a natural aptitude and exigency for existence in the body, in conjunction with which it makes up the substantial unity of human nature; though connaturally related to the body, it is itself absolutely simple, i.e. of an unextended and spiritual nature. It is not wholly immersed in matter, its higher operations being intrinsically independent of the organism; the rational soul is produced by special creation at the moment when the organism is sufficiently developed to receive it. In the first stage of embryonic development, the vital principle has merely vegetative powers; then a sensitive soul comes into being, educed from the evolving potencies of the organism -- later yet, this is replaced by the perfect rational soul, which is essentially immaterial and so postulates a special creative act. Many modern theologians have abandoned this last point of St. Thomas's teaching, and maintain that a fully rational soul is infused into the embryo at the first moment of its existence. THE SOUL IN MODERN THOUGHT Modern speculations respecting the soul have taken two main directions, Idealism and Materialism. Agnosticism need not be reckoned as a third and distinct answer to the problem, since, as a matter of fact, all actual agnosticisms have an easily recognized bias towards one or other of the two solutions aforesaid. Both Idealism and Materialism in present-day philosophy merge into Monism, which is probably the most influential system outside the Catholic Church. History Descartes conceived the soul as essentially thinking (i.e. conscious) substance, and body as essentially extende...
