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Scientific ReasoningChapter 2Scientists often tell us things
about the world that we would not otherwise have believed:
What exactly is the nature of scientific reasoning?How much
confidence should we place in the inferences scientists make?
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DeductionDeductive reasoning a type of reasoning in which the
existence of an appropriate relation between premises and
conclusion, namely that if the premises are true, the conclusion
must be true too:
Premise 1: All NCU teachers are ChristiansPremise 2: Jonathan is
a NCU teacherConclusion: Therefore, Jonathan is a Christian
Whether that premises are actually true is a different matter,
which doesnt affect the status of the inference as deductive.
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InductionInductive reasoning we move from premises about objects
we have examined to conclusions about objects we havent
examined.
The first five eggs in the box were rottenAll the eggs have the
same best-before date stamped on
them--------------------------------------------------------------------------------------------Therefore,
the sixth egg will be rotten
It is quite conceivable that the sixth egg (which we havent
examined) will be perfectly good. In other words, it is logically
possible for the premises of this inference to be true and yet the
conclusion false.
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When we reason deductively, we can be certain that if we start
with true premises, we will end up with a true conclusion.
On the contrary, inductive reasoning is quite capable of taking
us from true premises to a false conclusion.
Other examples of inductive reasoning:When you turn the steering
wheel of your car anticlockwise, you assume the car will go to the
left not the right.Newtons principle of universal gravitation every
body in the universe exerts a gravitational attraction on every
other body. Newton did not arrive at this principle by examining
every single body in the whole universe he couldnt possibly
have.
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How do we misuse inductive reasoning?You might read a newspaper
report that says that scientists have found experimental proof that
genetically modified maize is safe for humans. What this means is
that the scientists have tested the maize on a large number of
humans, and none of them have come to any harm. This does not prove
that the maize is safe (not in the strictest sense)The newspaper
report should really have said that scientists have found extremely
good evidence that the maize is safe for humans.The word proof
should strictly only be used when we are dealing with deductive
inferences.Scientific hypotheses can rarely, if ever, be proved
true by the data.
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Karl Popper
Poppers basic argument was that it is not possible to prove that
a scientific theory is true from a limited data sample, it is
possible to prove that a theory is false.
There is at least one metal that does not conduct electricity
this counterexample may be used to disprove a theory that states
that all metals conduct electricity.
The major problem with Poppers argument is that a scientist is
also interested in proving his/her own theory to be true setting up
the use of inductive inference.
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Humes Problem
Hume argued that the use of induction cannot be rationally
justified at all. Hume argued that we use induction all the time in
everyday life and in science, but he insists this was just a matter
of brute animal habit. If challenged to provide a good reason for
using induction, we can give no satisfactory answer.Uniformity of
nature (UN) The assumption that objects we havent examined will be
similar, in the relevant respects, to objects of the same sort that
we have examined:The fact that the sun has risen every day up until
now may not prove that it will rise tomorrow, but surely it gives
us very good reason to think it will.What has happened in the past
will happen in the future.It is easy to imagine a universe where
nature is not uniform, but changes its course randomly from day to
day.
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Humes Problem
Hume points out that our inductive inferences rest on the UN
assumption.Hume concludes that our confidence in induction is just
blind faith it admits of no rational justification whatever.Science
relies on induction, and Humes argument seems to show that
induction cannot be justified. If Hume is right, the foundations on
which science is built do not look as solid as we might have hoped
(Humes problem).Some people believe that the key to solving Humes
problem lies in the concept of probability.It is natural to think
that although the premises of an inductive inference do not
guarantee the truth of the conclusion, they do make it quite
probable.Strawson induction is one of the standards we use to
decide whether claims about the world are justified.
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Inference to the best explanation (IBE)
The cheese in the pantry has disappeared, apart from a few
crumbsScratching noises were heard coming from the pantry last
night._____________________________________________Therefore, the
cheese was eaten by a mouse.
This inference is non-deductive the premises do not entail the
conclusion.The cheese could have been stolen by the maid, who
cleverly left a few crumbs to make it look like the handiwork of a
mouse.The mouse hypothesis and the maid hypothesis can both account
for the missing cheese. Why do we regard the mouse hypothesis as a
better explanation of the data?Inductive inference is reserved for
inferences from examined to unexamined instances of a given
kind.
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Probability and induction
Frequency interpretation equates probabilities with proportions,
or frequencies. 1/10;1 in 4; 1 out of every 100 students at NCU is
disciplined.Subjective interpretation takes the probability to be a
measure of the strength of our personal opinions. It implies that
there are no objective facts about probability, independently of
what people believe. Example: I am very confident that Brazil will
win the World Cup; I am extremely confident that Jesus is coming
again; There is a low probability that a global environmental
disaster can be averted.Logical interpretation holds that a
statement such as the probability of life on mars is high is
objectively true or false, relative to the specified body of
evidence. A statements probability is the measure of the strength
of the evidence in its favour.
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Philosophers of science are interested in probability for two
main reasons:In many branches of science, especially physics and
biology, we find laws and theories that are formulated using the
notion of probability. (Mendelian Genetics)The hope that it might
shed some light on inductive inference, in particular on Humes
problem. At the root of Humes problem is the fact that the premises
of an inductive inference do not guarantee the truth of its
conclusion.
On the frequency interpretation, to say it is highly probable
that all objects obey Newtons law is to say it is highly probable
that all objects obey the law. But there is no way we can know
that, unless we use induction! For we have only examined a tiny
fraction of all the objects in the universe.
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The logical interpretation suggests that the premises of an
inductive inference cab make the conclusion highly probable, even
if they cannot guarantee its truth.
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Weighing the latest facts on seafood safety, health benefits
We've learned that some varieties of fish are low in fat and
contain oils that keep the heart healthy. But recent reports about
contaminants such as mercury and polychlorinated biphenyls, or
PCBs, have prompted some health experts to rethink their advice
about seafood. Lots of varieties of fish are safe, but some types
of seafood can be risky for certain groups of people.For Seattle
cardiologist Florence Sheehan, M.D., it isn't just her patients she
worries about. It's her family, too. "Ours has a history of high
cholesterol," Sheehan says. "So I eat fish frequently to keep my
cholesterol down."Lately, she finds herself scanning medical
journals and government advisories to stay abreast of fish safety
issues. She says that untangling the facts behind the latest
seafood scares isn't as complicated as it seems. "The key is to
place the benefits and risks into perspective," Sheehan says. "Lots
of varieties of fish are safe. It's just that some types of seafood
can be risky for certain groups of people." Here's a look at which
fish pose risks, and which ones are safe -- and good for you,
too.
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Since exposure to high levels of mercury can cause neurological
damage in a growing fetus, the Food and Drug Administration
continues to issue related seafood safety advisories to pregnant
women and young children. In March of 2004, the FDA updated that
advice with stricter, more specific rules: Pregnant women, or women
who plan to become pregnant, should avoid eating four fish with
high levels of mercury: swordfish, shark, tilefish, and king
mackerel. While fresh and canned tuna didn't make the FDA's list,
many experts say pregnant women may be better off limiting fresh
tuna steaks and canned albacore, or "white," tuna to one meal per
week or less, since these large fish can harbor mercury levels
close to the one part per million threshold the FDA deems safe.
(Canned light tuna is considered safe since it is made with smaller
skipjack fish that are low in mercury.) The American Medical
Association recently encouraged the FDA to require supermarkets to
post warning signs about mercury near canned tuna as well as at the
fish counter. CookingLight.com: Risks, benefits of 8 most popular
U.S. seafood
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Being aware of mercury is also a good idea for those who aren't
pregnant. When internal medicine specialist Jane M. Hightower,
M.D., performed a yearlong study of 123 of her patients, she found
that a steady diet of high-mercury fish caused serious symptoms
such as headaches, hair loss, problems with concentration, and high
blood levels of mercury. Fortunately, once these patients switched
to eating low-mercury varieties, symptoms began to disappear, and
blood mercury levels returned to a safe level.Purdue University
seafood expert Charles Santerre, Ph.D., thinks the key to
minimizing health risks for any food is to aim for variety. "If you
ate swordfish or shark or king mackerel every day, you could
experience mercury toxicity," Santerre says. "But if you eat them
once a month [and trade off with] some other low-mercury fish, it
shouldn't be a problem." However, the "sensitive population,"
including pregnant and nursing women, should always avoid
swordfish, shark, tilefish, and king mackerel, Santerre says. On
his list of safe, low-mercury options: shrimp, salmon, pollock,
farm-raised catfish, tilapia, flatfish (flounder, sole, plaice),
scallops, haddock, farm-raised trout, herring, crawfish, mullet,
oysters, ocean perch, sardines, squid, white fish, and
anchovies.
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If fish can harbor toxins, it seems plausible that the oils
extracted from fish to make supplements might be contaminated, but
that's not the case. "Fish oils are pure," says Connor. One recent
study tested 16 fish oil supplements sold in warehouse clubs,
pharmacies, and supermarkets, and none contained significant
amounts of mercury, PCBs, or the pollutant dioxin. Currently, the
American Heart Association recommends 1,000 milligrams of fish oil
supplements per week for people with heart disease. According to
Connor, supplements are a great way for nonfish lovers to tap into
the important heart-healthy benefits of omega-3 fatty acids.