Emergence is a notoriously slippery subject. Some say it seems to be nothing more than a shorthand way of describing the development of the universe and the rise of life on Earth. Others say it takes one into much deeper scientific and philosophical waters.

Charles Darwin might have articulated the most well-known scientific theory of emergence evolution in The Origin of Species. In the famous closing lines of his book, he wrote of his wonder that from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

Darwin aside, there is little doubt that common objects snowflakes, for example are examples of emergence.

Probe a little deeper into the literature on emergence and one can see there is a rich debate and discussion being conducted between and among scientists and philosophers. Those thinking about emergence are, to varying degrees, hopeful that a better understanding of emergence will lead to a richer understanding of cosmic and biological evolution or, more to the point, a better understanding of who we are as human beings.

In the recent history of the cosmos, life and eventually mind arose spontaneously from matter on this planet, and in the recent history of biological evolution, symbolic abilities and ethical experience dawned in one species, said Terrence Deacon, a professor of biological anthropology and linguistics at the University of California, Berkeley. Quite simply, the history of the world exhibits the emergence of these phenomena. So it isnt such a leap to think that this is also the best clue to understanding them. Ultimately, I feel confident that if we follow this clue, dont give up the quest prematurely and dont settle for pat pseudo answers, we will develop an understanding that is both useful and enlightening, he said.

Seeking the middle ground

The modern interest in emergence began a century ago when it was offered as a way to navigate between vitalism and reductionism. There was a three-way controversy in the 1920s, said Nancey Murphy, a professor of Christian philosophy at Fuller Theological Seminary in Pasadena, Calif. The vitalists were saying that you had to postulate a vital force or an entelechy which I think is just another word for an Aristotelian form in order to understand how you can get life out of nonliving material. That view has been as thoroughly rejected as anything can be in philosophy. The second possibility was that reductionism is true, and you could eventually explain living processes in simple, biochemical terms. The third suggestion was the postulation of emergence that you get something genuinely new when you get to the level of life, she said.

In the 1920s, British emergentist C. D. Broad said that the debate between reductionism and emergence in science really boiled down to whether matter and reality itself is arranged into different levels in which higher levels of increasing complexity are not reducible to lower levels. Others add, too, that these higher-level laws or properties have causal or top-down power over the lower-level laws and properties.

Technically, Darwinism is an example of weak downward causation, said Paul Davies, a professor of natural philosophy in the Australian Centre for Astrobiology at Macquarie University. The law of increasing complexity, if derivable from bottom-level laws, would be merely another weakly emergent law. I am suggesting that there is maybe a strongly emergent law of increasing complexity. Thus, neo-Darwinism might be adequate for biology, or inadequate but augmented by laws of complexity that are only weakly emergent the position of [biochemist Christian de Duve] and maybe [paleobiologist Simon Conway Morris] or require strongly emergent laws in addition.

Among those who think about emergence, said Australian philosopher David J. Chalmers, two main positions have developed: strong emergence and weak emergence. Supporters of weak emergence make the claim that the more fundamental theory can in principle explain the phenomena of the higher-level theory. And many also argue that emergent laws and properties, even if they do exist, dont cause anything on a lower level of reality to change in any way.

The sense of emergence that I approve of is what Ive called innocent emergence, said Daniel C. Dennett, director of the Center for Cognitive Studies and Austin B. Fletcher Professor of Philosophy at Tufts University in Medford, Mass. Dennett said his definition of an emergent phenomena is one that is startling, surprising, one that allows us to use a higher-level description to characterize it, but its not in principle unpredictable or irreducible or anything like that.

Those who favor strong emergence, meanwhile, search for examples of emergence where the emergent property, or alleged law, cannot be reduced in this way. To use a common emergentist phrase, the whole really is greater than the sum of the parts and the whole really causes things to happen among lower-level parts. In the broadest sense, as elements combine into complex structures, new properties emerge that are surprising and not present in earlier, simpler stages, that new things come about through the appearance of complex structures, said William Hasker, an emeritus professor of philosophy at Huntington University in Huntington, Ind. As such, these new things are not added from the outside but through the complex structure itself.

Science to the rescue

Supporters of weak emergence say that stronger forms leave the door open to an unwarranted intrusion of religion into science, even though many atheists also believe in strong emergence. More pointedly, they also argue that emergence can become a science stopper.

Sad to say, most of the people who use the concept of emergence have something much more mysterious in mind, maybe even mystical, and I dont think thats useful at all, said Dennett. I think that calling a phenomenon emergent should be the introduction to a [causal] investigation to understand what the property is and how it emerges and what causes it. If you just think youre explaining it by saying its emergent or youre just excusing yourself from the investigation, youre only causing trouble. Youre not helping, he said.

Scientists like Paul Davies are taking up Dennetts challenge. They want to put notions of strong emergence to the test. I long ago became exasperated that defenses of emergence were mostly just talk and propaganda and philosophy, with little or no science, Davies said. I have tried to focus the subject onto the question of whether emergence ultimately makes a difference, that is, does it have real effects that are not contained in the bottom-level laws already. Weak emergence will keep philosophers busy talking for a long time, but only strong emergence makes a real difference scientifically, Davies said.

In the journal Complexity, Davies recently published a paper the product of feedback from John D. Barrow, Leonard Susskind and others in which he proposed a scientific test for strong emergence. The idea is to show that causal closure the belief that physical events only have physical causes is false. If so, then non-physical causes become a possibility.

We are a long way from demonstrating causal closure scientifically and, as I have argued in my paper, there are sound scientific reasons for questioning it, Davies said. At the end of the day, we need to find experimental tests of these competing positions, or this stuff is all just words and politics. I propose just such a test in the form of a quantum mechanical experiment involving 400 entangled particles, he said, adding that he hoped that scientific advances will allow the experiment to be carried out within the next few decades.

Evolution gets deep

Scientists and philosophers engaged in emergence research remain committed to the long-standing emergentist view that reductionism and vitalism defined as life and mind entities or forces are both inadequate approaches for explaining past, present and future evolution.

If emergence is to make a splash among scientists in the 21st century, then it will have to be fashioned in a far different and more subtle form that takes science seriously, said Tim