TWO:Judged upon theoretical grounds, and leaving out the mechanical conditions of operation, it would at once be conceded that a proper plan would be to move the lightest body; that is, if the tools and their attachments were heavier than the material to be acted upon, then the material should be moved for the cutting action, and vice versa. But in practice there are other conditions to be considered more important than a question of the relative weight of reciprocating parts; and it must be remembered that in solving any problem pertaining to machine action, the conditions of operation are to be considered first and have precedence over problems of strain, arrangement, or even the general principles of construction; that is, the conditions of operating must form a base from which proportions, arrangements, and so on, must be deduced. A standard planing machine, such as is employed for most kinds of work, is arranged with a running platen or carriage upon which the material is fastened and traversed beneath the cutting tools. [130] The uniformity of arrangement and design in machines of this kind in all countries wherever they are made, must lead to the conclusion that there are substantial reasons for employing running platens instead of giving a cutting movement to the tools.The rules for distinguishing between truth and falsehood98 are given in the famous Epicurean Canon. On receiving an image into the mind, we associate it with similar images formerly impressed on us by some real object. If the association or anticipation (πρ?ληψι?) is confirmed or not contradicted by subsequent experience, it is true; false, if contradicted or not confirmed.187 The stress laid on absence of contradictory evidence illustrates the great part played by such notions as possibility, negation, and freedom in the Epicurean system. In ethics this class of conceptions is represented by painlessness, conceived first as the condition, and finally as the essence of happiness; in physics by the infinite void, the inane profundum of which Lucretius speaks with almost religious unction; and in logic by the absence of contradiction considered as a proof of reality. Here, perhaps, we may detect the Parmenidean absolute under a new form; only, by a curious reversal, what Parmenides himself strove altogether to expel from thought has become its supreme object and content.188
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