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ate the phenomenon by placing it among known circumstances, which can be then infinitely varied by introducing a succession of well-defined new ones. Observation cannot ascertain the effects of a given cause, because it cannot, except in the simplest cases, discover what are the concomitant circumstances; and therefore sciences in which experiment cannot be used, either at all, as in astronomy, or commonly, as in mental and social science, must be mainly deductive, not inductive. When, however, the object is to discover causes by means of their effects, observation alone is primarily available, since new effects could be artificially produced only through their causes, and these are, in the supposed case, unknown. But even then observation by itself cannot directly discover causes, as appears from the case of zoology, which yet contains many recognised uniformities. We have, indeed, ascertained a real uniformity when we observe some one antecedent to be invariably found along with the effects presented by nature. But it is only by reversing the process, and experimentally producing the effects by means of that antecedent, that we can prove it to be unconditional, i.e. the cause. CHAPTER VIII. AND NOTE TO CHAPTER IX.[1] THE FOUR METHODS OF EXPERIMENTAL ENQUIRY. Five canons may be laid down as the principles of experimental enquiry. The first is that of the Method of Agreement, viz.: _If two or more instances of the phenomenon under investigation have only one circumstance in common, the circumstance in which alone all the circumstances agree is the cause or the effect of the given phenomenon._ The second canon is that of the Method of Difference, viz.: _If an instance in which the phenomenon occurs and an instance in which it does not occur have every circumstance in common, save one, and that one occurs only in the former, that one circumstance is the effect, or the cause, or a necessary part of the cause, of the phenomenon._ These two are the simplest modes of singling out from the facts which precede or follow a phenomenon, those with which it is connected by an invariable law. Both are methods of elimination, their basis being, for the method of agreement, that whatever can be eliminated _is not_, and for that of difference, that whatever cannot be eliminated _is_ connected with the given phenomenon by a law. It is only, however, by the method of difference, which is a method of artificial experiment
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