As the growing mind of society deals with true beliefs they are piled up and classified in systems of science and philosophy: in which systems each belief or judgment strengthens and is strengthened by the rest. Even without systematisation, the mere structural similarity of judgments, formed unconsciously on the same implicit principles of causation and classification, throws them into those loose apperceptive masses which we call “common sense.” Such systems or masses, whether of science or of common sense, readily assimilate and confirm new inferences having the same character, and offer resistance to all inferences having a different structure, such as those about magic and spirits. The selective power of these apperceptive masses over novel ideas constitutes “understanding,” and is the plain solid man’s substitute for Logic; and so it is with many scientists, who often neglect the abstract study of Logic. For these systems or masses of experience are the substance of Logic and Methodology, which are their skeletons abstracted from them. They are the basis of all effective comparison and criticism; agreement or disagreement with them is the test of truth or error. It is the chief defect of common sense that the verification of its judgments depends almost entirely upon repetition of experiences (what Logicians call “simple enumeration”), without that analysis of observations which alone can show the necessary relations of facts; but this defect is in some measure remedied in good minds by that power of unformulated ideas of natural order, the result of unconscious analysis, which we call “good judgment”—a power which the fortunate possessor may be unable to explain. (2) Non-evidentiary causes of belief are all reducible to bad observations, imaginations, and the causes that excite imagination; and bad observations are caused by false imaginations as to the meaning of sense-data. If it should seem to any one that since imagination consists of ideas it must be by nature incompatible with intense belief, we must consider that memory, the effects of testimony, and inferences also consist entirely of ideas; so that in that character they do not differ from imagination. Even perception depends for its meaning upon implicit ideas, and erroneous perception is due to erroneous ideas. The weakness of imagination-belief which (despite its frequent intensity) always in time becomes manifest, is due to its not being constantly confirmed by experience.