268 I ATURE FEH. 18, 1939, \ 'oL. 143 Natural Philosophy and Human Cultures* By Prof. Niels Bohr, For. Mem. R.S. J T is only wiLh great ho.~ itai i on that I haYe for our general outlook i" common!.\' realized. it accepted a. kind invitaLion to address t his is scarcely yet so as rogardH tho unsuspectt'd assembly of distinguished reprcsenlati,·cs of tho epistemological lesson which tho opening of quite anthropological and ethnographical sciences of now realm s of phyHical ro::;oarch has given us in which I , as a physicist,. have of course no first-hand tho most recent years. Our penetration into the knowledge. Still, on this specia,l occasion. when world of atoms. hitherto closed to t.he oyo of man cYon tho historical smroundings speak to everyone is indeed an adventure wh ich may bo compared of us about aspoct.s of lifo other than those discussed with the great journoyil of discovery of the aL tho regular congress proceedings, it might. circumnavigators and t h(' bol<l explorations of perhaps bo of interest to try w.i t.h a few words Lo astronomers into t. ho depths of colosLial space direct attention to the epistemological aspect of As is well !mown. the man·cllou development of the latest dc,·elopmont of natural philosophy and tho art of physical experimentation has not only it s bearing on general human problems. Notwith­ romO\'Od t ho last traces of tho old holiof that the "tanding t ho great separation between ou r different coarse n r~s of our ~o;cnsrs would for over prcren hm nchcs of knowledge, the new lesson which has us from obtaining direct. information nbou hcon impressed upon phy,;icists regarding t.hc i.ndi,idual atoms, hut has o,·cn shown us that the raution wit h which all usual com·entions must be atoms themsel>cs consiHt of still smaller corpu>clel appli ed as soon as we aro not concerned with which can be isolated and tho properties of whiti o,·eryday oxpericnco, may, indeed, bo suited to can be innstigated separately. At the same time remind us in a noYel way of tho dangers, well wo h<n·o. howc\er. in this fascinating fi~ld of known lo humanists, of judging from our own cxporioncc been taught that the la.ws of Xatme I point of view cultures developed within other hitherto known, which constitute the grand i societies. edifice of classical physics, aro only valid wbell Of course it is impossible to distinguish sharply dealing with bodies consisting of practically • between natural philosophy and human culture. infinite numbers of atoms. The new knowledgt Tho physical sciences are, in fact, an integral part concerning the bcha,·iour of single atoms and of our oivilizat.ion, not only beca.uso om· over atomic corpuscles has, in fact., revealed an UJI. incrca. ing mastery of the forces of Xa Luro has so expected limit for t ho suhdi,~i s ion of all physilll C'O mplet r l.'· changed the material conditions of act.ions extending far beyond t.ho old doctrine of l·•• lifo, but a lso because the study of t hose sciences t ho limited di,·isibility of m.altor, and ghing err11 I,. has contributed so much to clarify t ho background atomic process a peculiar individual chnracl« IJ .• of our own existence. \Vh at has it. not meant in This discovery bas, in fa,ct, yielded a quite new ,..•• '.. this respect., that we no more consider oursoh-es basis for the u ndorstanding of tho intrinsic stability a;; pri\Tilcgcd in linng altho cent ro of the uni,·er ~e. of atomic structures, which, in t.ho last ralOrl, surrounded by less fortunate societies inha.biting condit.ions the rcgularit.ies of a ll ordinary experiellf8. the edges of t.he abyss, but t hat t hrough the How radical a change in our aLt itude towards 1M deYclopmonl of astronomy and geography wo have description of Nat uro this dovolopmont of at011110 realized that wo aro all sharing a sm<tll spherical physics has brought abou t is perhaps most clearly planet of the solar system, which again is only a illustrated by tho fact that oven tho principle Kma ll part of still huger system s. How forrofnl an causali ty, so far considorocl as the unquestioned admonition about tho relativity of all human foundation for all interpretation of natural ph8110o judgements ha,-o we not also in our days received mona, has proved too narrow a framo to embrue t h.rough tho renewed rcYision of t.ho presuppositions t ho pecu I iar regularities governing indi\id nndcdying tho unambiguous uso of oven our atomic proces ·es. Certainly C\7eryono will most elementary concepts such as space and limo, stand that ph_picisls haYe needed Yery which, in disclosing t.he osson t.ial dopondcnco of reasons to renounce tho idea.! of causality every physical phenomenon on tho point of view but in tho study of atomic phenomena. we of tho observer, has contributed so largely lo the repeatedly been taught that questions which unity and beauty of our whole world-picture. believed to have received long ago their \ Vh ilo the importance of t hese great achiovomonts answers had most unexpected surpri ·, • \duress giwn at Kronborg Castl<· during tlw cxcuf'oi on to EJ,ino r<' ;;tore for us. You will smely all have of t he int.cmntional Congress of An thropological and Ethnoiu~ical ~cirncr•, ! ·oJ)('nhngen, .\ ugu<t , 1938. about thr riddlcs regarding the most clem~n Xo. 3616, FEB. 18, 1939 J\ATURE 269 properties of light and matter which have puzzled interacting wit h t he objects also enter essentially. physicists so much in recent years. Tho apparent This last fact gives tho straightforward explanation contradictions which we have met in this respect of the apparent contradictions which appear when are, in fact, as acute as those -whi ch gave rise to results about atomic objects obtained by different the developm ent of tho theory of relativity in t he experimental arrangements are tentatively com­ beginning of this century, and have, just as the bined into a self-contained picture of the object. latter, only fo und their explanation by a closer Information reg<trding the behaviour of an aamination of the limitation imposed by the atomic object obtained under definite experimental DeW experiences thcrnselYcs on tho unambiguous conditions, may, however, according to a termino­ ue of the concepts ontoring into t he description logy often used in atomic physics, bo adequately ahhe phenomena. While in relativity theory the characterized as complementary to any information decisive point was tho recognition of tho essentially about tho same object obtained by some other different ways in which observers moving relatively cxpori.montal arrangement excluding t ho fulfilment to each other will describe the beha,~iour of of tho first conditions. Although such kinds of pren objects, t ho elucidation of the paradoxos information cannot be combined into a single i atomic physics has disclosed tho fact t hat the picture by moans of ordinary concepts, t hey m&'mmla.ole interaction botwoen the objects and represent indeed equally essential aspects of any lbe measuring instruments sets an absolute limit lmowledge of the object in question which can bo to the possibility of speaking of a behaviour of obtained in tlus domain. It is t he recognition of iom.ic objects which is independent of tho means such a complementary character of the mechanical i observation. analogies by which one has tried to visualize the We are here faced with an epistemological individual radiative effects which has, in fact, led JIOblem quite now in natural philosophy, where to an entirely satisfactory solution of the riddles •description of experiences has so far boon based of the properties of light alluded to above. In the 1p011 the assumption, already inherent in ordinary same way, it is only by taking into considera­ lllventions of language, that it is possible to tion tho complementary relationship between the sharply between the behaviour of different experiences concerning the behan our of and tho moans of observation. This atomic corpuscles, that it has been possible to •mpt1on is not only fully justified by all eYcry­ obtain a clue to tho understanding of the striking experience, but oven constitutes tho whole contrast botweon the properties of ordinary of classical physics, which, just through t he mechanical models and the peculiar laws of of relativity, has recoi,'ed such a wonderful stability governing atomic structures which form • J)letiion. As soon as we begin to deal, however, tho basis for every closer explanation of t ho phenomena like individual atomic processes specific physical and chemical properties of matter. duo to their very nature, are essentially Of course I have no intention, on this occasion, •muled by the interaction between t ho objects of entering more closely into such details, but I question and the measuring instruments hope that I have boon able to give a sufficiently for tho definition of tho oxporimontal clear impression of tho fact that wo aro horo in wo aro forced to examine more no way concerned with an arbit rary renunciation tho question of what kind of knowledge as regards t ho dotttiled analysis of tho almost obtained concerning t hese objects. In this overwhelming richness of our rapidly increasing we must, on one hand, realize that tho oxperionco in tho realm of atoms. On the contrary, of overy physical experiment-to gain know­ we havo to do with a rational development of under reproducible and communicable con­ our means of classifying and comprehending new •&-teaoves us no choice but to uso everyday experience which, due to its very character, finds eventually refined by t ho terminology of no place within the frame of causal description physics, not only in all accounts of t he suit.ed only to accow1t for the behaviour of objects llll'llction and manipulation of tho measuring so long as this behaviour is independent of tho but also in tho description of the means of obscn'ation.