What will future historians remember about the impact of science during the last decade of the 20th century? They will not be much concerned with many of the marvels that currently preoccupy us, such as the miraculous increase in the power of home computers and the unexpected growth of the Internet. Nor will they dwell much on global warming, the loss of biodiversity and other examples of our penchant for destruction. Instead, the end of 20th century will be recognized as the time when, for better or worse, science began to bring about a fundamental shift in our perception of ourselves. It will be the third time that science has forced us to re-evaluate who we are. The first time, of course, was the revolution that began with Copernicus in 1543 and continued with Kepler, Galileo and Newton. Despite the Church's opposition, we came to realize that the Earth does not lie at the centre of the universe. Instead we gradually found we live on a small planet on the edge of a minor galaxy, circling one star in a universe that contains billion of others. Our unique position in the universe was gone for ever. A few centuries later we were moved even further from stage centre. The Darwinian revolution removed us from our position as a unique creation of God. Instead we discovered we were just another part of the animal kingdom proud to have 'a miserable ape for a grandfather', as Thomas Huxley put it in 1850. We know now just how close to the apes we are—over 90% of our genes are the same of those of the chimpanzee. Increasing knowledge of our own genetics is one of the driving forces in the third great conceptual shift that will soon take place. Others are the growing knowledge of the way our minds work, our new ability to use knowledge of the nervous system to design drugs that affect specific states of mind and the creation of sophisticated scanners which enable us to see what is happening inside our brains. In the third revolution we are taking our own selves to pieces and finding the parts which make up the machine that is us. Much of the new knowledge from genetics, molecular biology and the neurosciences is esoteric. But its cultural impact is already running ahead of science. People begin to see themselves not as wholes with a moral centre but the result of the combined action of parts for which they have little responsibility. It's Nobody's Fault is the title of a popular American book on 'difficult' children. Many different children, the book explains, are not actually difficult but are suffering from Attention Deficit Disorder (ADD). There is nothing wrong with them or the way they have been brought up. Rather, the part of the brain which controls attention is short of a particular neurotransmitter. You might, as many people do, question the way in which the disorder has been diagnosed on such a staggering scale. But that is not the point. The cultural shift is that people are not responsible for their disorders, only for obtaining treatment for the parts of them that have gone wrong. Even when a treatment is not to hand, the notion that we are made of 'clusters of functions' remains strong. Genetic analysis supports this view. A gene linked to alcoholism has been located and a Gallup poll has revealed that the great majority of Americans consider alcoholism to be a disease There are claims of genes too for obesity, homosexuality and even for laziness. Some claims about genes may be silly. Or you may think that the current conceptual shift is just a re-run of old arguments about the relative roles of nature and nurture. Instead, take one drug, Viagra, as an example of the new way of thinking about ourselves. If you suffer from impotence, it might have a variety of physiological causes. Or you might just be anxious about sexual performance. But Viagra does not make such fine distinctions: it acts at the level of the chemical reactions that control t