[On January 21st, Steven Pinker and Steven Rose debated each other in an event chaired by Susan Blackmore and held at London University's Institute of Education under the sponsorship of Dillon's and The London Times. Part I of "The Two Steves," was published on EDGE 36 (March 10th) and is available on the EDGE site. In Part II Pinker and Rose answer questions from the audience.]

QUESTION for STEVEN PINKER: What do you believe consciousness is?

STEVEN PINKER: There is an extensive discussion of consciousness in the book. Consciousness is a word that refers to a number of different concepts. There's Freud's distinction between the conscious and unconscious mind, which I relate, following a number of other cognitive scientists, to the fact that no computational system can make all its information available to all of its processes. Thus there is a division in the human brain between the kind of information that we can verbally report on and that can affect our day-to-day decision making, and the kind that goes on "beneath the level of consciousness," such as the control of individual muscles in arms and legs or the rules of syntax that govern how we put sentences together. That's, I think, a tractable definition of consciousness, and it can be readily explained by the fact that the particular sequence of muscle movements is not relevant to my global course of planned action, and so therefore should be sealed off and not allowed to interfere with that planning process.

There are other definitions of consciousness, such as the philosophical concept of "qualia," or pure subjective experience: why red looks red to me, or whether my red is the same as your red. I don't have an evolutionary, or neural, or any kind of explanation as to the origin of that sense of consciousness.

ROSE: I don't regard consciousness as a property locked inside the brain of an individual. I regard it as a process which emerges in interaction between individuals, particularly humans, during their development, and the society and culture in which they're embedded. Therefore consciousness, in a very interesting sort of way, is not a brain property alone; it involves many many other features as well, and we reduce it excessively-and I don't think Steve is as guilty of this as many of my neuro-scientific colleagues are, in trying to argue that it's simply the reverse of being asleep, or unconscious. Or make the Freudian distinction. I think there are richer meanings; it's a process, not a thing.

QUESTION for Pinker and Rose: The parts of the brain which distinguish us from the animals are the least modular, and that's the frontal lobes, which take up 30% of the brain. The frontal lobes have the capacity to modulate and even change the physical structure of the brain. Posterior structures, for instance, are extremely flexible; you can cut out quite large chunks of them and they can reorganize. Similarly, the growing evidence for plasticity generally in the cortex, for instance, the use of apparently visual areas ... in blind people who are not using them gives a very different picture of, if you like, culture and society shaping, the brain-particularly, for instance, the growth of intelligence as society has developed over the last 50 or 60 years. It is quite a different picture of determinance of behavior and brain function than the picture of these rather crude and easily overridable systems of ancient structures of evolutionary adapted brain.

STEVEN PINKER: It is certainly true that the brain has a great deal of plasticity. I think of each one of these subsystems or faculties as systems that are designed to learn, that are designed to shape themselves in interaction with the environment. But it's not true that these faculties are infinitely plastic, and that the brain can do whatever it wants with itself. One example is the difference between spoken language and written language. All children learn to speak without lessons, spontaneously, by exposure to a community of other people, whereas to learn to read requires extensive practice, artificial curricula, and has a high failure rate. If the brain were completely plastic there should be no difference between reading and speech. There is a huge difference, and that is likely to characterize other mental faculties as well. But it certainly is true that they all are designed to learn and interact with the environment.

ROSE: I think the dialogue between specificity and plasticity in the development of the brain is much the most important and interesting thing that we need to understand. Of course the brain cannot be infinitely plastic; our eyes as we develop need to wire up to the visual cortex in the brain in a fairly ordered and systematic sort of way, or we couldn't preserve binocularity, we couldn't have a visual analyzing system of the sort that we've got. At the same time we have to have brains that are modified by experience. That's plasticity, and the capacity for both specificity and plasticity is there genetically to start with, so I entirely agree with you, and I think it's a mistake to have to think in terms of modularity, to an excessive degree, when one's concern is much more complex functions than simply visual analyzing functions.

The Debate Continues