Kant offered a brilliant solution of the problem in his _Natural History of the Heavens_ (1755), a work embodying the celebrated nebular hypothesis rediscovered forty years later by Laplace. It has been well observed that great philosophers are mostly, if not always, what at Oxford and Cambridge would be called "double-firsts"--that is, apart from their philosophy, they have done first-class work in some special line of investigation, as Descartes by creating analytical geometry, Spinoza by applying Biblical criticisms to theology, Leibniz by discovering the differential calculus, Locke by his theory of constitutional government, Berkeley by his theory of vision, Hume by his contributions to history and political economy. Kant's cosmogony may have been premature and mistaken in its details; but his idea of the heavenly bodies as having originated from the condensation of diffused gaseous matter still holds its {88} ground; and although the more general idea of natural evolution as opposed to supernatural creation is not modern but Greek, to have revived and reapplied it on so great a scale is a service of extraordinary merit. The next great event in Kant's intellectual career is his rejection of Continental apriorism in metaphysics for the empiricism of the English school, especially as regards the idea of causation. For a few years (1762-1765) Kant accepts Hume's theory that there is nothing in any succession of events or in change generally to prove on grounds of pure reason that there must be more in it than a customary sequence. To believe that anything may happen without a cause does not involve a logical contradiction; and at that time he believed nothing to be known _à priori_ except that the denial of which involves such a contradiction. But on reconsidering the basis of mathematical truth it seemed to him to be something other than the logical laws of Identity and Contradiction. When we say that seven and five are twelve we put something into the predicate that was not affirmed in the subject, and also when we say that a straight line is the shortest distance between two points. Yet the second proposition is as certain as the first, and both are certain in the highest degree, more certain than anything learned from experience, and needing no experience to confirm them.