Much of the initial progress in a science derives from passive observation of a natural system. This approach is severely limited in that one remains at the mercy of the system for what qualitative and quantitative properties it chooses to reveal. Nevertheless, passive observation is the first step in all areas of research and remains of ultimate importance in some (e.g., astronomy). A more pragmatic approach is one in which the experimenter plays an active role by forcing information out of the object of his or her affection. The experimenter first tickles the system with a forcing function thought to be appropriate by passive observation and intuition. Every so often the experimenter will even be inclined to try anything. This is not as bad as it sounds because the "anything" selected will be inseparable from previous experience and will invariably be better than doing nothing. The observed response function to the chosen excitation signal is analyzed and this information is then used to design new active probes. Such a closed-loop modus operandi is shown in Figure 1.1. Block diagram illustrating modification of experimental design using feedback from previous experiments. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315274263/1022c1cb-68c4-4170-89e2-a38cf9102755/content/fig1_1.tif"/>