Porphyry of Tyre, who lived in the 3rd century, may be called the founder of both Arabian and Christian scholastics. He was an Alexandrian, but of peripatetic rather than platonic opinions. In the Isagoge, or Introduction to the Categories, already mentioned as translated by Boetius about 500 A.D., he set forth plainly a problem which during the Middle Ages rent Western Europe asunder; a problem which, says John of Salisbury[10], engaged more of the time and passions of men than for the house of Cæsar to conquer and govern the world; one indeed which even in our day and country is not wholly resolved. The controversy lay between the Realists[11] and the Nominalists; and the issues of it, in the eleventh century,—at which time the “Dark Ages” passed into the earlier of the two periods of the Middle Ages,—were formulated on the realist side by William of Champeaux, while the Breton Rousselin, or Roscellinus, had the perilous honour of defining them on behalf of the nominalists[12]. To see the depth of the difference we must step back a little, to a time when metaphysics and psychology were not distinguished from other spheres of science[13], and all research had for its object the nature of being. Plato himself held ideas not as mere abstractions but in some degree as creative powers; and we shall see how potent this function became in the thought of the Middle Ages when, in the ardour of research into the nature of being, the modes of individuating principles were distinguished or contrasted with an ingenuity incomprehensible to Plato or Aristotle, or at any rate undesired by these greater thinkers. Aristotle avoided the question whether form or matter individuate; he held that there is no form and no matter extrinsic to the individual. But by the medieval realist every particular, every thing, was regarded as after some fashion the product of universal matter and individual form. Now “form” might be regarded, and severally was regarded, as a shaping, determinative force or principle, pattern type or mould, having real existence apart from stuff, or, on the other hand, as an abstract principle or pattern having no existence but as a conception of the mind of the observer. The realists roundly asserted that form is as actual as matter, and that things arise by their participation—without whiteness no white thing, without humanity no man; and not individuals only: for the realist, out-platonising Plato, genera and species also had their forms, either pre-existent (“universalia ante rem”), or continuously evolved in the several acts of creation (“universalia in re”). Indeed for the extreme realist every “predicamental modality” was “aliquid ens separatum”; for instance, the soul, the active intellect, the passive intellect, and so on: conversely, by fusing idea with will, for other philosophers realism would get pushed back into efficient reason or divine will, and almost vanish[14]. By this latter route the Sorbonne, originally opposed to the