of different breeds are all perfectly fertile.
TP1: P1, P2, and P3, then there’s good evidence that our domestic breeds are not
descended from separate wild species.
TP2:If P3, P4, and P5, then there’s good evidence that our domestic breeds are descended
from the rock pigeon or Columba livia with its geographical subspecies.
TP3: If there’s good evidence that our domestic breeds are not descended from separate
wild species, and there’s good evidence that our domestic breeds are descended from the
rock pigeon or Columba livia with its geographical subspecies, then C.
C: All our domestic breeds are descended from the rock pigeon, or Columba livia with
its geographical subspecies.
Plausibility of Premises.
P1: This premise should either be a “don’t know” (check with a biologist or biology
text...) or “plausible” based on the source, and the fact that in a serious book a mistake on
such a basic claim would be quickly revealed, with highly embarassing consequences for
the author.
P2: Much the same answer here.
P3: Again, much as P1.
P4: Ditto (except this time you could also check with a pigeon breeder or enthusiast).
P5: As P4.
TP1: Seems likely- a scenario to counter this evidence is hard to imagine. (Could be
“don’t know” plus a check with biologists.)
TP2: As TP1.
TP3: This is a difficult one. It’s one thing to recognise that there is evidence for a view,
and against the (main) alternative to it, but it’s another to conclude that the view in
question is correct. There seems to be no way to guarantee that this premise will be
correct. On the other hand, it seems likely that it is correct. So I’m inclined to call it
plausible.
Independence:
P1: It’s clear that we (or others) can gather evidence for this that will not depend on any
assumptions about where our current pigeon breeds come from (the evidence will draw
on the difficulty of domesticating species in general, and birds in particular...)
P2: Similar to P1, though here the evidence draws on our knowledge of species of
pigeons found in the wild, which we have reason to believe is at least pretty close to
complete.
P3: Same story (evidence here based on straightfoward study of various pigeon breeds
and comparing them with known wild species).
P4: Ditto (here pigeon breeders and enthusiasts can provide direct confirmation).
P5: Ditto (just as P4).
Interest of Conclusion:
Interesting to biologists, perhaps to pigeon breeders, to people interested in evolution and
how far a species can change under selection...
Summary:
This argument is pretty persuasive, though (see the comment on TP3) certainly not a sure
thing.