The last and most glorious treatise in philosophy of which some knowledge can be given at the end of a school course is that of natural theology. If it is true, as they say, that St. Thomas Aquinas at the age of five years used to go round to the monks of Monte Cassino pulling them down by the sleeve to whisper his inquiry, "quid est Deus"? it may be hoped that older children are not incapable of appreciating some of the first notions that may be drawn from reason about the Creator, those truths "concerning the existence of God which are the supreme conclusion and crown of the department of physics, and those concerning His nature which apply the truths of general metaphysics to a determinate being, the Absolutely Perfect." [1--Cardinal Mercier, "Natural Theology," Introduction.] It is in the domain of natural theology that they will often find a safeguard against difficulties which may occur later in life, when they meet inquirers whose questions about God are not so ingenuous as that of the infant St. Thomas. The armour of their faith will not be so easily pierced by chance shots as if they were without preparation, and at the same time they will know enough of the greatness of the subject not to challenge "any unbeliever" to single combat, and undertake to prove against all opponents the existence and perfections of God. For instruction as well as for defence the relation of philosophy to revealed truth should be explained. It is necessary to point out that while science has its own sphere within which it is independent, having its own principles and methods and means of certitude, [1--De Bonald and others were condemned and reproved by Gregory XVI for teaching that reason drew its first principles and grounds of certitude from revelation.] yet the Church as the guardian of revealed truth is obliged to prosecute for trespass those who in teaching any science encroach by affirmation or contradiction on the domain of revelation. To sum up, therefore, logic can train the students to discriminate between good and bad arguments, which few ordinary readers can do, and not even every writer. Ethics teaches the rational basis of morals which it is useful for all to know, and psychology can teach to discriminate between the acts of intellect and will on the one hand and imagination and emotion on the other, and so furnish the key to many a puzzle of thought that has led to false and dangerous theorizing.