Confirmation, Falsification, Fallibility...Count Rumford’s (aka Benjamin Thompson’s) experiment Friction from boring out a cannon heats the metal, but there is no source from which

FalsificationConfirmation

Confirmation, Falsification, Fallibility

http://philosophy.ucsd.edu/faculty/wuthrich/

12 Scientific Reasoning

Acknowledgements: Bill Bechtel

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http://philosophy.ucsd.edu/faculty/wuthrich/


Basics and examplesOvercoming the overly simplistic: auxiliary hypotheses

The simple model of falsifying a hypothesis

The basic idea that false predictions count against the truth of ahypothesis is captured in the following argument schema:

(1) If the hypothesis is true, then the prediction will be true.(2) The prediction is not true.

∴ The hypothesis is not true.

This argument form is modus tollens. It is valid.

The question is whether the premises are true.

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A compelling simple example of falsification

QuestionWhere do plants get their body mass from?

Natural assumption: from the soil

In 1649, Jan Baptista van Helmont grew a willow tree for fiveyears in a measured amount of soil, adding only water.

The tree increased in weight by 164 pounds while the soildiminished by only two ounces.

(1) If soil is the source of the mass of trees, then the weightof the soil should diminish as the tree grows.(2) The weight of the soil does not diminish as the treegrows.

∴ The soil is not the source of the mass of trees.

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Galen’s (129-216) two bloods

According to Galenic physiology, arteries andveins each carried different types of bloodaway from the heart.

Venous blood carried nutrients fromthe liver through the right side of theheart to the body.

Arterial blood is vivified by the lungsand carried from the left side of theheart to the body.

Slight seepage from right to left

Heart operated to suck blood in fromthe veins (and ultimately from the liver)

Both types of blood were consumed bythe body’s tissues.

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William Harvey’s (1578-1657) evidence against Galen

Harvey determined that the valves inthe veins would only permit flow intothe heart, not out.

But the Galenic theory predicted thatblood could flow away from the heart inthe veins.

(1) If the Galenic theory wereright, valves should permit out-ward flow from the heart into theveins.(2) Valves do not permit outwardflow from the heart.∴ The Galenic theory is wrong.

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An assumption of the Galenic theory is that all the contents of arterialand venous blood originate in food and is dispersed.

Prediction: the mass of food and drink must equal the mass ofthe material in the arterial and venous blood.

Harvey measured the amount of blood in the heart at a time(approx. 2 ounces) and multiplied by number of heart beats anhour (2,000), estimated that 40 pounds of blood sent out perhour

This exceeded the amount of food and liquid a personconsumes—and where does it all go?

(1) If the Galenic theory is right, people need to replenishthe stuff of blood from food and drink.(2) People do not eat or drink enough to replenish the stuffof blood.

∴ The Galenic theory is wrong.

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Holding on to hypotheses despite falsification

It is infrequent that a scientist will give up a hypothesis as soon as aprediction fails. Why?

An accepted hypothesis often has lots of evidence in itsfavour—things it explains.

Even a flawed hypothesis is better than no hypothesis at all. ⇒Without an alternative theory, stay with what has worked so far.

Often the prediction depends not just on the hypothesis, but onother, auxiliary conditions. These rather than the hypothesisitself could be what is false.

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The endurance of the caloric theory of heat

The ability to heat a cold body by placing it next to a warm onesuggested the hypothesis that heat is a fluid.

In the 18th century heat (caloric) was treated like other newlydiscovered fluids such as electricity.

⇒ suggests that heat can be conserved

Complexity: different substances warmed differently by samesource of heat: 1 liter of 100◦C water will warm a liter of 0◦Cwater to 50◦C, but 0◦C mercury to approx. 97◦C.

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Count Rumford’s (aka Benjamin Thompson’s) experiment

Friction from boring out a cannon heatsthe metal, but there is no source fromwhich caloric is transferred.

⇒ Maybe the caloric is contained in pocketswithin the heat.

But slowly boring with a dull knifeproduced more heat than rapid boring,which would unseal from pockets.

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Rumford’s reasoning

(1) If caloric (a fluid) is respnsible for heat, drilling a cannonwith a bit should not produce heat.(2) Drilling a cannon with a bit did release heat (lots of it).

∴ Caloric is not responsible for heat.

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Endurance of caloric theory

Caloric has been named by Antoine Lavoisier (1743-1794) after herejected phlogiston. After Lavoisier was executed, Rumford marriedMme Lavoisier:

I think I shall live to drive caloric off the stage as the lateLavoisier drove away [phlogiston]. What a singular destinyfor the wife of two Philosophers!

But caloric survived Rumford (and Mme Lavoisier divorced him).

Caloric theory had a great deal of explanatory power such as theability to explain the movement of heat through a vacuum.

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The final demise of caloric

James Joule (1818-1889) discovered in1840 that an electric current I flowingthrough a resistance R generated a rateof heating I2R

Joule calculated the mechanical energyneeded to produce one BTU of heat

Relating heat to mechanical energyprovided an alternative conception ofheat

⇒ With the development of an alternativetheory linking heat to mechanical energy,the caloric theory was finally displaced.

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The gap between hypothesis and prediction

Few interesting hypotheses lead to prediction all on their own.

There are typically a host of other factors at work:

Other hypotheses—auxiliary assumptions—that areassumed to be true and required to make the prediction.Features of the observational or experimental design thataffect the prediction.

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Option:Reject the auxiliary assumptions or the experimental procedure

(1) If the hypothesis is true AND if all auxiliary hypothesesneeded to make the prediction are true AND if theexperimental setup is adequate, then the prediction will betrue.(2) The prediction is not true.

∴ EITHER the hypothesis is false, OR an auxiliaryhypothesis is false, OR the experimental setup is notadequate.

Challenge: when to reject one of the auxiliary hypotheses or theexperiment, and when to reject the main hypothesis?

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Basics and Harvey againMore examples

The fallacious version of confirming a hypothesis

(1) If the hypothesis is true, then the prediction is true.(2) The prediction is true.

∴ The hypothesis is true.XXXXX

This is the form affirming the consequent, and is invalid!

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Using a prediction not otherwise expected to be true

(1) If the hypothesis were not true, then the predictionwould not be true.(2) The prediction is true.

∴ The hypothesis is true.

This argument is valid, but is it sound?

⇒ We now have to make sure that the first premise is true.

Problem: typically there will be alternative hypotheses (someslight variants of the one under consideration) that make thesame prediction.

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Refining the argument for confirmation

(1) If the hypothesis under investigation were notapproximately true and there is not a plausible alternativehypothesis that is true, then this prediction would be veryunlikely to be true..(2) The prediction is true.

∴ The hypothesis is approximately true or a plausiblealternative hypothesis is true.

If there are not plausible alternatives, then we can conclude ourhypothesis is approximately true.

But what if an alternative is discovered next month?

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Harvey’s alternative hypothesis

Rejecting Galen’s hypothesis, Har-vey proposed that

there is only one kind ofblood, and that

it circulates out from the heartin the arteries and return tothe heart in the veins.

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Harvey’s proposal

Since all things, both argument and ocular demonstration,show that the blood passes through the lungs and heart bythe force of the ventricles, and is sent for distribution to allparts of the body, where it makes its way into the veins andporosities of the flesh, and then flows by the veins from thecircumference on every side to the centre, from the lesserto the greater veins, and is by them finally discharged intothe vena cava and right auricle of the heart, and this in sucha quantity or in such a flux and reflux thither by the arteries,hither by the veins, as cannot possibly be supplied by theingesta, and is much greater than can be required for merepurposes of nutrition; it is absolutely necessary to concludethat the blood in the animal body is impelled in a circle, andis in a state of ceaseless motion.

(Harvey (1628), On the movement of the heart and blood in animals)

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How to secure positive evidence?

Harvey could not see the connecting capillaries. What kind ofprediction could he make on the basis of his hypothesis that would bespecific to it (i.e., not expected on other grounds)?

What would happen if Harvey is right and you permit blood toflow through arteries in your arm, but not the veins?

Harvey predicted and demonstrated swelling of the limbs.

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Harvey’s circulation experiments

By bandaging the upper arm(Fig. 1), the veins swell up.

The valves then show up aslumps (G, O, H).

By pressing a finger along thevein (Figs. 2, 3) from onevalve to the next in thedirection away from the heart(from O to H), the section ofthe vein between the valvesis emptied of blood.

This is because valve O willnot allow blood flow awayfrom the heart.

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It helps to have friends in high places!

Harvey is here demonstrating the heart to King Charles, to whom he was personalphysician. Nonetheless, Harvey lost a lot of patients for his radical ideas.

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Resistance and slow acceptance

“In the olden days patients were healed without theknowledge of the circulation, and that therefore, thisdoctrine, even if true, would be useless.” (James Primrose,1630)

“If Harvey’s idea of the circulation was correct, there wouldbe many concoctions of the blood, an unlikelyphenomenon, especially if one upheld the philosophicalprincipal that in nature, simplicity and perfection remaininviolate.” (Caspar Hofmann, 1636)

In 1637, Descartes accepted circulation, but proposed that the heartwas a furnace that heated the blood.

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Confirming a mental rotation hypothesis

ThesisPeople use mental images and operate on them in problem solving.

PredictionIf asked whether two rotated figures match, response times will varywith degree of rotation.

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(1) If an image is not beingrotated, reaction times shouldnot correspond to degree ofrotation.(2) Reaction times do corre-spond to degree of rotation.

∴ An image is being rotatedmentally.

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Stephen Kosslyn on mental scanning

Subjects memorize thismap with landmarkssuch as a hut, aswamp, and a well.

With the map gone,subjects were asked toform an image of themap. Focus on the welland image a speckthere.

Now move the speck ina straight line to thetree.

More on Kosslyn and his experiments

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http://en.wikipedia.org/wiki/Stephen_Kosslyn



The time it tookKosslyn’s subjects toperform the scancorresponded to thedistance on the map.

(1) If subjects were not mentally scanning and moving thespeck in their mental map, reaction times should notcorrespond to distances between objects.(2) Reaction times do correspond to distances betweenobjects.

∴ Subjects are mentally scanning and moving the speck intheir mental map.

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Hypothesis: perception affected by one’s beliefs

ThesisHuman perception is affected by the subject’s beliefs.

PredictionJudgments of the size of a cardboard circle are quite accurate butjudgments of the size of a coin is affected by its value.

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Jerome Bruner’s results

Size of coinssignificantlyoverestimated

Up through quarter,overestimate larger forlarger valued coins

PredictionPoor children willoverestimate more than rich.

More on Bruner’s classic experiment

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http://psychclassics.asu.edu/Bruner/Value/



Poor children overestimatedthe sizes of coins more thanrich children.

(1) If value did not in-fluence perception, thenthere should be no over-estimates of coin sizes..(2) There are overesti-mates of coin sizes.∴ Value does influenceperception.

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Evaluating hypotheses: the simple and the subtle

In principle the logical evaluation of a hypothesis given evidenceis simple:

If a hypothesis predicts something not otherwise expected,we tend to think it is true.If a hypothesis makes a false prediction, that counts againstits truth.

But most interesting hypotheses in science both make new,unexpected predictions that turn out to be true and makepredictions that turn out to be false.

⇒ No simple recipe for deciding whether to reject a hypothesisonce it has made a false prediction or to hold onto it because ofits record of true predictions.

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Fallibility

Issues in science are seldom resolved by decisive experimentsthat once and for all falsify or confirm a hypothesis.

Hypotheses often take a long period to becomeestablished—and once established, a long period is oftenrequired to overthrow them.

All judgments in science are fallible—subject to revision withmore evidence.

Yet, making the best judgment about our hypotheses isimportant.

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Confirmation, Falsification, Fallibility...Count Rumford’s (aka Benjamin Thompson’s) experiment Friction from boring out a cannon heats the metal, but there is no source from which