illusion, although older than Plato, was first applied on a great scale in Plato's philosophy, of whose influence on seventeenth-century thought this is not the only example. We shall find it to some extent countervailed by a revived Aristotelian current in the work of the metaphysician who now remains to be considered. LEIBNIZ. G. W. Leibniz (1646-1716), son of a professor at the University of Leipzig, is marked by some of the distinguishing intellectual characters of the German genius. {58} Far more truly than Francis Bacon, this man took all knowledge for his province. At once a mathematician, a physicist, a historian, a metaphysician, and a diplomatist, he went to the bottom of whatever subject he touched, and enriched all his multifarious studies with new views or with new facts. And as with other great countrymen of his, the final end of all this curiosity and interest was to combine and reconcile. One of his ambitions was to create a universal language of philosophy, by whose means its problems were to be made a matter of mathematical demonstration; another to harmonise ancient with modern speculation; a third--the most chimerical of all--to compose the differences between Rome and Protestantism; a fourth--partly realised long after his time--to unite the German Calvinists with the Lutherans. In politics he tried, with equal unsuccess, to build up a Confederation of the Rhine as a barrier against Louis XIV., and to divert the ambition of Louis himself from encroachments on his neighbours to the conquest of Egypt. It seems probable that no intellect of equal power was ever applied in modern times to the service of philosophy. And this power is demonstrated, not, as with other metaphysicians, by constructions of more or less contestable value, however dazzling the ingenuity they may display, but by contributions of the first order to positive science. It is now agreed that Leibniz discovered the differential calculus independently of Newton; and, what is more, that the formulation by which alone it has been made available for fruitful application was his exclusive invention. In physics he is a pioneer of the conservation of energy. In geology he starts the theory that our planet began as a glowing molten mass derived from the sun; and the modern {59} theory of evolution is a special application of his theory of development.