With Zeno we close our survey of the second grand line of independent inquiry by which philosophy sought to solve the problem of the universe. The reader will be struck with the resemblance which subsists between the history of its development and that of the modern Idealist school. Pythagoras was the Descartes, Parmenides the Spinoza, and Zeno the Hegel of the Italian school. In this survey of the speculations of the pre-Socratic schools of philosophy, we have followed the course of two opposite streams of thought which had their common origin in one fundamental principle or law of the human mind--the _intuition of unity_--"or the desire to comprehend all the facts of the universe in a single formula, and consummate all conditional knowledge in the unity of unconditioned existence." The history of this tendency is, in fact, the history of all philosophy. "The end of all philosophy," says Plato, "is the intuition of unity." "All knowledge," said the Platonists, "is the gathering up into one."[460] [Footnote 459: Cudworth's "Intellectual System," vol. i. p. 518.] [Footnote 460: Hamilton's "Metaphysics," vol. i. pp. 67-70 (English edition).] Starting from this fundamental idea, _that, beneath the endless flux and change of the visible universe, there must be a permanent principle of unity_, we have seen developed two opposite schools of speculative thought. As the traveller, standing on the ridges of the Andes, may see the head-waters of the great South American rivers mingling in one, so the student of philosophy, standing on the elevated plane of analytic thought, may discover, in this fundamental principle, the common source of the two great systems of speculative thought which divided the ancient world. Here are the head-waters of the sensational and the idealist schools. The Ionian school started its course of inquiry in the direction of _sense_; it occupied itself solely with the phenomena of the external world, and it sought this principle of unity in a _physical_ element. The Italian school started its course of inquiry in the direction of _reason_; it occupied itself chiefly with rational conceptions or _à priori_ ideas, and it sought this principle of unity in purely _metaphysical_ being. And just as the Amazon and La Plata sweep on, in opposite directions, until they reach the extremities of the continent, so these two opposite streams of thought rush onward, by the force of a logical necessity, until they terminate in the two Unitarian systems of _Absolute Materialism_ and _Absolute Idealism_, and, in their theological aspects, in a pantheism which, on the one hand, identifies God with matter, or, on the other hand, swallows up the universe in God.