I. Idealism—Different forms of idealism: subjective idealism, objective idealism: The whole of existence resolved into a mode of mental existence—Value of idealism considered from point of view of the religious sentiment—Most specious of contemporary idealisms: Possibility of universal progress in the hypothesis of radical spontaneity and of “freedom”—Reconciliation between determinism and the conception of freedom—Moral idealism as a possible substitute for religious sentiment: Dependence of the universe on the principle of goodness. II. Materialism—Difficulty in defining absolute materialism: Matter—The atom—Nebular hypothesis—Hydrogen—Necessity of supplementing materialism by some theory of the origin of life—The latest conception of materialism: Conception of infinite divisibility and infinite extensibility. III. Monism and the fate of worlds—Current of contemporary systems toward monism—Scientific interpretation of monism—The world conceived monistically as a becoming and as a life—Scientific formulæ for life—Progress consists in the gradual confusion of these two formulæ—That the rise of morality and religion can be accounted for without the presupposition of any final cause—Metaphysical and moral expectations in regard to the destiny of the world and of humanity, it may be, founded on scientific monism—Facts which appear to be inconsistent with these expectations—Pessimistic conception of dissolution that is complementary to the conception of evolution—Is the immanence of dissolution demonstrable?—Natural devices for the perpetuation of the “fittest”—Rôle of intelligence, of numbers, etc.—Calculation of probabilities—Is eternity _a parte post_ a ground of discouragement or of hope—Probable existence of thinking beings in other worlds: the planets, possibility of the existence of beings superior to man—Survival of the conception of gods—Hypothesis of intercosmic consciousness and of a universal society.