3. EXPERIMENT.
In observation we simply watch the phenomenon; in experiment we make it. In experiment we not only observe, but we manipulate the circumstances so as to present the phenomenon under the most favorable conditions for observation. “In observation,” says Mill, “we find an instance in nature suited to our purposes”; whilst in experiment, by an artificial arrangement of circumstances, we make an instance suited to our purpose. In observation we watch for causes; in experiment we work for effects. We may thus define experiment as the act of making phenomena occur for the purpose of watching for effects. In experiment there is much which is merely observation. In fact experiment is observation in which the phenomenon is artificially produced. For the sakeof definiteness, however, any observation which involves a manipulation of circumstances, may be designated as experimental.