THREE:We have seen how, for the antithesis between Form and Matter, was substituted the wider antithesis between Actuality and Possibility. Even in this latter the opposition is more apparent than real. A permanent possibility is only intelligible through the idea of its realisation, and sooner or later is certain to be realised. Aristotle still further bridges over the interval between them by a new conceptionthat of motion. Motion, he tells us, is the process of realisation, the transformation of power into act. Nearly the whole of his Physics is occupied with an enquiry into its nature and origin. As first conceived, it is equivalent to what we call change rather than348 to mechanical movement. The table of categories supplies an exhaustive enumeration of its varieties. These are, as we have already mentioned, alteration of quality or transformation, increase or decrease of quantity, equivalent to growth and decay, and transport from place to place. Sometimes a fourth variety is added, derived from the first category, substance. He calls it generation and destruction, the coming into existence or passing out of it again. A careful analysis shows that motion in space is the primordial change on which all others depend for their accomplishment. To account for it is the most vitally important problem in philosophy.By cutting up some of the longer essays into parts, Porphyry succeeded, much to his delight, in bringing the whole number up to fifty-four, which is a product of the two perfect numbers six and nine. He then divided them into six volumes, each containing nine booksthe famous Enneads of Plotinus. His principle of arrangement was to bring together the books in which similar subjects were discussed, placing the easier disquisitions first. This disposition has been adhered to by subsequent editors, with the single exception of Kirchhoff, who has printed the works of Plotinus according to the order in which they were written.418 Porphyrys scrupulous information has saved modern scholars an incalculable amount of trouble, but has not, apparently, earned all the gratitude it deserved, to judge by Zellers intimation that the chronological order of the separate pieces cannot even now be precisely determined.419 Unfortunately, what could have been of priceless value in the case of Plato and Aristotle, is of comparatively small value in the case of Plotinus. His280 system must have been fully formed when he began to write, and the dates in our possession give no clue to the manner in which its leading principles were evolved.420
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