The Nature Of Science : Testing Hypothesis (1972)

foreign [Music] much of the work done by scientists has to do with the forming of hypotheses useful hypotheses that help describe what the observed world might be like as an aid to forming such hypotheses a scientist may try to get away from his laboratory and colleagues for a while to a place where he can think undisturbed by outside distractions when it comes time to test his hypotheses however he must subject them to observed data to the world of his senses scientific hypotheses don't gain much respect until they've been tested by observations probably the most familiar way of doing this is by means of experiments experimental procedures vary according to the hypotheses to be tested yet they follow certain basic principles principles which we can illustrate with a classic series of Investigations done by Louis Pasteur and others during the 18th and 19th centuries one of these studies had to do with samples of broth made up of meat or vegetable juices when such a broth was left to stand exposed to the air for a few days it often became clouded with swarms of microscopic organisms bacteria yeasts and moles some investigators of pastors day hypothesized that these living organisms formed spontaneously from the non-living broth other scientists including Pasteur hypothesized that the organisms formed only from other living organisms broth simply provided nourishment testing this latter hypothesis required an identification of the variables involved one variable was whether organisms did form or not in a broth after it had been left to stand for a few days a drop of the broth under a microscope reveals some of the many organisms that were sometimes found the forming of microorganisms according to the hypothesis depended on another variable the prior presence of living organisms in the broth from which new ones might grow to test this hypothesis there had to be some way of getting broth that already contained some living microorganisms and also some broth that did not but how could a scientist know if he had broth that did not contain microorganisms early in the experiment any organisms present would be very few in number they would be difficult to see even with a microscope and almost impossible to find how then did Pasteur test his hypothesis how did he test to see if microorganisms had to be present before new microorganisms would form what sorts of experiments could he do one experiment was set up with two flasks of broth the liquid in one of the flasks was boiled it was assumed that boiling would kill any microorganisms that might be present microorganisms in the unboiled broth would not be killed a controlled experiment the heated broth was allowed to cool and both flasks were left to stand for a few days the result organisms formed in both flasks their appearance in the unboiled broth could have resulted from other live microorganisms but what about in the second flask perhaps scientists reasoned after the broth was boiled additional organisms may have fallen into the liquid from the air to control for this possibility another experiment was devised in which the broth in both flasks was boiled outside air was allowed to enter one flask without any sort of special treatment but the air leading into the other flask was passed through a heated tube to kill organisms that might be present in the air as well as in the liquid a more careful control of the conditions so that live organisms might be present in one flask but not in the other after the liquid had been boiled for several minutes the burners below the flasks were removed allowing the flasks to cool however the burners were kept on below the metal tube to kill organisms that might be present in the air under these conditions organisms formed as before in the broth that had been exposed to unheated air but in the flask receiving heated air no organisms appeared this seemed to show that heating broth and air killed organisms more important it seemed to show that organisms could not form from air and broth alone but did it Advocates of spontaneous generation argued that heating air did more than just kill microorganisms it tortured the air they said destroying its regenerative properties in heating the air an unwanted variable had been added to control this variable still other experiments were done instead of passing the air through a heated tube it was passed through a cotton filter the idea was to remove organisms from the air without heating it or otherwise altering it with this Arrangement both flasks were allowed to cool and stand once again after a few days organisms grew as before in the broth that had been exposed to untreated air but in the flask of filtered air no organisms appeared this seemed to back up the hypothesis that organisms form from other organisms at least for a particular kind of broth when this experiment was repeated with other kinds of nutrient materials however conflicting evidence was found in some cases no microorganisms appeared in other cases microorganisms did appear as a consequence Pastor was led to perform many more intricate and refined experiments as he sought to confirm his hypothesis that microorganisms derive from other microorganisms in the process he established principles of experimental procedure stating hypotheses clearly in terms that can be tested isolating variables performing controlled experiments to find out the effects of unwanted and unknown variables these principles of procedure scientists continue to follow in their investigations today you follow these principles or should follow them whenever you test hypotheses by experiment experiments are generally the surest safest way of testing hypotheses because conditions can be set up to control variables being tested and to reduce interference from unwanted variables it's not always possible however to test hypotheses by experiment for instance in studying the origins of living things we come across hypotheses which cannot be tested experimentally a classic example has to do with an idea held by many people in the 18th century that all species of living things were created at the same time all plants and all animals were thought to have been created at the same moment in history scientists of the 18th century however were finding fossils of unknown species of plants and animals in layers of rock believed to be millions of years old they were not finding fossils of known present-day species in these same rocks such observations LED some scientists to hypothesize that there had been a gradual evolution of living things Through the Ages how could they test this hypothesis they couldn't very well set up a controlled experiment the changes believed to have taken place in nature occurred over an interval of millions of years these changes were believed to have been influenced by many natural occurrences changing levels of the Seas volcanic eruptions and earthquakes and changes in climate alternating between ice ages and warm tropical ages scientists could never hope to recreate such changing conditions on the massive scale of nature the only way they could test their hypothesis was on the basis of naturally occurring evidence to do this they made a prediction they predicted that if there has been a gradual evolution of living things then the simplest organisms should be found in the oldest rocks and the most complex organisms should be found in the newest most recently formed rocks they predicted also as a consequence of the same hypothesis that there should be a pattern of evolution showing a series of gradual successive changes in the structures of organisms found examining the evidence inspecting thousands of different fossils from various ages they found that the evidence supported their predictions example after example was found that seemed to connect ancient plants and animals through a series of slight successive changes with plants and animals of modern times on the basis of millions of bits of such evidence scientists throughout the world became convinced that there has been a gradual evolution of living things from one species to another today most scientists accept this as an accurate description of what has happened although no experiment has ever been performed to substantiate this the belief in evolution is based entirely on naturally occurring evidence such evidence is used in modern science to test other hypotheses that cannot be validated by experiment astronomers observe natural events in distant galaxies to test hypotheses relating to the formation of stars and planets hypotheses about effects of living conditions on life expectancy are also tested using naturally occurring evidence the same methods are used to find out how people are influenced by the social groups and organizations they belong to you use naturally occurring evidence whenever you go on a field trip to learn about scientific hypotheses or to test those you've read about naturally occurring evidence and experimental evidence these are the kinds of data relied on by scientists to test hypotheses to see how closely they fit The observed world and to suggest new tests that will hopefully lead to revised more useful hypotheses in the future [Music] [Applause] [Music]