The extent and minuteness of observation which may be requisite, and the degree of decomposition to which it may be necessary to carry the mental analysis, depend upon the particular purpose in view. The first and most obvious distinction between Observation and Experiment is, that the latter is an immense extension of the former. It not only enables people to produce a much greater number of variations in the circumstances than nature spontaneously offers, but, moreover, in thousands of cases, to produce the precise sort of variation which they are in want of for discovering the law of the phenomenon; a service which nature, being constructed on a quite different scheme from that of facilitating their studies, is seldom so friendly as to bestow upon them. Whatever phenomenon varies in any manner whenever another phenomenon varies in some particular manner, is either a cause or an effect of that phenomenon, or is connected with it through some fact of causation.
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