But the distinction between individual human souls is not a specific or formal distinction. These, though spiritual, are not _pure_ spirits. They are spiritual substances which, of their very nature, are essentially ordained for union with matter. They all belong to the same species—the human species. But they do not constitute individuals of this species unless as existing actually united with matter. Each human soul has a transcendental relation to its own body, to the “_materia signata_” for which, and in which, it was created. For each human soul this relation is unique. Just as it is the material principle of each human being, the matter as allied to quantitative dimensions, that individuates the man, so it is the unique relation of his soul to the material principle thus spatially determined, that individuates his soul. Now the soul, even when disembodied and existing after death, necessarily retains in its very constitution this essential relation to its own body; and thus it is that disembodied souls, though not actually allied with matter, remain numerically distinct and individuated in virtue of their essential relation, each to its own body. We see, therefore, that human souls, though spiritual, are an entirely different order of beings, and must be conceived quite differently, from pure spirits. We must be content with this brief exposition of the Thomistic doctrine on individuation. A discussion of the arguments for and against it would carry us too far.(148) There is no doubt that what _reveals_ the individuality of the corporeal substance to us is its material principle, in virtue of which its existence is circumscribed within certain limits of time and space and affected with individual characteristics, “_notae individuantes_”. But the Thomistic doctrine, which finds in “_materia signata_” the formal, intrinsic, constitutive principle of individuation, goes much deeper. It is intimately connected with the Aristotelian theory of knowledge and reality. According to this philosophy the formative principle or ἔιδος, the _forma subtantialis_, is our sole key to the intelligibility of corporeal things: these are intelligible in so far forth as they are actual, and they are actual in virtue of their “forms”. Hence the tendency of the scholastic commentators of Aristotle to use the term “form” as synonymous with the term “nature,” though the whole nature of the corporeal substance embraces the material as well as the formal principle: for even though it does, we can understand nothing about this “nature” beyond what is intelligible in it in virtue of its “form.” The material principle, on the other hand, is the potential, indeterminate principle, in itself unintelligible. We know that in ancient Greek philosophy it was regarded as the ἄλογον, the surd and contingent principle in things, the element which resisted