Philosophy has often been defined as the quest or the vision of the world's unity. We never hear this definition challenged, and it is true as far as it goes, for philosophy has indeed manifested above all things its interest in unity. But how about the VARIETY in things? Is that such an irrelevant matter? If instead of using the term philosophy, we talk in general of our intellect and its needs we quickly see that unity is only one of these. Acquaintance with the details of fact is always reckoned, along with their reduction to system, as an indispensable mark of mental greatness. Your 'scholarly' mind, of encyclopedic, philological type, your man essentially of learning, has never lacked for praise along with your philosopher. What our intellect really aims at is neither variety nor unity taken singly but totality.[Footnote: Compare A. Bellanger: Les concepts de Cause, et l'activite intentionelle de l'Esprit. Paris, Alcan, 1905, p. 79 ff.] In this, acquaintance with reality's diversities is as important as understanding their connexion. The human passion of curiosity runs on all fours with the systematizing passion. In spite of this obvious fact the unity of things has always been considered more illustrious, as it were, than their variety. When a young man first conceives the notion that the whole world forms one great fact, with all its parts moving abreast, as it were, and interlocked, he feels as if he were enjoying a great insight, and looks superciliously on all who still fall short of this sublime conception. Taken thus abstractly as it first comes to one, the monistic insight is so vague as hardly to seem worth defending intellectually. Yet probably everyone in this audience in some way cherishes it. A certain abstract monism, a certain emotional response to the character of oneness, as if it were a feature of the world not coordinate with its manyness, but vastly more excellent and eminent, is so prevalent in educated circles that we might almost call it a part of philosophic common sense. Of COURSE the world is one, we say. How else could it be a world at all? Empiricists as a rule, are as stout monists of this abstract kind as rationalists are. The difference is that the empiricists are less dazzled. Unity doesn't blind them to everything else, doesn't quench their curiosity for special facts, whereas there is a kind of rationalist who is sure to interpret abstract unity mystically and to forget everything else, to treat it as a principle; to admire and worship it; and thereupon to come to a full stop intellectually. 'The world is One!'--the formula may become a sort of number-worship. 'Three' and 'seven' have, it is true, been reckoned sacred numbers; but, abstractly taken, why is 'one' more excellent than 'forty-three,' or than 'two million and ten'? In this first vague conviction of the world's unity, there is so little to take hold of that we hardly know what we mean by it.