In science the meaning of the word 'explain' suffers with civilization's every step in search of reality. Science can not really explain electricity, magnetism, and gravitation their effects can be measured and predicted, but of their nature no more is known to the modem scientist than to Thales who first speculated on the electrification of amber. Most contemporary physicists reject the notion that man can ever discover what these mysterious forces 'really' are. Electricity, Bertrand Russell says, 'is not a thing, like St. Paul's Cathedral it is a way in which things behave. When we have told how things behave, when they are electrified, we have told all there is to tell.' Until recently scientists would have disapproved of such an idea. Aristotle, for example, whose natural science dominated Western thought for two thousand years, believed that man could arrive at an understanding of reality by reasoning from self-evident principles. He felt, for example, that it is a self-evident principle that everything in the universe has its proper place, hence one can conclude that objects fall to the ground because that is where they belong, and smoking goes up because that is where it belongs. The goal of Aristotelian science was to explain why things happen. Modem science was born when Galileo began trying to explain how things happen and thus originated the method of con- trolled experiment which now forms the basis of scientific investigation. The aim of controlled scientific experiments is ______.