FORE:We have already observed that Scepticism among the ancients was often cultivated in connexion with some positive doctrine which it indirectly served to recommend. In the case of its last supporters, this was the study of medicine on an empirical as opposed to a deductive method. The Sceptical contention is that we cannot go beyond appearances; the empirical contention is, that all knowledge comes to us from experience, and that this only shows us how phenomena are related to one another, not how they are related to their underlying causes, whether efficient or final. These allied points of view have been brought into still more intimate association by modern thought, which, as will be shown in the concluding chapter, has sprung from a modified form of the ancient Scepticism, powerfully aided by a simultaneous development of physical science. At the same time, the new school have succeeded in shaking off the narrowness and timidity of their predecessors, who were still so far under the influence of the old dogmatists as to believe that there was an inherent opposition between observation and reasoning in the methods of discovery, between facts and explanations in the truths of science, and between antecedence and causation in the realities of Nature. In this respect, astronomy has done more for the right adjustment of our conceptions than any190 other branch of knowledge; and it is remarkable that Sextus Empiricus, the last eminent representative of ancient Scepticism, and the only one (unless Cicero is to be called a Sceptic) whose writings are still extant, should expressly except astronomy from the destructive criticism to which he subjects the whole range of studies included in what we should call the university curriculum of his time.301 We need not enter into an analysis of the ponderous compilation referred to; for nearly every point of interest which it comprises has already been touched on in the course of our investigation; and Sextus differs only from his predecessors by adding the arguments of the New Academy to those of Protagoras and Pyrrho, thus completing the Sceptical cycle. It will be enough to notice the singular circumstance that so copious and careful an enumeration of the grounds which it was possible to urge against dogmatismincluding, as we have seen, many still employed for the same or other purposes,should have omitted the two most powerful solvents of any. These were left for the exquisite critical acumen of Hume to discover. They relate to the conception of causation, and to the conception of our own personality as an indivisible, continuously existing substance, being attempts to show that both involve assumptions of an illegitimate character. Sextus comes up to the very verge of Humes objection to the former when he observes that causation implies relation, which can only exist in thought;302 but he does not ask how we come to think such a relation, still less does he connect it with the perception of phenomenal antecedence; and his attacks on the various mental faculties assumed by psychologists pass over the fundamental postulate of personal identity, thus leaving Descartes what seemed a safe foundation whereon to rebuild the edifice of metaphysical philosophy.To construe the term "transmission of power" in its full sense, it will, when applied to machinery, include nearly all that has motion; for with the exception of the last movers, or where power passes off and is expended upon work that is performed, all machinery of whatever kind may be called machinery of transmission. Custom has, however, confined the use of the term to such devices as are employed to convey power from one place to another, without including organised machines through which power is directly applied to the performance of work. Power is transmitted by means of shafts, belts, friction wheels, gearing, and in some cases by water or air, as various conditions of the work to be performed may require. Sometimes such machinery is employed as the conditions do not require, because there is, perhaps, nothing of equal importance connected with mechanical engineering of which there exists a greater diversity of opinion, or in which there is a greater diversity of practice, than in devices for transmitting power.
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