At Göttingen, Jordan was an assistant to mathematician Richard Courant and later to Born.

According to Max Jammer (The Philosophy of Quantum Mechanics, p. 161), Jordan declared, with emphasis, that observations not only disturb what has to be measured, they produce it! In a measurement of position, for example, as performed with the gamma-ray microscope, "the electron is forced to a decision. We compel it to assume a definite position; previously it was, in general, neither here nor there; it had not yet made its decision for a definite position.... If by another experiment the velocity of the electron is being measured, this means: the electron is compelled to decide itself for some exactly defined value of the velocity; and we observe which value it has chosen. In such a decision the decision made in the preceding experiment concerning position is completely obliterated." According to Jordan, every observation is not only a disturbance, it is an incisive enchroachment into the field of observation: "we ourselves produce the results of measurement" [Wir selber rufen die Tatbestände hervor] (Erkenntinis, 4, 215-252, 1934).

Jordan went further, arguing that there were times when a quantum system effectively observed itself, by collapsing into a specific state rather than remaining in a superposition of states. This does not need any "conscious observer," as had been argued by John von Neumann and Eugene Wigner, but it does need decoherence (and collapse) of the wave function that prevents further interference of various possibilities.

Jordan connected the decoherence with thermodynamic increase in the entropy (it is also connected with the increase of information in the measurement that will be recognized by the conscious observer). He noted that every microphysical observation leaves some sort of macrophysical record (containing information). Indeed, if it did not, there would be nothing to be observed by the conscious observer.

In more orthodox formulations of quantum mechanics one is accustomed to say that the process of observation (or measurement) makes the photon decide between the two possibilities-or makes any other observable take one of its different eigenvalues. But I think that what is here called "observation," must not be interpreted as any mental process, but as a purely physical one; we may better call it, following Margenau (3), the preparation of a state, chosen from those which correspond to a certain operator or observable. The essential point seems to me to be that this process must be a macrophysical one. Macrophysics by definition deals with objects or processes which allow an application of the traditional concept of reality. It is essential that we may think of a macrophysical object as existing independently of any process of observation. Certainly we know of the planet Pluto only because we possess astronomical observatories; but we believe Pluto to have existed already in the time of homo neandertalensis. This is what we call, in the German literature, "Objektivierung," to think of objects as existing independently of the processes of observation. Or to put it otherwise: It belongs to the definition of macrophysics that we are here never faced with the characteristic microphysical features of complementarity.

Now we have indeed in each case of microphysical observation and measurement a situation in which the microphysical object of observation makes a track of macrophysical dimensions. Usually this is made possible by an avalanche process set off by the microphysical object of observation. To induce this track (giving a macrophysical record of the microphysical decision), is - I think - in some cases identical with the decision itself...

Let us first consider what might appear to be a difficulty. A silver grain in a photographic plate - or any other object suited to allow a macrophysical track to be produced by a microphysical decision - is nothing other than an accumulation of microphysical individuals. If we try to give a complete description of the silver grain, then we have to mention its atoms and their wave functions - and we are faced again with those difficulties which we tried to avoid by emphasising the macrophysical character of the silver grain.

This leads us to acknowledge that it is both possible and necessary to formulate a physical axiom not formulated hitherto. Above we held it to be part of the definition of macrophysics, to show no complications in the manner of complementarity, but to allow a complete "objectivation" of phenomena in space and time. But usually one defines macrophysics only by stating that it deals with great numbers of microphysical individuals - and this is another and a different definition. We need therefore a special axiom to express the empirical fact that these two definitions define the same thing - that really each large accumulation of microphysical individuals always shows a well defined state in space and time that a stone never, unlike an electron, has indeterminate coordinates. One often vaguely believes this to be guaranteed already by Heisenberg's Δp Δq > h; but in fact this relation only provides a possibility and not a necessity for the validity of our axiom. Let us assume that, in our experiment involving the photon, the photographic plate be removed, but that we have an arrangement whereby a macrophysical stone will fall according to the decision of the photon. Then, if we strictly assume v. Neumann's view, the stone comes to possess a wave function which makes it undecided whether it does fall or does not, and an observer has the opportunity to compel the stone to a decision by the mental process of forgetting that interference between the two wave functions of the falling stone would be possible. Schrödinger's famous cat is another illustration of this point.

H. D. Zeh says of such things that "they are never observed in nature."

I think we can summarize the situation by saying that indeed a new feature - to be formulated by a new axiom - lies in the fact that such things do not happen; all formulations of quantum mechanics hitherto given do not suffice to exclude them. We are unable to make a clock with a hand which does not always point to a definite figure on the dial. This is a well known fact, but a fact of which present theory gives no sufficient account.

It seems possible to give a still more precise meaning to our new axiom. Let us look at a special case. The emission of an alpha particle by a nucleus (this nucleus may be assumed to be infinitely heavy and to be located at a definite point) is regulated by a spherical wave. Now it is doubtless possible that by some suitable arrangement we could cause interference between alpha emissions in widely different directions, as in the case of photon emission by an atom. But if we let this emission take place in a Wilson-chamber, we always get the picture of a Wilson track showing the particle to have taken a well defined direction. Why is that?

One will scarcely doubt that the thermal motion of the gas molecules must play a decisive role in this instance. I will not discuss here the application of v. Neumann's and v. Weizsäcker's ideas to this case. My own opinion is this. We have to see the cause of the phenomenon not in any "perception," nor any mental process, nor in the fact that drops of water are formed - for surely in the absence of water (though then any direct observation would be difficult) the particle would have a definite direction of emission and we would have tracks of ionisation in the gas. The decisive point seems to be that in consequence of the gas temperature all possibilities of interference between wave functions of different atoms are destroyed. For if we were to fill the chamber not with ordinary gas but with liquid helium at the temperature T = 0, I do not see why interference of alpha emission over wide angles should not remain possible.

Returning again to our photon, we may say that the Nicoll itself would be able to make the two waves φ and ψ incoherent, provided the Nicoll had a sufficient degree of Brownian movement. Generally we can regard Brownian movement as that factor which is suited to create incoherence and to destroy every possibility of interference.

An irreversible event (wave-function collapse) followed by entropy radiated away are the two essential steps in any measurement

If this idea is correct, then we see that thermodynamics is involved in quantum mechanical observation; and this is in harmony with a fact showing irreversibility to be connected with observation: We draw from an observation consequences about the probabilities of experiments to be made afterwards; we cannot reverse this relation.

But while thermodynamics is essential for the concept of observation and measurement, this concept itself seems to me to be indispensable in thermodynamics and in the notion of entropy. The relation of thermodynamics and quantum mechanics - especially thermodynamical statistics and quantum mechanics - has been the object of much discussion. Let us mention here only the first and the last stages of the subject.

1) Pauli (5) emphasised that even in quantum theory there remains the necessity of an "hypothesis of elementary disorder," which has to be acknowledged as an additional axiom besides the "pure" quantum mechanics as formulated by the Schrödinger equation. Our macrophysical axiom mentioned above stands in close connection with this axiom of elementary disorder, governing each thermodynamic system; indeed, we may also say each macrophysical system.

2) During the last years Born (1) and Green, in a series of papers, developed a fascinating account of thermodynamical statistics based upon quantum mechanics. Those results of their endeavour which are related intimately to our question here may be formulated in two theses:

A) Quantum mechanics in its full content implies irreversibility as a necessary consequence.

B) But "pure" or "restricted" quantum mechanics, which applies only the Schrödinger equation without the concepts of preparation of states, observation., measurement or "decision", would not do so.

Point A) has been emphasised by Born himself.

Point B) requires some comment in order to show that it is really in accord with Born's statement and not in any contradiction with it. Born's exposition allows us to see with great clarity where the concept of "decision" comes to play its role: The notion of transition probabilities is used - they are given by his formula [23], (1) which is derived from [21]. This is exactly the point in which we are interested here: It was the whole purpose of our discussion to show the inadequacy of the statement that the intensities of the photon waves φ and ψ are probabilities (of transition or of decision - this is only a verbal difference), and to look for the physical process which makes these waves incoherent.

("On the process of measurement in quantum mechanics," Philosophy of Science, 16, 1949, pp. 269-278 (PDF)

In the earliest beginnings of a rational approach to natural phenomena we encounter a very modern idea: the atomic theory was developed in antiquity; it formed the basis of a philosophical Weltanschauung as we should call it today. Its creator was Democritus, a solitary thinker, understood by next to none of his contemporaries, regarded as a madman by his neighbors and fellow citizens. Democritus had his philosophical precursors, and there were a few successors who attempted to develop his ideas. But essentially the ancient atomic philosophy was all his own.

We have come to take his doctrine so much for granted that we scarcely appreciate its great intellectual audacity. Democritus taught that all matter — we still use the term today, but the very concept of matter was first defined through Democritus' ideas — consists of innumerable tiny particles, which he held to be immutable, hence indestructible and untreatable. In moving through empty space, colliding and exerting a mechanical action upon one another, these particles give rise to what should be regarded as the real, objective world; our crude senses give us only a blurred, imprecise, complexly veiled picture of this objective world of indestructible atoms.

When Democritus in his solitude developed these ideas, the ancient mythological view of the world was still accepted by his contemporaries, who merely shook their heads at him. They still believed in demons, nymphs, demigods and other mythological beings, to whose arbitrary intervention they attributed all conspicuous natural phenomena. In his atomic philosophy, Democritus for the first time expounded the great idea of a nature governed by law. Essentially he was the founder and first proponent of scientific thinking.

Like other intellectual achievements of antiquity, the atomic philosophy was almost entirely forgotten for many centuries. But in the age of the Renaissance, when the Western spirit sought new inspiration in the cultural heritage of antiquity, the old atomic philosophy was rediscovered. It has become a fruitful source of scientific ideas.

In the centuries that have elapsed since then nearly all our physicists have derived significant guidance from the atomic theory. The history of chemistry would be scarcely conceivable without the notion of atoms. For when such men as Boyle and Dalton strove to emerge from the confusion of alchemistic doctrines, in which the facts of experience were shrouded and almost totally concealed by mythological, symbolic, and allegorical thinking, the concept of the atom was the thread that led them out of the labyrinth to clear ideas about the nature of chemical processes. In the field of physics, the atomic theory proved particularly fruitful when heat phenomena were being investigated during the age of the steam engine. With the help of brilliant mathematical analyses Maxwell in England, Boltzmann in Germany, Gibbs in America showed heat to be a hidden, statistically irregular motion of the infinitesimal atoms which constitute matter.

But despite the signal success of the atomic theory in physics and chemistry, a few outstanding physicists and chemists at the beginning of the twentieth century criticized it sharply. They pointed out that after almost two thousand years of speculation there was no really cogent proof of the existence of atoms. They admitted that the atomic hypothesis had provided any number of fruitful ideas, but insisted that it had never been conclusively proved. No one could refute their contention that the whole atomic theory was probably nothing more than an idle speculation and an ancient fallacy, and consequently the results obtained on the basis of it might well be illusory.

This was the situation at the beginning of our century: despite everything that had been accomplished by the atomic theory since the Renaissance, no one could say with certainty that any such things as atoms existed. It is not surprising that these outspoken critics created a panic among their colleagues; the very foundations of physics and chemistry, which for centuries had been regarded as secure, seemed to have been shaken.

And yet this criticism proved beneficial. Great minds brought new zeal to what had become the crucial problem in physics: to prove the reality of the atoms; to devise experiments which would show irrefutably that the atoms actually exist.

This problem, as we know, was solved. We cannot relate in detail how the physicists of our century succeeded in breaking into the realm of the atoms, how they bored their way into this profound, hidden stratum of material reality. One of the crucial proofs, in any event, was arrived at by Einstein, who found that certain hitherto neglected experimental data ("Brownian motion") lent unquestionable support to the theory developed by Maxwell, Boltzmann, and Gibbs that heat was simply a motion of countless infinitesimal atoms.

Another important achievement was the discovery of the electron, which was effected in several steps, the most important being the investigation of cathode rays.

But the inquiry into the reality of the atom carried on in the early part of our century accomplished far more than to prove that Democritus was right. It also gave us a thorough knowledge of the atoms. As the physicists of our day explored this hidden stratum, a whole new world opened before them — and science garnered in the richest harvest of its entire history. Not only has the existence of atoms, doubted by excellent authorities only fifty years ago, been securely demonstrated; but in addition the whole realm of the atoms has been thoroughly explored and illuminated; its secrets have been unveiled, its riddles solved.

Many outstanding scientists share in the glory of these discoveries. And indeed, it would seem as though physics had attracted a conspicuous proportion of the outstanding minds of our age. The names are so numerous that we cannot begin to mention all of them. Suffice it to say that Lord Rutherford achieved epoch-making results in investigating the structure of the atom. For what we call an atom today has itself a structure; it is by no means an ultimate, simple component of matter. We habitually call the smallest unit of a chemical element an atom. But these atoms, it has been found, are themselves composed. We term their ultimate components "elementary particles" — and it is these ultimate units of matter that correspond to Democritus' conception of "atoms."

The matter of all chemical elements consists of three varieties of elementary particles: electrons, protons, and neutrons. The structure of an atom — taking the concept in the modern sense — may be described as follows. In the center is the much smaller atomic nucleus, composed of protons and neutrons. Nearly the entire mass of the atom is contained in this nucleus. But the volume of the atom is made up almost entirely of its electrons, although these constitute less than one-thousandth of its mass. When atoms are combined into molecules through chemical processes, the nuclei of the atoms involved remain totally unchanged; certain changes occur only in the relations between the electrons. This fact is in keeping with the fundamental law of chemistry that in all chemical reactions the quantities of the elements involved remain unchanged — for the chemical nature of an atom is conditioned by the structure of its nucleus (or more precisely, by the number of protons in the nucleus).

So far everything seems perfectly simple, although these seemingly simple results required no end of the most ingenious and painstaking calculation and experiment. But what is really impressive about modern atomic research is this: that it disclosed natural laws of an entirely different order from those previously investigated in the world of larger, cruder bodies. The first indication of the totally novel conditions that had so long lain undiscovered in the inaccessible substratum of the atomic world was detected by Max Planck at the very beginning of our century. Compared with the conclusions that were subsequently derived from it, Planck's discovery was only a first step. But it was the decisive step. With it physics entered on wholly new and unsuspected paths.

Planck's so-called quantum theory formulated the natural law that had to do with the peculiarly discontinuous reactions of atoms, electrons, protons, and so on. It was taken up by the leading physicists of our time, who amplified it and through it achieved remarkable triumphs. The development of the quantum theory, indissolubly linked with the atomic discoveries it has made possible, strikes us as the most dramatic event in the history of physics since 1900. Einstein's theory of relativity, although not directly connected with the quantum theory, has also considerably helped the physicists who were seeking to solve the riddles raised by the quantum jumps, by the continuous reactions of the atoms. For Einstein's example encouraged them to an unprecedented boldness of thought. Moreover, Einstein himself made revolutionary contributions to the quantum theory. And the great discoveries of other investigators in this field — here we can mention only Niels Bohr and Werner Heisenberg — brought astounding results.

The atomic world held many surprises in store. And strangest of all, it was found that the principle of causality, the basic principle of all science up until then, is no longer applicable to this realm, where it is replaced by general statistical laws. The quantum physicists did not draw these far-reaching inferences until the problems connected with atomic quanta had been solved so clearly and conclusively as to leave no possible room for doubt. Scientific thinking had entered upon a new age. Not since Democritus had there been such an upheaval in scientific ideas.

If today we think back to Democritus, it is because we are in a better position than ever before to appreciate the towering stature of this solitary thinker. There is no similar case in the whole history of science: A dreamer far in advance of his time puts forth a theory which cannot be proved until more than two thousand years later; but then it is confirmed. And meanwhile this theory has influenced, inspired, shaped all scientific thought.

But our present epoch, which has confirmed the atomic theory, also marks the end of the scientific era dominated by Democritus' ideas. On the one hand we have established the reality of the atoms, so proving once and for all that Democritus was right; but on the other hand the investigation of the atomic quanta has proved that the laws governing the atomic world are of an entirely different nature than Democritus and his successors supposed. The transcending of the principle of causality characterizes this century as an incomparable turning point in the development of human thought.

By 1927 the quantum physicists, overcoming gigantic difficulties, had solved all the basic problems relating to the electronic husks of the atoms. Since then vast efforts have been devoted to the nuclei.

In a short time a new branch of physics has grown from small beginnings. In regard to fundamentals, nuclear physics has registered no such revolutionary advances as the earlier investigation of the electronic husks of the atoms. But all of us know that the technical applications of nuclear physics have been no less revolutionary than the theoretical findings of quantum physics. Both aspects of scientific activity, the theoretical penetration that leads to deeper insights and the practical mastery of nature through the technical exploitation of these insights, have been prodigiously realized in these modern developments. In this retrospective view of the remote and recent past we have attempted to gain a profounder understanding of our own times. Today the world is stricken with anxiety over the terrible possibilities of atomic warfare. But at the same time the transformation that has occurred in our scientific view of the world gives us food for reflection along entirely different lines. In the era inaugurated by Democritus our scientific thinking was forced into a definite philosophical position. But in recent years the dangerous one-sidedness of a mechanistic view of nature, as propounded by Democritus, has become evident. The great turning point in scientific knowledge that we are experiencing today opens up new perspectives for our reflection on the problems of nature and man, world and God.

Since then far better and sharper microscopes have been devised, and every optical advance has enabled biologists to discover smaller structures in the organism. In this respect organic nature differs strikingly from the inorganic world: the dead matter of a well developed crystal reveals a high degree of uniformity, disturbed only by small accidental irregularities. But when we closely examine the living organisms every fragment, however small, shows us ingenious new structural details.

In recent years the electronic microscope has developed a power of enlargement far greater than that of any optical microscope; and once more, almost as in Leeuwenhoek's day, a new realm of more minute organic structures has been opened up to us. But we know that even the smallest organic particles now perceptible to us involve further structural riddles; organic life is ingeniously articulated down to the very molecules and atoms.

The incredible intricacy of a living cell is revealed not only by microscopic examination; it is manifested, perhaps still more strikingly, in the facts of heredity. In every case a huge number of hereditary characters must be embedded in a single tiny germ cell. We know today that the nucleus of the germ cell is the bearer of heredity; modern genetics has led us deep into the amazing complexity and lawfulness of the hereditary process.

The study of heredity has been combined with the study of mutations to form a single science of impressive stature. This new science of genetics is entirely an achievement of our own twentieth century. For it was in 1900 that the laws of heredity, previously investigated by Mendel but disregarded by his contemporaries, were rediscovered. The new science of genetics began its fabulous course just as Max Planck was making his great discovery which has played so vital a role in the development of atomic physics.

The Mendelian laws were only the beginning, the gateway to this new science which has very largely laid bare the intricacies of the hereditary process and the laws which govern its operation. Two men chosen at random, who do not happen to be identical twins, differ in hundreds of inherited characters; but far greater of course is the number of inherited characters which are common to these two men but distinguish them from the various representatives of the animal kingdom. The mechanism of heredity, residing in the nucleus of a germ cell, must be fantastically complex to embody all these hereditary characters despite its minuteness. To investigate and decipher this vastly intricate mechanism, invisible to the naked eye, represented a great challenge to the restless human mind.

The geneticists concentrated their main effort on the observation of a single object, a small species of fly known as Drosophila, and the impressive results achieved were largely due to this astute method. Of course many other objects, both animal and vegetable, were examined; this was the only way of making sure that the laws of heredity were all-embracing, that they were valid for all organic life. But since, despite the universality of these laws, every animal and vegetable species offers its own highly complex peculiarities, a thorough understanding of heredity and its occasional changes, or mutations, could be arrived at only if a number of able scientists concentrated on a single species that was particularly favorable for investigation.

An experiment in genetics can be conclusive only if large numbers of individuals are bred and examined; only then can the statistical findings provide reliable answers to the many complex questions involved. Geneticists all over the world bred several hundred million drosophilae and examined each individual separately. The result is that today we have a detailed picture of the mechanism governing heredity. The cell nucleus contains the so-called chromosomes, spiral-shaped threads consisting of numerous genes, which are lined up in chains. Each individual gene is the bearer of a single hereditary trait: every gene is a catalyst which makes it possible for certain ferments to be produced in the life process of the cell; and these ferments in turn are catalysts which determine and regulate very complex processes that take place when a new organism grows from the fertilized germ cell. Before the germ cell splits in two, as it does in the process of growth, the chromosomes double in number; thus each of the two new cells receives its full complement of the genes that guide its vital processes.

This duplication of the chromosomes makes the new cells exact copies of the parent cell, so assuring the faithful transmission of hereditary characters. But a gene can occasionally incur a sudden small change, which we call a mutation. The changed, mutated form is then duplicated; the change that has occurred in the gene is inherited — a new hereditary character has come into being.

These mutations of course attracted much attention among geneticists; if there were no mutations at all, no science of genetics would be possible, for it is only the existence of hereditarily dissimilar individuals that enables us to attempt crosses and so investigate the laws of heredity. All inherited differences result ultimately from mutations in the germ cells. And the entire development of organic life over millions of years has only been made possible by mutations that increase the complexity of the germ cell, giving rise to higher and more complex forms and species.

Mutations occur in all animal and plant species. But as a rule they are very infrequent, and the most painstaking attention is needed if we are to observe them. It was a great step forward for genetics when Muller, the Nobel prize winner, found that the frequency of mutations in Drosophila could be appreciably increased by artificial means. When the flies are subjected to X rays the mutations become more frequent. This applies of course not only to flies but also to every other species whose germ cells are struck by radiation, and that is why it is dangerous for men to work with X rays for protracted periods unless careful protective measures are taken.

Of course the possibility of inducing mutations by radiation was very welcome to geneticists, partly because it provided them with more abundant material for experiments and partly because it enabled them to study the mutation process itself more closely and obtain new information regarding the nature of the genes. In this direction important discoveries were made by Timofeeff-Rossovsky, the Russian geneticist, who lived for many years in Germany. He found that mutation can be induced in a gene if a single electron is detached from it.

This makes it plain that the individual gene is an extremely fine structure. Actually we must conceive of it as a single molecule — a very large molecule, of course, containing many thousands, perhaps hundreds of thousands of atoms, but in any event a single molecule — with a very definite architecture, a wonderfully developed edifice of atoms, in which no detail can be changed without effecting a mutation and a change in the corresponding hereditary character.

Even more clearly than microscopic examination, these considerations show that organic life is as finely structured as physical possibility permits. And here we have a basic insight. To understand this we must turn for a moment to the other great scientific achievement of our century, the exploration of the atoms and their reactions. The two new sciences, as I have said, were inaugurated at the same time: both modern genetics and modern atomic physics date from the beginning of our century.

It is true that the notion of the atom as such is far older; it was worked out in ancient times by Democritus. But although the doctrine of Democritus provided guidance for the whole development of Western science beginning with the Renaissance, there was still disagreement among physicists in 1900 as to whether we should take the concept of the atom quite seriously. It is only in our century that the reality of the atoms has been proved convincingly, that these infinitesimal basic structures of matter have been investigated experimentally.

But this advance brought about a profound — and quite unsought-for — change in our view of nature. Up to our century scientists held a very definite conception of natural law, a conception first formulated in Democritus' atomic philosophy. The following example may serve to illustrate it.

The movements of the planets and their moons are such that we can predict them: solar eclipses, lunar eclipses, and the entire movement of the other planets and moons can be calculated in advance by the astronomer. Here then there is no possibility of surprising, unforeseen events; everything operates with the certainty of clockwork. The course of things is predetermined and immutable. Anyone familiar with the natural laws involved can calculate in advance what will happen.

Democritus' view of nature led us to suppose that all natural processes were governed by such necessity, that all nature operated like a machine. For Democritus taught that nature seen in its objective reality, unclouded by our inadequate sense organs, consists in a multiplicity of atoms moving in empty space. He conceived of these atoms as minute bodies which, like the huge heavenly bodies, are subject to the laws of mechanics. Thus in accordance with the laws of nature every atom must effect a predetermined motion, and like the planetary system the total system of these atoms is subject to compelling necessity. Nature as a whole and in every infinitesimal detail moves like clockwork, like a machine.

We shall not deny that these ideas of Democritus represented a great and fruitful achievement. At a time when the old mythological view of nature was still very much alive Democritus fashioned his picture of a nature governed by law, a picture so profound that the scientists of two millennia followed him in the conviction that all natural processes are predetermined and that there are no exceptions or gaps in this determinism.

But now that we have at last learned to look into the realm of the atoms and recognize the atoms as realities, we can no longer let our imagination decide how these atoms are constituted and according to what laws they react. We confront the reality and must accept what it shows us.

The result is an almost overpowering surprise: the natural laws that govern the realm of the atoms do not provide for an unbroken determinism; they are so entirely different from anything that the human imagination had expected as to justify Heisenberg, the eminent physicist, in saying that the physics of the atoms and quanta has definitively refuted the principle of causality.

Nevertheless, these laws are perfectly clear and exact; but they are statistical laws, which determine only the average reaction of large quantities of atoms, while within this frame each individual atom remains inalienably free and unpredictable. If, for example, we have before us a milligram of radium atoms — a milligram will comprise innumerable atoms — natural law determines how many of them will disintegrate in the next second; for that is a statistical question, a question applying to the mass process. But if we take a single radium atom, the law leaves it open as to when this single atom will disintegrate — perhaps today and perhaps in thousands of years. And it is not human ignorance that makes this prediction impossible. It is objectively uncertain when the disintegration will take place; natural law as such sets down only statistical decrees for the atoms.

At first sight this discovery in the field of physics has nothing to do with biology. But considered in conjunction with it, the findings of genetics open up totally unexpected perspectives. When a mutation occurs in the gene of a germ cell, this is an event of far-reaching consequence: on the basis of this mutation the organism growing from the germ cell may deviate very materially from the form it would otherwise have assumed; for the organism this mutation is of fateful importance and may entirely change its life. But researches in genetics have taught us that the gene in which the mutation has occurred is only a single molecule. If we consider this in the light of what has just been said about the world of atoms, it becomes clear that our mutation lies outside the mechanical course of natural events; considered as an individual occurrence in the realm of atomic physics, it is not predetermined.

Thus the great discovery of atomic physics, the discovery of natural processes which are not subject to mechanical necessity, reaches over into biology; the new view of nature at which we arrived through modern physics leads our thinking into entirely new paths.

But the doctrines of scientific materialism are historically much older. They begin with the atomic philosophy of Democritus; and they were fully developed in the Western world two hundred years ago when the physician and philosopher La Mettrie published his sensational book, L'homme machine —"Man Is a Machine."

With ruthless logic La Mettrie drew the consequences from the view of nature which had been conceived by Democritus and which had seemed to find increasing confirmation as scientific inquiry advanced. The essence of this view is that all events in nature are prescribed and predetermined by law, and that this determinism is complete and unbroken. We actually observe such predetermination in the movements of the planetary system; and the calculation of the planetary movements, particularly by Newton, did much to encourage and advance scientific thinking. All the triumphs of physics and chemistry up to the end of the last century strengthened the conviction, born in the days of Democritus, that natural law is synonymous with unbroken determinism.

La Mettrie accepted this view of natural law. Carrying it to its utmost logical consequence, he inevitably came to regard man as a machine or a cog in a machine. Convinced that man was not excepted from the validity of the natural laws, he concluded that every human action is absolutely determined by the events that went before it, and that there is no break in this chain of determination. Every atom is subject to law, whether it is a component of a lifeless stone or of a human brain; every atom of the human body must therefore move according to unswerving mechanical necessity; its behavior, like the motion of the planets, is clocklike and predetermined. How a man will act, what he will say, what facial expression he will assume, all this is determined by the mechanics of the atoms that make up his body.

This doctrine of La Mettrie contains the core of the materialistic view of nature. If we wish to know what scientific materialism is, we need merely read this book that appeared in the year 1748, for in it the fundamental tenets of materialism are set forth with the utmost clarity. To be sure, these doctrines did not attract wide support until much later, in Haeckel's day; but in this book they are fully formulated. All nature is predetermined, a gigantic clockwork; man is part of the clockwork, he too is a mechanism, a machine — this is the essence of the materialistic philosophy.

When Democritus developed his atomic philosophy, he denied the pagan gods: the world he outlined, a world in which the atoms move according to their own mechanical law, left no room for the intervention of gods and demons. When La Mettrie, following in Democritus' footsteps, defined the materialist view of nature, he denied the creator God of the Christian religion; he also denied the human soul and freedom of the human will. A nature operating according to mechanical law suffers no intervention of creative power; man conceived as a machine has no free will.

It is perfectly understandable that a wave of irreligion should have passed over the world once Darwin's discoveries had shown that the development of organic life is a process governed by natural law. Even without Haeckel's impassioned materialism, many men would have drawn the same conclusion.

It was not until our century that this Democritean view of nature was replaced by something very different. Democritus' picture of the world was a product of bold imagination and penetrating philosophical thought; it guided and fructified natural science for two thousand years, because it contained profound truths. And yet in crucial points it proved to be not only inadequate but actually misleading. The new view of nature that has arisen in our century is not a product of human invention, it was found in reality. The old concept of natural law was too narrow and specialized. True, there is such a lawfulness in nature and it is very important. But everything is not governed by laws of this kind, by natural laws which determine the course of events in every detail. Only observation, only large bodies and quantities of matter, consisting of huge numbers of atoms, disclose a seemingly perfect, unbroken determinism, because statistical laws wholly fixate the average event. But when we look more sharply when we observe the individual atoms that make up the huge bodies and quantities of matter, we find on all sides free individual decisions which are not determined by natural law.

This modified view of nature broadens the scope of biology. Such essential factors in the biological process as the occurrence of mutations prove to be acts of decision outside of any predetermined clockwork. Organic life partakes then of the same freedom and spontaneity that physicists have found at the root of material being.

This negates the scientific foundation of La Mettrie's thesis that man is a machine. The very basis of the materialist doctrine proves to be a fallacy resulting from a narrow and obsolete concept of natural law.

It was not until a hundred and fifty years later that La Mettrie's ideas were widely publicized by Haeckel and critically influenced the beliefs of large numbers of people. And it will be a long time before today's new insights become a common possession, before they can exert their full effect on men's ideas and attitudes. Today we shall do well to be cautious in judging the philosophical implications of the new knowledge, to avoid anticipating the future. But even so, we cannot help feeling that our epoch has achieved momentous insights which set a term to an intellectual development that has been in progress for two thousand years, and will guide our thinking into new channels. From an intellectual point of view the scientific refutation of materialism is surely as overwhelming an accomplishment as the invention of the atom bomb.

We may say then that the attempt to prove man a machine, to deny him free will, has been refuted by the sheer facts of science. But this still does not solve the problem which Darwin's theory of evolution raises for the religious man. It is a problem which is not to be evaded, but to be considered in all seriousness.

We now know that the behavior of an individual organism — regardless of whether animal or man — is not exclusively determined by mechanical necessity; we can no longer, with La Mettrie, forbid the soul or the will to intervene in the fixed and predetermined movements of the body's atoms. But what of the long history of organic life on our earth, the course of development which in millions of years has brought forth our present wealth of forms from the lowly one-cell organisms? If we restrict ourselves to scientific truth, must we look on this vast history of organic development as mechanically conditioned and determined, as a self-contained process closed to the intervention of any creator? Or can we once again find gaps in the chain of determination and show that events outside the realm of mechanical necessity are requisite to the actual development?

This is a strictly scientific question that must be solved independently of philosophical presuppositions. But our answer will have philosophical as well as scientific implications. Of course we cannot expect the scientist to prove either that phylogeny, the history of organic development, is independent of a creator or that the divine hand definitely entered into it. The two scientific possibilities are these: either, as was formerly believed, the course of phylogeny is a matter of mechanical necessity; or else the history of organic development is conditioned by events which demonstrably lie outside the realm of mechanical determination.

If inquiry should lead us to the second of these conclusions, a religious man might, without coming into conflict with science, regard these "events outside the realm of mechanical determination" as a creative intervention.

The statistical nature of the laws that have now been revealed to us makes it seem difficult to give this second answer scientific support. Even if we must grant the individual organism a certain freedom to make unpredictable decisions, the decisions remain within the framework of statistical law; as soon as we consider numerous organisms, such as a large herd of animals, the average behavior will be prescribed by the statistical laws; individual freedom of decision will be submerged by the statistical mass. Must, then, the destinies of the largest existing groups of similar animals or plants, namely the species as a whole, be governed almost entirely by predetermined necessity?

If this is not the case, it is because the most insignificant events in the reproductive activity of plants and animals can multiply like snowballs and produce tremendous effects. A certain mutation, which gives an organism an appreciable head start over the rest of its species, need occur but once, and conceivably this one organism will multiply stupendously, giving rise to a new and superior species that will assert itself on the stage of phylogenetic history and affect its course for millions of years to come.

But are there actually such cases? The scientists investigating Drosophila know that in this species certain mutations occur over and over again. Among a million flies which reproduce we find a very definite, statistically calculable frequency of mutated progeny; this frequency of course is predetermined. As we have said, it can be influenced experimentally by X rays, and this effect too is determined by law.

These frequent mutations, whose frequency makes them subject to statistical laws, play a great part in phylogenetic development; the modern efforts to provide a secure foundation for Darwin's theory of natural selection and to define it more closely draw on the entire knowledge of modern genetics.

But now we must ask: Are there also instances of extremely rare mutations, which have occurred only a few times throughout the millions of years of a species' lifetime with all the innumerable individuals this represents, or which perhaps have occurred but once?

This is only the first half of the question; the other half will follow. But first it should be remarked that this first half of the question is addressed not so much to the biologist as to the physicist, the student of molecules and quanta. And the physicist can give a very simple answer: It is certain that the various mutations will occur with every conceivable degree of frequency; there must also be very infrequent mutations, including those which occur but once in the entire history of a species.

And now comes the second half of the question: Can we maintain and prove — or are we at least justified in presuming — that such extremely rare mutations have played a decisive role in the phylogenetic history of life? It is assuredly a theoretical possibility; but in so weighty a question we must not judge hastily — real proofs are required.

We cannot yet prove this hypothesis. We may only say that for the present nothing argues against it, while various facts support it. Consider for example the strange experience of the American fruit growers who were attempting to combat certain insect pests with poison. Suddenly a new strain of insects immune to the poison made its appearance and spread over a large region. If we look into the observations recorded at the time, we find substantial reason to suppose that this new strain of insects developed from the offspring of a single individual — that the mutation which produced the new strain occurred but once in the space of several decades among many millions or billions of individuals.

To this example we might add several others, equally favorable to our hypothesis — but the scientific decision in this important question is reserved for the future. There is only one point in which we may be almost certain of the truth of our hypothesis.

In the first section of this chapter we followed the history of organic life back to the remote past. We saw the earth as it was 500 million years ago. Organic life was then limited to the waters; the plants had not yet developed beyond the stage of marine algae; the animal world was represented by the archaic trilobites, while vertebrates were entirely lacking. When we look still further back into the past and attempt to investigate epochs prior to the Cambrian, fossil remains become less and less frequent. But even so there must have been a long period of organic development before the Cambrian; the creatures then alive were very primitive and simple compared to later species, but they were complex enough to presuppose a long preceding history covering millions of years. The investigation of these early developments holds an irresistible fascination. For the deeper we can penetrate into the nebulous past, the closer we come to that mysterious event which constituted the starting point of all organic development on our earth: the origin of life itself.

A consideration of these questions will take up a part of our last section; and in this connection we shall come back to the question we have just been discussing. But we must still regard it as a hypothesis — strongly supported but not definitively proved — that phylogeny, the history of the development of organic life on our planet, was affected and guided at decisive points by events lying outside the realm of mechanical determination. If future scientific inquiry should definitively prove this assumption, we shall be able to say that creation and evolution are no longer conflicting, antithetical conceptions — that a religious man may fully accept the Darwinian theory and still see the Creator at work in the wonderful development that has taken place over millions of years of the earth's history.