FORE:To make this plainer, the laws of forces, the proportion of parts, strength of material, and so on, are subjects of general knowledge that may be acquired from books, and understood without the aid of an acquaintance with the technical conditions of either the mode of constructing or the manner of operating machines; but how to construct proper patterns for castings, or how the parts of machinery should be moulded, forged, or fitted, is special knowledge, and must have reference to particular cases. The proportions of pulleys, bearings, screws, or other regular details of machinery, may be learned from general rules and principles, but the hand skill that enters into the manufacture of these articles cannot be learned except by observation and experience. The general design, or the disposition of metal in machine-framing, can be to a great extent founded upon rules and constants that have general application; but, as in the case of wheels, the plans of moulding such machine frames are not governed by constant rules or performed in a uniform manner. Patterns of different kinds may be employed; moulds may be made in various ways, and at a greater and less expense; the metal can be mixed to produce a hard or a soft casting, a strong or a weak one; the conditions under which the metal is poured may govern the soundness or shrinkage,things that are determined by special instead of general conditions.Plotinus, as we have said, starts with the Aristotelian account of Matter; but by a process of dialectical manipulation, he gradually brings it into almost complete agreement with Platos conception; thus, as usual, mediating between and combining the views of his two great authorities. In the first place, he takes advantage of Aristotles distinction between intelligible and sensible Matter, to strip the latter of that positive and vital significance with which it had been clothed in the Peripatetic system. In the world of Ideas, there is an element common to all specific forms, a fundamental unity in which they meet and inhere, which may without impropriety be called their Matter. But this Matter is an eternal and divine substance, inseparably united with the fixed forms which it supports, and, therefore, something which, equally with them, receives light and life and thought from the central source of being. It is otherwise with sensible Matter, the common substance of the corporeal elements. This is, to use the energetic expression of our philosopher, a decorated corpse.481 It does not remain constantly combined with any form, but is for ever passing from one to another, without manifesting a particular preference for any. As such, it is the absolute negation of Form, and can only be conceived, if at all, by326 thinking away every sensible quality. Neither has it any quantity, for quantity means magnitude, and magnitude implies definite figure. Aristotle opposed to each particular form a corresponding privation, and placed Matter midway between them. Plotinus, on the other hand, identifies Matter with the general privation of all forms. It is at this point that he begins to work his way back to the Platonic notion of Matter as simple extension. There must, after all, be something about Matter which enables it to receive every kind of quality and figure,it must have some sort of mass or bulk, not, indeed, in any definite sense, but with an equal capacity for expansion and for contraction. Now, says Plotinus, the very indeterminateness of Matter is precisely the capacity for extension in all directions that we require. Having no principle of stability, but being borne towards every form, and easily led about in all directions, it acquires the nature of a mass.482
129
