Such a cross-examination involved the mastery of all the openings for doubt, difficulty, deception, or refutation, bearing on the subject: openings which a man is to profit by, if assailant--to keep guarded, if defendant. Now when we survey the Greek negative philosophy, as it appears in Plato, Aristotle, and Sextus Empiricus--and when we recollect that between the second and the third of these names, there appeared three other philosophers equally or more formidable in the same vein, all whose arguments have perished (Arkesilaus, Karneades, Ænesidêmus)--we shall see that no man has ever been known competent both to strike and parry with these weapons, in a manner so skilful and ready as to amount to knowledge in the Platonic sense. But in so far as such knowledge is attainable or approachable, Plato is right in saying that it cannot be attained except by long dialectic practice. Reading books, and hearing lectures, are undoubtedly valuable aids, but insufficient by themselves. Modern times recede from it even more than ancient. Regulated oral dialectic has become unknown; the logical and metaphysical difficulties--which negative philosophy required to be solved before it would allow any farther progress--are now little heeded, amidst the multiplicity of observed facts, and theories adapted to and commensurate with those facts. This change in the character of philosophy is doubtless a great improvement. It is found that by acquiescing provisionally in the _axiomata media_, and by applying at every step the control of verification, now rendered possible by the multitude of ascertained facts--the sciences may march safely onward: notwithstanding that the logical and metaphysical difficulties, the puzzles ([Greek: a)pori/ai]) involved in _philosophia prima_ and its very high abstractions, are left behind unsolved and indeterminate. But though the modern course of philosophy is preferable to the ancient, it is not for that reason to be considered as satisfactory. These metaphysical difficulties are not diminished either in force or relevancy, because modern writers choose to leave them unnoticed. Plato and Aristotle were quite right in propounding them as problems, the solution of which was indispensable to the exigencies and consistent schematism of the theorising intelligence, as well as to any complete discrimination between sufficient and insufficient evidence. Such they still remain, overlooked yet not defunct. [Side-note: Plato's _idéal_ philosopher can only be realised under the hypothesis of a pre-existent and omniscient soul, stimulated into full reminiscence here.]