We typically associate the word “science” with a person in a white coat doing experiments in a laboratory. Ideally, experiments should play as big a role in the human sciences as they do in the natural sciences; but in practice this is not usually the case. The are at least three reasons for this.1．Human scientists are often trying to make sense of complex real world situations in which it is simply impossible to run controlled experiment.2．The artificiality of some of the experiments that can be conducted may make the behavior of the participants abnormal.3．There are moral reasons for not conducting experiments that have a negative effect on the people who participate in them.Faced with the above difficulties, what are human scientists to do? One solution is to wait for nature to provide the appropriate experimental conditions. We can, for example, learn something about how a normal brain functions by looking at people who have suffered brain damage; and we can gain some understanding into the roles played by genes and the environment by studying twins, who have been separated at birth and brought up in different families. In the case of economics, economic history can provide us with a bank of-admittedly not very well-controlled-experimental data.However, human scientists do not just sit around waiting for natural experiments to arise. They also think of some experiments of their own. Suppose you want to know how a baby sees the world. We cannot, of course, ask the baby since it has not yet learnt to speak. So it might seem that all we can do is guess. People usually won’t change their mind until it was found out that babies tend to stare at surprising things longer than at unsurprising ones. This key understanding was like opening a window on to the developing mind. There was now a way of testing babies’ expectations and getting some idea of how they are six months old, babies can already do the following things: figuring out that objects consist of parts that move together being aware of the difference between living and non-living things and even doing simple arithmetic work.