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THREE:It is, at any rate, certain that the successors of Aenesid��mus adhered to the standpoint of Pyrrho. One of them, Agrippa, both simplified and strengthened the arguments of the school by reducing the ten Tropes to five. The earlier objections to human certainty were summed up under two heads: the irreconcilable conflict of opinions on all subjects; and the essential relativity of consciousness, in which the percipient and the perceived are so intimately united that what things in themselves are cannot possibly be discovered. The other three Tropes relate to the baselessness of reasoning. They were evidently suggested by Aristotle��s remarks on the subject. The process of proof cannot be carried backwards ad infinitum, nor can it legitimately revolve in a circle. Thus much had already been admitted, or rather insisted on by the great founder of logic. But the Sceptics could not agree to Aristotle��s contention, that demonstration may be based on first principles of self-evident certainty. They here fell back on their main argument; that the absence of general agreement on every point is fatal to the existence of such pretended axioms. A still further simplification was effected by the reduction of the five Tropes to two��that all reasoning rests on intuition, and that men��s intuitions are irreconcilably at variance with one another.300 As against true science, the sceptical Tropes are powerless, for the validity of its principles has nothing to do189 with their general acceptance. They are laid before the learner for his instruction, and if he chooses to regard them as either false or doubtful, the misfortune will be his and not theirs. But as against all attempts to constrain belief by an appeal to authority, the Tropes still remain invincible. Whether the testimony invoked be that of ancient traditions or of a supposed inward witness, there is always the same fatal objection that other traditions and other inward witnesses tell quite a different story. The task of deciding between them must, after all, be handed over to an impersonal reason. In other words, each individual must judge for himself and at his own risk, just as he does in questions of physical science.Let the reader compare a hammer with a wheel and axle, inclined plane, screw, or lever, as an agent for concentrating and applying power, noting the principles of its action first, and then considering its universal use, and he will conclude that, if there is a mechanical device that comprehends distinct principles, that device is the common hammer. It seems, indeed, to be one of those provisions to meet a human necessity, and without which mechanical industry could not be carried on. In the manipulation of nearly every kind of material, the hammer is continually necessary in order to exert a force beyond what the hands may do, unaided by mechanism to multiply their force. A carpenter in driving a spike requires a force of from one to two tons; a blacksmith requires a force of from five pounds to five tons to meet the requirements of his work; a stonemason applies a force of from one hundred to one thousand pounds in driving the edge of his tools; chipping, calking, in fact nearly all mechanical operations, consist more or less in blows, such blows being the application of accumulated force expended throughout a limited distance.