Induction is systematic observation of a given class of phenomena. It consists in bringing together a variety of facts and instances, carefully and patiently viewing them in all possible lights to discover from a comparison of the whole what, if any, new principle is elicitable. Induction is an experiment with a number of facts, to see if any general result can be arrived at. Thus observation is of three kinds--discursive, experimental, and inductive. For brevity of speech, we use respectively the terms observation, experiment, and induction, as the names of the three recognised modes of investigation. But it facilitates a clear view of this subject, to note that experiment and induction are but phases of observation--and that observation is the great source of the discovery of truth. Discursive observation and experiment are the sources of facts or particular truths. Nature, poetically says Dr. Reid, is put to the question by a thousand observations and experiments, and forced to confess her secrets. Out of these secrets induction gathers its general truths, which become the premises of argument. Facts, like stones, are of little service while scattered--it is in the edifice raised by them that their value is apparent. They have been compared to blocks, upon _one_ of which, if a person stand, he has but a partially increased view; but when many are piled up, a person from their summit commands the prospect round. Particular truth seldom proves anything but itself. Argument is proving something else, and we have seen that that which is proved must be _contained_ in something which proves it. In other words, an argument is an assertion or denial of something substantiated by other things--by facts. Gall _observed_ the peculiar formation of a certain head, but the one fact proved nothing, except that the head had a certain form. It was a barren observation, except that it suggested to his imagination the hypothesis that the peculiar form of the head might be caused by peculiarity of mind. This set him upon the experiment of observing the habits and dispositions of the individual in order to test his hypothesis. But the one fact of finding a peculiarity proved nothing new of any value. The two facts, though incident, were hardly convincing. They proved only that a peculiar head was accompanied in one case by peculiar habits--but whether one was the cause of the other, or whether the phenomena were in any way connected, still remained unknown. When, however, Gall, Spurzheim, and others, had travelled through Europe, making observations and experiments, and at last putting all the facts and instances together, and carefully and patiently viewing them in all possible lights, and finding that they shadowed forth that the brain was the organ, the map and measure of intelligence, they inducted a general truth, which enters the lists of argument and takes its place as an addition to our metaphysical and moral treasures.