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Scientific Reasoning Chapter 2 Scientists often tell us things about the world that we would not otherwise have believed: What exactly is the nature of.

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Presentation on theme: "Scientific Reasoning Chapter 2 Scientists often tell us things about the world that we would not otherwise have believed: What exactly is the nature of."— Presentation transcript:

1 Scientific Reasoning Chapter 2 Scientists 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?

2 Deduction Deductive 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 Christians Premise 2: Jonathan is a NCU teacher Conclusion: Therefore, Jonathan is a Christian Whether that premises are actually true is a different matter, which doesn’t affect the status of the inference as deductive.

3 Induction Inductive reasoning – we move from premises about objects we have examined to conclusions about objects we haven’t examined. The first five eggs in the box were rotten All 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 haven’t 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.

4 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. Newton’s 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 couldn’t possibly have.

5 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.

6 Karl Popper Popper’s 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 Popper’s argument is that a scientist is also interested in proving his/her own theory to be true – setting up the use of inductive inference.

7 Hume’s 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 haven’t 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.

8 Hume’s 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 Hume’s 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 (Hume’s problem). Some people believe that the key to solving Hume’s 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.

9 Inference to the best explanation (IBE) The cheese in the pantry has disappeared, apart from a few crumbs Scratching 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.

10 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 statement’s probability is the measure of the strength of the evidence in its favour.

11 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 Hume’s problem. At the root of Hume’s 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 Newton’s 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.

12 The logical interpretation suggests that the premises of an inductive inference cab make the conclusion highly probable, even if they cannot guarantee its truth.

13 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.

14 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. seafoodCookingLight.com: Risks, benefits of 8 most popular U.S. seafood

15 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.

16 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.


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