A clear conception of the universe as Milton's inner eye beheld it, and of his religious and philosophical opinions in so far as they appear in the poem, is indispensable for a correct understanding of "Paradise Lost." The best service to be rendered to the reader within such limits as ours is to direct him to Professor Masson's discussion of Milton's cosmology in his "Life of Milton," and also in his edition of the Poetical Works. Generally speaking, it may be said that Milton's conception of the universe is Ptolemaic, that for him sun and moon and planets revolve around the central earth, rapt by the revolution of the crystal spheres in which, sphere enveloping sphere, they are successively located. But the light which had broken in upon him from the discoveries of Galileo has led him to introduce features not irreconcilable with the solar centre and ethereal infinity of Copernicus; so that "the poet would expect the effective permanence of his work in the imagination of the world, whether Ptolemy or Copernicus should prevail." So Professor Masson, who finely and justly adds that Milton's blindness helped him "by having already converted all external space in his own sensations into an infinite of circumambient blackness through which he could flash brilliance at his pleasure." His inclination as a thinker is evidently towards the Copernican theory, but he saw that the Ptolemaic, however inferior in sublimity, was better adapted to the purpose of a poem requiring a definite theatre of action. For rapturous contemplation of the glory of God in nature, the Copernican system is immeasurably the more stimulating to the spirit, but when made the theatre of an action the universe fatigues with its infinitude-- "Millions have meaning; after this Cyphers forget the integer." An infinite sidereal universe would have stultified the noble description how Satan-- "In the emptier waste, resembling air, Weighs his spread wings, at leisure to behold Far off the empyreal heaven, extended wide In circuit, undetermined square or round, With opal towers and battlements adorned Of living sapphire, once his native seat; And fast by, hanging in a golden chain, This pendant world, in bigness as a star Of smallest magnitude close by the moon." This pendant world, observe, is not the earth, as Addison understood it, but the entire sidereal universe, depicted not as the infinity we now know it to be, but as a definite object, so insulated in the vastness of space as to be perceptible to the distant Fiend as a minute star, and no larger in comparison with the courts of Heaven--themselves not wholly seen--than such a twinkler matched with the full-orbed moon. Such a representation, if it diminishes the grandeur of the universe accessible to sense, exalts that of the supersensual and extramundane regions where the action takes its birth, and where Milton's gigantic imagination is most perfectly at home.