THREE:It is strange, considering the simplicity of construction and the very important office filled by machines for cutting on plane surfaces, that they were not sooner invented and applied in metal work. Many men yet working at finishing, can remember when all flat surfaces were chipped and filed, and that long after engine lathes had reached a state of efficiency and were generally employed, planing machines were not known. This is no doubt to be accounted for in the fact that reciprocal movement, except that produced by cranks or eccentrics, was unknown or regarded as impracticable for useful purposes until late years, and when finally applied it was thought impracticable to have such movements operate automatically. This may seem quite absurd to even an apprentice of the present time, yet such reciprocating movement, as a mechanical problem, is by no means so simple as it may at first appear.At this last point we touch on the final generalisation by which Plato extended the dialectic method to all existence, and readmitted into philosophy the earlier speculations provisionally excluded from it by Socrates. The cross-examining elenchus, at first applied only to individuals, had been turned with destructive effect on every class, every institution, and every polity, until the whole of human life was made to appear one mass of self-contradiction, instability, and illusion. It had been held by some that the order of nature offered a contrast and a correction to this bewildering chaos. Plato, on the other hand, sought to show that the ignorance and evil prevalent among men were only a part of the imperfection necessarily belonging to derivative existence of every kind. For this purpose the philosophy of Heracleitus proved a welcome auxiliary. The pupil of Socrates had been taught in early youth by Cratylus, an adherent of the Ephesian school, that movement, relativity, and the conjunction of opposites are the very conditions under which Nature works. We may conjecture that Plato did not at first detect any resemblance between the Heracleitean flux and the mental bewilderment produced or brought to light by the master of cross-examination. But his visit to Italy would probably enable him to take a new view of the Ionian speculations, by bringing him into contact with schools maintaining a directly opposite doctrine. The Eleatics held that existence remained eternally undivided, unmoved, and unchanged. The Pythagoreans arranged all things according to a strained and rigid antithetical construction. Then came the identifying flash.132 Unchangeable reality, divine order,208 mathematical truth��these were the objective counterpart of the Socratic definitions, of the consistency which Socrates introduced into conduct. The Heracleitean system applied to phenomena only; and it faithfully reflected the incoherent beliefs and disorderly actions of uneducated men. We are brought into relation with the fluctuating sea of generated and perishing natures by sense and opinion, and these reproduce, in their irreconcilable diversity, the shifting character of the objects with which they are conversant. Whatever we see and feel is a mixture of being and unreality; it is, and is not, at the same time. Sensible magnitudes are equal or greater or less according as the standard of comparison is chosen. Yet the very act of comparison shows that there is something in ourselves deeper than mere sense; something to which all individual sensations are referred as to a common centre, and in which their images are stored up. Knowledge, then, can no longer be identified with sensation, since the mental reproductions of external objects are apprehended in the absence of their originals, and since thought possesses the further faculty of framing abstract notions not representing any sensible objects at all.