The goal of all scientific investigation is to predict the future. Prediction is a part of every field of science. The chemist who develops a new drug must be able to predict its effects on the human body. The biologist must predict the likelihood of a particular child inheriting haemophilia or another genetically linked disease. No prediction of the future behaviour of nature is 100 percent certain. The scientist must always be ready to adjust and refine predictions based on new observations.