I ROBERT S. MULLIKEh The U n i w ~ s n yof Chicago Chicago, lllinais 60637
Interview with Robert S. Mulliken NORMAN H. NACHTRIEB The Universw of Chicago
. : Chicago, Illinois 60637
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Nachtrieb: Let me start out bv askino. Robert. about vour earlv life m Newburyport. Massachusens? Muttten: The oooulation rs almost me same nowas in 1800 /about 15.000): it's three miles from the ocean and a very nice place to be brought up. Back in 1800 it was one of the larger seaporrs, as we# as a center of ship building, and with commerce a1 over the world. Those were the days of sailhg ships. My grandfather was a ship captain. h fact, he went to sea instead of going to college as his father had h m . Nachtrieb: But your own father was Dr. Samuel Mulliken. I know this because when I was a student, the best book I hadon the subject of organic quaiitative analysis was one called ''A Method for the Mentification of Pore Organic Compounds" by S. P. Mulliken. That was your father. Mulliken: Yes, and I read the proof of those volumes (there were several), so that organic chemical names were not frightening to me. He was a member of the M.LT. facully. We were within long commuting distance of Cambridge. Nachtrieb: So it is quite natural that when you began to think of college you went to M.l T. Mulliken: Yes. also there was a soecial fund called the Wheelrioht Fund, which provided for young men from ~ewburypogto go to college, especially to M.I.T. And in high school there was a very good scientific course in preparation for this. The course included ohvsics, mathematics, biobov. French, German and En&sh. iaddeda year of Latin asan extra. Nachtrieb: You got your Bachelor's degree at M.I.T. around 1917. Was that in Physics or in Chemistry? Mulliken: in Chemistry. in those days hardlyany American students took a degree in physics. Physicists were a rarity. Anyway, chemistryseemed to be the sublect, although at one time I looked into chemical engineering and found it quite enjoyable. One summer I spent with the Chemical Engineering Practice School. We worked at a number of chemical installations in Massachusens and Maine. We spent about a week at each place. At that time the First Worid War was in its conchding stages. Aftergraduation if seemed to be a question of what was appropriate. i went into a laboratory for making poison gases under the direction of James B. Conant. We had samples of every kind
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by Norman H. Nachtrieb
known to man, in one small laboratory, at the American Universw in Washington, D.C. Nachtrieb: The laboratory was part of the Chemical Warfare Service? Mulliken: It wasn't so at first but afterward the personnel were transferred to the CWS under Lt. Conant. I was inducted into the CWS as a private, and later was promoted to private first class. I didn't do much of anylhlng because i indiscreetly spilled mostam' gas and later got the flu. Soon after, the war was over. Nachtrieb: What are these books on your desk, Robert? Mulliken: They have, you will see, a highly personal quality and are part of a family heritage. This is a book which first belonged to my grandfather. Here is his signature. This book, "Conversations on Chemism, " went through n u m s editions, this one dated 1830, and two others with the same title dated 1814 belonged to my great aunts. The original was published in England. The author explained that she hadgone to some lectures by Humphrey Davy at the Royal Institution, but didn't understand them very well until after several discussions with a friend. Then she dec M , h e / ( to get out a book which would prepare other people to understand the lectures. The book takes the form of "conversations" between "Mrs. 6.'' and three young ladies. Later my father came across "Conversations on Chemistry", and wrote his name, dated 1879. next to his father's. Times had changed, and he took chemistry seriously. He and Arthur Noyes, the wellL!mown physical chemist, also a native of Newburyport, were boyhood friends and they used this book as a reference to do experiments. Nachtrieb: Before they were really introduced to this subject in school? Mulliken: Oh yes. i don't know whether they learned anything about it in school but they did experiments at home. First, in my grandfather's house and then in the Noyes' home. Not much different from any youngster nowadays with his first chemistry set. Later. both Noyes and my father went to M.I.T. for a BS, andafter that both of them went to Leipzig fora German PhD. Nachtrieb: You got your Bachelor's degree in chemistry at MILT. around 1917. What were the events that led you to the University of Chicago?
First, let's return to my high school days. Imuch enjoyed reading about the recent developments h science. Ithink there was one book in particular by Robert Kennedy Duncan. All this must have been the background for my graduation essay on "The Electron. What it is and What it Does." I was the class salutatorlan, this role being awarded to the class member with the second best record. Much new had recently been learned about the electron, and it seemed to provide an especially interesting subject at that time (1913). so Ichose it as subject for my graduationessay. Nachhieb: So for 61 years the electron has been the thing that has intriguedyou most among scientific subjects. Mulbken: h a way, yes. Ithought that after the electron the next thing to understand should be the nucleus and Professor W. D. Harklns at the University of Chicago seemed to be the only person in this country who was interested In the nucleus. That Is why Icame to Chicago, although there was a brlef interlude with the New Jersey Zinc Company. Idid some analyses with obvbus reluctance so they put me on a little research topic of compounding rubber, putting zinc oxide and carbon black into rubber to see how it would behave. Then Icame to Chicago. Nachtrleb: If Iam not mistaken, for your PhD dissertation you did the first work on the separatbn of isotopes. Dempster, In Chicago's Department of Physics had been one of the first to show that there were lsotopes. AMwugh Iguess S M y had done some work about the same tlme, but Harkins had assigned you the problem of separating the isotopes of chlorine. Is that right? Mulliken: No, as a matter of fact Iwas on the mercury problem. Someone else was and had been working with Harkins on the chlorine isotopes. But Bronstedand Hevesy in Copenhagen had obtainedsuccess In partlally separating mercury lsotopes, and what Idid was at first not a great deal more than a repeat of what they had done. Soon, however, Idevelopedthe matter further. Nachtrleb: How much of an enrichment of mercury, or how much of a separatbn of the isotopes were you able to accompkh? Mulllken: Oh, a great many parts per milfion of density change, perhaps as hgh as 100 w 150, much more than enough so that a good density determination couid show separatbn. Harkins had a pycnometer and Icould fill it wlth mercury and get the density accurate within 1part per million. h various experiments Ifound out several Interesting things. For example, Ifound that in every ordinary distlllation of mercury you are llkely to get a considerable sepa10, 15 or more parts per millbn. If you boil ration-5, down the residue to a very small volume, the resulting small quantity of heavy fraction will show quite a considerable enrichment. These effects are maximized by having a dirty surface on the mercury. Such a surface acts as a dfision membrane. Nachtrieb: Here is an interesthg variatbn! An impurity, a thin layer of dross on the surface actually constituted a diffusion banier allowing some isotope separation. Mulliken: Yes. So, since then in practice the density of mercury during purification by muitiple distillatbn Is not very well standardzed. Every time mercury Is Fractbnally distilled there is Inevitablysome separation. Nachtrleb: The unbelievable part is that this preceded, by some 35 years, the barrier layer separation of uranium isotopes during the Manhattan District work. Mulliken: Iproposed and tried experimentallya method ofseparating isotopes by centrifuging; however, Igot no separatbn, concluding that the centrifuge was too crude. My centrlfuge seemed to be a better shaker-mixer than separatbn for isotopes! Nachtrieb: Was this work done during the days of your PhD research? Mulliken: No. afterward. Also afler the doctoralresearch. 1buiit the Mulliken:
first mercury isotope factory. The mercury was subjected to a successbn of units. The combined process gave 140% efficiency over that of a single process. The first process was an ineversible evaporatbn from a flask. The vapor then rose through the Inside of a long diFFusion membrane made of finer mPer with edges sealed together with sodium silicate. ThsvaPor whicn diffused through the membrane and the undiffused residoe were separately condensed. Thus the resuits of two separation processes were added together. Neither one was 100% efficient, but the overall efficiency was greater than 100% for a single process. There are the basic chemical engineerlng principles ofa multiple stage separation. Nachtrleb: Ithink this must come as a surpfise to many that Professor Mulllken was an experimentalist to begh with. Now what turned you away from a very promising beginning In experimentalphysicalchemistry7 Mulllken: Well, Ican't really claim to be an experimentalist. Ifelt at the tlme that Ihadjust barely gotten a grip on experlmental work. Neither was Ia proper theorist. Iguess Iam a middleman, or an interpreter. Really, my first experience at what was supposed to be research was with ProFessor Noyes at M.LT. approximately during my junior year. He wanted me to do something with cobalt complexes. Idid look up a vast amount of literature but in the end he was extremely disappointed. 1 didn't seem to be very experimentally minded. h fact, he thought my prospects were very dim. Nachtrieb: You mean as an experimentakt? Mulllken: As a matter of fact as a research sclentist. You can see that Ifelt a little better after working on those isotopes of mercury. But at the same time Itrled to think of allsorts of ways of separating isotopes and wrote several theoretical articles on that subject Iproposed some methods of centrifuging (in spite of my mercury experience) which if Iam am not mistaken are now actuallv . beino used or about to be used, on a large scale. Professor Beams developed the centrifugal methods in ~ractlcevery skillfully. Ieven considered; photochemic.& method ofseparation, but did not publishanything ~.on that. Nachtrieb: Your work was obviously of very high callber. Were you not awardeda NationalResearch Councilfellows hi^? Mulllken: After receiving my PhD in 1921, 1did become a National Research Fellow at Chicago and worked on isotope separation. But when Itried to get the Fellowshlp renewed, the Board mid me that for a renewal Imust work in some other field and go elsewhere than Chicago. Iproposed working .wlth Rutherford In England, but the Board said I was too ignorant of the necessaryphyslcaltechniques. So 1proposed working at Harvard on isotope effects in band spectra. Ithink that it was probably a suggestion from my friend, Norman Hilberry, which led to to an acceptable project. Iconcluded that In the electronic spectrum of a molecule containing an isotopic atom there shouM be an isotope effect. One shouid be able to see the isotopes separately in the spectrum. This proved to be the case when Ilooked at the spectrum of boron nitride. There was an abundance of Isotope bands. Nachtrieb: That is, the boron isotopes, O ' B and "B? Mulliken: Yes. They hadn't been noticed before. Some work had been done before on isotopes of chlorine h hydrogen chloride in the infrared, but not on electronic spectra. This IS the way /got "diverted'lnto the field of electronic spectra. Further, Ihad eagerly read the papers of Lewis and Langmuir on valence theory in the Kent Laboratory library. Nachtrieb: When was it. then. that you went abroad to Mttingen and Copenhagen? Iremember reading of some kind of a literature "hassle" involvingboron spectra. Mulliken: it was the summer of 1925 which Idevoted to this trip abroad. Sam Allison, a student of Harkins. went on the same trip. Incidentally. Alllson and Pan Jenkins had foC iowedme to Harvardfrom Harklns' lab.
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The choice of boron nitride as the first spectrum to lo& at for isotoDes tumed out to have been a very good thing for an extraneous reason. lt was obtained byJevons England by letting boron chloride into active nitrogen. Now, active nitrogen proved to be a very fine source for a variety of spectra, so we obtained many spectra using it. The nitric oxide spectrum, for example, is beautifully obtained by having a linle air impurity in the nitrogen. The "hassle" was a controversy with Jevons. I said that the source of the spectrum was actually boron oxide and not boron nitride. He said "Of course it is boron nitride. You put boron chloride into active nitrogen, what else can it be?" But I said the isotoDe effect theow shows that it is oxygen not nitrogen. At that time Kemble at Haward and Birge at California, and I believe Leigh Page were wriffing a report on molecular spectra. I got into correspondence with Birge and conversations, of course, with Kembie. I wrote a letter to Nature and Jevons wrote replies about this boron oxide/boron nitride controversy. 1 also was in communication with Birge. He said, "Why don't you write to Jevons instead of writing to Nature?" So Idid. Nachtrieb: Andavoida polemic in the literature! Muliiken: When I visited Europe in the summer of 1925, 1 must have written a lot of letters to many people to arrange for visits. i visited practically all the people who were working actively on molecular spectra, and also the best known people working on atomic spectra, such as Fowler in England and Paschen in Germany, also Bohr in Copenhagen. As you know, my deductions were correct, not Jevons: When I got h touch with Jevons, he toM me that it wasn't his idea to write those letters to Nature, but it was the Head of the Physics Department who said he shouM do it-E. N. deC. Andrade, at that time at Woolwich Arsenal, London. Nactrieb: Then your travels took you to Gattlngen, or was that at a subseouent time? Mulliken: My first visit to Giittingen was at that time. Franck and Born were active there. You miaht find some other interactions with well known peopli of interest. h England I visited Oxford and at Cambridge talked wiih C. P Snow who was doing research there. At London, Jevons introduced me to Lord Rayleigh who had done very much on active nitrogen. At Oxford I met T. R. Merton and many others. Merton was half spectroscopist and half country oentleman. Nachtrieb: Rayleigh must have been a very impressivegentleman. Mulliken: Reasonably so. Now let me add somethino about Lord Rayleigh. The one I met was the son of the e&cially well known Lord Rayleigh. He was called "Strutf" before be became Lord Rayleigh. At that time he did most of his work on active nitrogen. Nachtrieb: h your visii to Gottingen. you met Heisenberg; and I suppose, Hund and Pauii? Muliiken: I am not sure Heisenberg was there then. Max Born and Franck, Oldenberg. Fri. Sponer, and many others were there. A h Kondratiev and Semenov from Russia. Frk edrich Hund and I talked very extensively and developed many things together. He had the quantum mechanics before I knew much about the subject. He could interpret in quantum mechanical terms some of the things that I had M e d out pretty empirically with the he@ of the OM quantum theory. Nachtrieb: You and he developed what is now called the Hund-Mulfiken Theow. I wonder if vou miaht comment on some of the broader concerns of science. What wouM you think is most characteristic of science? What sets it apart from other kinds of intellectual discipfines, or other human occuoations? Mulfiken: h e answer is very simple, and I don't understand why people don't know about if. People in general don't seem to realize that science is basically honest and scienrists are looking for the truth. At least insofar as ii is possible to
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define truth. . . scientific truth is the truth about nature, including men as pai? of nature. Psople seem not to know or maybe understand that the first rule of science is to be completely honest. Nachtrieb: It takes a different kind of temperament, it seems to me, to be a scientist. At least, one has to acquire this temperament. One needs great patience; one needs a great deal of courage to undettake a foray into an unknown area. You're a l m s t always convinced that what you know at this moment isn't the whole truth, too. Isn't that SO? Mulfiken: That is always hue. lt's incomplete. Oh, yes, it is necessary not to be discontented with not knowing everything, with not being absolutely certah. I wrote somewhere about how I learned about being careful. That is part of the thing, being exceedingly careful. That attribute i learned in the study of a subject which I did not hke at all . . . quantitative analysis. I found it was necessary to be sure and not lose a drop of anything. This means great patience too, as well as care. i reaiized afterwards that I learned something . . . discipline, very rigorous discipline was necessary. But so it is for the best success in any kind of endeavor (business, ballet dancing, what not). Nachtrieb: Wouid you characterize science as being a conservative kind of subject, or is it for the lack of a better word, a kind of revolutionary attitude that is required in science? Or, is it something of both. Mulliken: Both. Conserve whatever has been found to be true, but be willing to entei?ain the most radical revolutionary new ideas about how things might be understood and explained. This, of course, was illustrated in Einstein's relativity, and in quantum mechanics, and now in astrophysics and in high energy physics there ere many developments which are a great strain. A great strain on understanding. We see that there is much there that we don't understand. Nachtrieb: How much does intuition play a part in the development of scientific theory. would you say? Mulliken: My idea of intuition is that it is merely an excsedingly careful. . . an excruciatingly detailed cross-examination of all the facts. Nachtrieb: So it's a kind of distillation of what you know, perhaps not proved, but based upon the best evidence at hand. i think the average person, not the scientist let's say, commonly thinks of a scientist's way or proceeding as going very logically from step to step. rarely ever making a mistake because he knows the goal is predictable. Almost as though it were a lock-step . . . almost a mechanical process. Mulliken: I wouid certainly not agree with that as the way scientists proceed. Whether it corresponds to the average person's opinion I do not know. There are many times which are very discouraging when something doesn't work. . . when some idea doesn't work. This is true in experimental as weil as theoretical science. if it's the application of weilestablished principles such as the Schrdinger equation. then it is a matter of first thoroughly understanding the theory and then being careful in using it . . . not making mistakes . doing it right, And solving a variety of practicaiproblems, oflen dfficult. But if it is the matter of finding some new fundamental principles that is an entirely different matter. A little treatise by Dirac has some interesting things to say about the subject: "Development of Quantum Theory" by P. A. M. Dirac. Nachtrieb: You must have read Thomas Kuhn's book on the Scientific Revolution and how it takes great courage to work outside the p a r a m m established in a given scientific area. That is certainly true, isn't it? Muiliken: Yes, but also it is very hard to get away from habits. For instance, atthough i dealt with the boron isotopes in 8 0 , it just didn't occur to me that oxygen too might have isotopss. Later / looked at the spectrum of oxygen gas (0~) and gave a new and valuable ex~lanation,but there were
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some weak parts of the spectrum which I couldn't explain. But the explanation became obvious as soon as Giauque and Johnston pointed it out. I had always thought of oxygen as ' % Oand it didnr occur to me that " O and could also be present. These isotopes explained the weak parts of the @ spectrum which /didn't explain. Nachtrieb: What soti of a social or political environment. in a national sense, do you think is conducive to the development of science? ls democracy a necessary kind of milieu in which to have science flourish . . or would a fascist or a communist regime be just as comfotiabie a place for a scientist to develop in? Mulliken: Democracy is naturally better, but it is not impossible, except when some dogmatism gets t w strong, for science to develop, as we see in Russia. But I think that the democratic environment is a natural home for freedom, the freedom of thought, the freedom of dlscussion and so on which permit the scientist more rea&ly to bring out his views. Nachtrieb: Robert, I would like to ask you how important models are in your view of the development of science . . . mechanical modsls, a picture . . . a physical picture of sornething that goes on in nature. Mulfiken: 1 never fiked models. A model is often an approxinWdion . . . a hypothetical approximation of some sort. Well I never liked models, but this is probably to a conskierable extent a case of puritanicalperfectionism. Nachtrieb: But that isn't to say that one doesn't have a kind of mental picture of what physically transpires in a process at the atomic level. MuHiken: Well, to begin, in some fields of study where things are in a very primitive state, some kind of a model is necessary which m y not be taken very seriously as to its fundamental significance. In high energy physics people are talking about quarks. I am not sufficiently familiar with the field to know whether they should be fisted under the heading of "model" or whether there is enough evidence that they are real. Nachtrieb: Quarks . . . some fraction of what we have supposed to be the indivisible electron charge . . . I notice on your blackboard some things I would call models. . . hexagons. They represent benzene rings. In a sense they are models, aren't they? Mulliken: I think we could say they are also approximate pictures such as an electron microscope would reveal. They might also be classified under the heading of language. They are symbols, just as a wordis a symbol for some idea. Nachtrieb: You have written recently on some views you have about the world's human population. I wonder if you would fike to comment on population growth and what we could do about it . . . whether there is much hope that we can do very much . . . about controXing the size of the human population on the earih. MuXUen: lt has been and still is very unfashionable to talk about things that shwM be done. lt is much more fashionable to say that it is impossible to do something about some of the very serious problems. But this population problem is one which will blow us up. it is a population bomb unless we do something about L But how do we do anything abour it? lt seems that an Indivkfual can't do very much. If some people work on it . . . talk about i t . . present some Utopian goal, we can hope that a solution is achieved before something more catastrophic happens. Of course. eventually it will be straightened out, by catastrophe if no other way. There is an inevitable growth which will bring us to. let us say. 7 billion in the year 2000. We can't do much to prevent something fike that, so I take that as a starting point and say that we ought then to get gradually back to 1 binion, at the most, for the total world population. We have, amwrg others, the problems of fimited resources and pollution . . . problem for a long time ahead if we are interested in our descendants. Of course many
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people are not. Nachtrieb: What would say to a young person starting in science today? Is it a good vocation to enter, w is it one that is unusually uncertain? Is it a field that has as many bright prospects now as you thought it had when you decided to become a scientist? Mulfiken: I don't see why not. I don't see why it is not. The world is full of thinos to learn Of course you may . imaqine . that in time, it might be a few hundred years or a thousandyears, that we miqht know so much that we won't want to know much more. Nachtrieb: Do you think there is a Droner balance given today in science to the expioration of scientific fundamentals as compared with the application of science to technological .. prodiems7 Mulfiken: It is very hard to know just what is the proper balance. The amount of basic science is not such a large fraction. on the whole. When I read about Research and Development I feel rather unhappy about the lack of general understanding of the difference between science and technology. . . the lack of general understanding. On the other hand, from pure science to applied science there is no sharp boundary. Often times our impulses to go into some fisM of science arise from practical problems. Anyone who acts as a consultant finds that there are many practical problems which would beneM from more fundamental understanding and which may fall in the realm of pure science. Much of what is probably properly ca1W basic science is, from a more fundamental viewpoint, just applied science. All of the calculations that are made about molecules are just applying the appropriate wave equations to chemistry. lsn't this applied science? Thus, any architecture, whether molecular (atoms) architecture, or housing (bricks and steel) architecture . . . is a combinstion of basic and applied Scientists should have mutual respect, regardless of their areas of interest. ANproblams whether socalled basic or applied, can be interesting. Nachtrieb: Once you get Into problem, virtually any problem, it should be interesting. Mulllken: That is true, once you get into it! Let's look at the question of garbage, as an example. Too few scientists have concerned themseives with the problem. Nachtrieb: Garbage appears to be more than a few quantum jumps from the "Hund-Mulfiken Theory". lassume you are concerned with energy If properly exploited . . . conservedand used it would help in our present energy need. Mulliken: Yes, contributions to the energy problem . . . also contributions to the raw materials problem. And there are also negative contributions of what to do with aX this stuff. . . pollution and environmentalproblems. Nachtrieb: As you l w k back over SO years of contributions to quantum mechanics . . and they are enormous . . . how near do you think we may be, or are we at all near to the ~ossibifityof calculating the .properties of molecules and . Umir reactions with one another so that the experimental scientist might not have to go into the laboratory to determine the empirical condirnns for a reaction? Mulfiken: Chemists are only beginning to be aware of the possibilities of using machine computations as an alternstive to experiments, and increasingly in obtaining results for systems (e.g. radicals) where experiment is more difficult w nearly impossible. Computations on molecular wave functions show the intimate structure of many molecules ieading to better understanding of their reactions. But also from the computed wave function many of the prop&ies of molecules (or radicals) can easily be computed, e.g. di001s and ouadru~olemoments, electric and maonetic susceptibifities, polarization properties, and spectroscopic constants. Prowess is beina made on ~otentialsurfaces in chemical reactions, in short, paths and energy in reactions. Although what has already been done is substantial. it represents only a very small fraction indeed of what can
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and will be done. I envisage hundreds or thousands of computing chemists using computations in fieu of experiment. However, in experimental work t w , computing machines are being and will be used more and more to take care of many of the chores. Nachtrieb: What do you think are the great areas that should r e ceive scientific aitention in the fuiure? Could you mention one or two of these? Mulliken: Well. i have always said that psychology has a great future. I mean social psychology. Of course that would be very much an applied science . . . applM to human activities . . . and then there is political science. Oh. if we could make pohics more scientitic! But i think we are making some progress in . . . for example, this plan of having scientists anached to congressmen. it could be an internship with young scientists so anached. Politics and even psychology are really muitivariant discipiines compared to chemistry or physics. where one has the experimentai situation rather well under controland can even run controls. These end other social sciences are in a very different category from the natural sciences. They are areas where the uncertainty is greater than the certainty. In other words, the probable error of the thing or idea one is trying to quantify may be greater than the thing itself.
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Nachtrieb: Because the "noise level" is high it doesn't mean that one should write off the study as something that is not worthwhilejust because it's difficu#. Mulliken: I agree! If is a great mistake for sociai scientists to iry to coby the me&s of physical scientists and biological scC entists . . . just to copy the same methods and criteria for quantitative accuracy. Nachtrieb: Chemistry occupies a rather interesting position in the sciences. So many things are derivative from chemistry and the roads cross from one science to another through chemistry. For example, asironomy, these days, has its chemical input. The interaction of chemistry with biology is an area which is well secured and weli on the way to im prove man's lot. Mulliken: This is true but the ouestion of what is imwrtant is almost impossible to answer. if you look at the universe it doesn't appear that life is very Therefore, why is bioio.. . important. . gy important and in particular why is human life important? Nachtrieb: But you obviously have faith that they are vitally importani! Mulliken: Well, let's say they are interesting. . . naturally to human beings.
