It is these irreducible elements--these great provinces in human experience, in reality, in the system of reason--that correspond to the more important of what are known as Platonic ideas. As ultimate constituents of the actual world they are in the narrower sense inexplicable. One does not amount to an exact sum of some others, nor is one got from another by the simple process of subtraction. But if they cannot be explained, by being reduced to multiples of some one basis, they can be comprehended in the respective implication and explication they exhibit with their co-realities. They can be correlated, reduced, and unified: we may even say, they can be identified; but if we use such a term, we must mean that there is some totality beyond and above them in which they all find a place and all are harmonious; in which all when brought to their Truth are really one and the same. This birthright of human nature in all ages and countries--this central essence of man's spirit--is the realm of Platonic ideas. They are the great elements, or constituent members, of humanity and of reality: the framework of his mind and of the world. How in each case they may be wrought out in detail, to what degree they may here be evolved, and there stunted, is a matter of historical research. And, in a sense, even it is not wrong to try to trace them one to another: to explain them, as the phrase is, one by another. For they are essentially connected: they are members of one system: they are unified and harmonised in a way for which even the word 'organism' is wholly insufficient. They are the poles and lines on which the tent of human life, of intelligent life, is stretched: but they are also the invisible ties which bind together the earth and heavens, and all that is therein.