When Rachel Carson made us aware of the dangers arising from the mass application of toxic chemicals, she presented her arguments in the manner of an advocate, not a scientist. In other words, she selected the evidence to prove her case. The chemical industry, seeing its livelihood threatened by her action, responded with an equally selective set of arguments, chosen in defense. This may have been a fine way of achieving justice, and perhaps in this instance it was scientifically excusable; but it seems to have established a pattern. Since then a great deal of scientific argument and evidence concerning the environment is presented as if in a courtroom or at a public enquiry. I cannot say too often that, although this may be good for the democratic process, it is bad for science. Truth is said to be the first casualty of war. Being used selectively in evidence to prove a case in law also weakens it.

During the present century a few physicists have tried to define life. Bernal, Schroedinger, and Wigner all came to the same general conclusion, that life is a member of the class of phenomena which are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form. This definition is not only difficult to grasp but is far too general to apply to the specific detection of life. A rough paraphrase might be that life is one of those processes which are found whenever there is an abundant flow of energy. It is characterized by a tendency to shape or form itself as it consumes, but to do so it must always excrete low-grade products to the surroundings.

We can now see that this definition would apply equally well to eddies flowing stream, to hurricanes , to flames, or even to refrigerators and many other man-made contrivances. A flame assumes a characteristic shape as it burns, and needs and adequate supply of fuel and air to keep going, and we are now only too well aware that the pleasant warmth and dancing flames of an open fire have to be paid for in the excretion of waste heat and pollutant gases. Entropy is reduced locally by the flame formation, but the overall total of entropy is increased during the fuel consumption.

Explosions are seldom one hundred per cent efficient. When a star ends as a supernova, the nuclear explosive material, which includes uranium and plutonium together with large amounts of iron and other burnt-out elements, is distributed around and scattered in space just as is the dust cloud from a hydrogen bomb test. Perhaps the strangest fact of all about our planet is that it consists largely of lumps of fall-out from a star-sized hydrogen bomb. Even today, aeons later, there is still enough of the unstable explosive material remaining in the Earth's crust to enable the reconstitution on a minute scale of the original event.

Binary, or double, star systems are quite common in our galaxy, and it may be that at one time our sun, that quiet well-behaved body, had a large companion which rapidly consumed its store of hydrogen and ended as a supernova. Or it may be that the debris of a nearby supernova explosion mingled with the swirl of interstellar dust and gases from which the sun and its planets were condensing. In either case, our solar system must have been formed in close conjunction with a supernova event. There is no other credible explanation of the great quantity of exploding atoms still present on the Earth. The most primitive and old-fashioned Geiger counter will indicate that we stand on fall-out from a vast nuclear explosion. Within our bodies, no less than three million atoms rendered unstable in that event still erupt every minute, releasing a tiny fraction of the energy stored from that fierce fire of long ago.

The history of Earth's climate is one of the more compelling arguments in favour of Gaia's existence. We know from the record of the sedimentary rocks that for the pst three and a half aeons the climate has never been, even for a short period, wholly unfavorable for life. Because of the unbroken record of life, we also know that the oceans can never have either frozen or boiled. Indeed, subtle evidence from the ratio of the different forms of oxygen atoms laid down in the rocks over the course of time strongly suggest that the climate has always been much as it is now, except during glacial periods or near the beginning of life when it was somewhat warmer. The glacial cold spells--Ice Ages, as they are called, often with exaggeration--affected only those parts of the Earth outside latitudes 45 degrees North and 45 degrees South. We are inclined to overlook the fact that 70 per cent of the Earth's surface lies between these latitudes. The so-called Ice Ages only affected the plant and animal life which had colonized the remaining 30 per cent, which is often partially frozen even between glacial periods, as it is now.

At the end of the last century Boltzman made an elegant redefinition of entropy as a measure of the probability of a molecular distribution. It may seem at first obscure, but it leads directly to what we seek. It implies that the probably life or one of its products, and if we find such a distribution to be global in extent then perhaps we are seeing something of Gaia, the largest living creature on Earth.

But what, you may ask, is an improbable distribution of molecules? There are many possible answers, such as the rather unhelpful ones: an ordered distribution of improbable molecules (like you, the reader), or an improbable distribution of common molecules (as, for example, the air). A more general answer, and one useful in our quest, is a distribution which is sufficiently different from the background state to be recognizable as an entity. Another general definition of an improbable molecular distribution is one which would require the expenditure of energy for its assembly from the background of molecules at equilibrium. (Just as our sand-castle is recognizably different from its uniform background, and the extent to which it is different or improbable is a measure of the entropy reduction or purposeful life-activity that it represents.)

We now begin to see that the recognition of Gaia depends upon our finding on a global scale improbabilities in the distribution of molecules so unusual as to be different and distinguishable, beyond reasonable doubt, from both the steady state and the conceptual equilibrium state.

One of the most characteristic properties of all living organisims, from the smallest to the largest, is their capacity to develop, operate, and maintain systems which set a goal and then strive to achieve it through the cybernetic process of trial and error. The discovery of such a system, operating on a global scale and having as its goal the establishment and maintenance of optimum physical and chemical conditions of life, would surely provide us with convincing evidence of Gaia's existence.

Cybernetic systems employ a circular logic which may be unfamiliar and alien to those of us who have been accustomed to thing in terms of the tranditional linear logic of cause and effect...

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The attainment of any skill, whether it be cooking, painting, writing, talking, or playing tennis, is all a matter of cybernetcis. We aim at doing our best and making as few mistakes as possible; we compare our efforts with this goal and learn by experience; and we polish and refine our performance by constant endeavor until we are satisfied that we are as near to optimum achievement as we can ever reach. This process is well called learning by trial and error.

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...Human and other animals intentionally seek a warmer or cooler environment, as the case may be, in their ceaseless pursuit of the goal of maximum comfort. If necessary, the local environment is modified to reduce exposure to bearable limits. We wear clothes and build houses; other animals grow fur or seek and make burrows. These activities constitute and additional mechanism of temperature control, which is vital when conditions pass beyond the capacity of internal regulation.

Let us turn for a moment to the philosophical aspect of the subject and consider the problem of pain and discomfort. Some of us are so conditioned to regard unendurable heat, cold, or pain of any kind as in some measure of a punishment or visitation from on high for sins of omission or commission that we are inclined to forget that these sensations are all essential components of our survival kit. If shivering and cold were not unpleasant we would not be discussing them, since our remote ancestors would have died of hypothermia.

Let us again look at the laws of thermodynamics. It is true that at first sight they read like the notice at the gait of Dante's Hell; but in fact, tough as they are and although like income tax they cannot without penalyt be evaded, they can with forethought be avoided. The Second Law states unequivocally that the entropy of an open system must increase. Since we are all open systems, this means that all of us are doomed to die.

In science, simultaneous macroscopic and microscopic exploration is quite customary, especially in biology. Molecular biology, for example, which derived from the application of chemical analysis to biological problems and led to the discovery of DNA and its function as the carrier of information for every form of life, has developed independently from physiology, which concerns the whole animal and the way it functions as an integrated living system. In like manner, the difference between the Gaian notion and the ecological notion of our planet derives in part from their history. The start of the Gaia hypothesis was the view of the Earth from space, revealing the planet as a whole but not in detail. Ecology is rooted in the down-to-Earth natural history and the detailed study of habitats and ecosystems without taking the whole picture. The one cannot see the trees in the wood. The other cannot see the wood for the trees.

What is remarkable about man is not the size of his brain, no greater than that of a dolphin, nor his loose incomplete development as a social animal, nor even the faculty of speech or his ability to use tools. Man is remarkable because by the combination of all these things he has created an entirely new entity. When socially organized and equipped with technology even as rudimentary as that of a Stone Age tribal group, man has the novel capacity to collect, store, and process information, and then use it to manipulate the environment in a purposeful and anticipatory fashion.

We stand upright-after a little practice-on a ship that rolls because we possess an array of sensory nerve cells buried in our muscles, skin, and joints. The function of these sensors is to provide a constant flow of information to the brain about the movements and location in space of the various parts of our bodies, as well as the environmental forces currently acting on them. We also have a pair of balance organs associated with our ears which work like spirit-levels, each having a bubble moving in a fluid medium to record any change in the position of the head; and we have our eyes to scan the horizon and tell us how we stand in relation to it. All this flow of information is processed by the brain, usually at an unconscious level, and is immediately compared with our consciously intended stance at the time. If we have decided to stand level in spite of the ship's motion, perhaps to look at the receding harbour through binoculars, this chosen posture is the reference point used by the brain to compare with departures from it caused by the rolling of the vessel. Thus our sense organs continually inform the brain about our stance, and counter-instructions pass constantly from the brain down the motor nerves to the muscles. As we tip from the vertical, the push-and-pull of these muscles changes, so as continuously to maintain the upright position.

This process of comparing wish with actuality, of sensing error and then correcting it by the precise application of an opposing force enables us to stand erect. Walking or balancing on one leg is more difficult and takes longer to learn; riding a bicycle is even trickier, but this also can become second nature through the same active control process which keeps us upright.