Of Brentano’s researches in _metaphysics_ and in the _theory of knowledge_ it must also be said that hitherto they remain still unpublished, though they are familiar to a greater or smaller circle of disciples. In this latter sphere are to be mentioned particularly his inquiries respecting the nature of our insight into the law of causality, the logical justification of induction, the _a priori_ nature of mathematics, and the nature of analytic judgments. In ontological questions also psychognosie has proved fruitful to the investigator in leading him to an understanding and to an analysis based upon experience, of the most important metaphysical notions, as, for instance, causality, substance, necessity, impossibility, etc., notions which some, despairing of the task rightly insisted upon by Hume, of showing their origin to be based upon perception and experience, have sought to explain straight away as _a priori_ categories. For the rest, Brentano, in regard to metaphysics, is a decided theist. He is an adherent of the theory of evolution, while denying that accidental variations and natural selection in the struggle for existence render explicable the phenomena of evolution and the teleological character of the organism, basing his objections, among other things, upon the fact that this attempt at a solution not only leaves unexplained the first beginnings of an organism, but also takes too little account of the fact that with the increasing perfection and complication of the organism it becomes more and more improbable that an accidental variation will lead to an improvement upon that which already exists. And yet if there is to be progress, the organisms which, in the struggle for existence, survive must not only be more perfect than those which perish, but also more perfect than the organisms through which they themselves are descended. Brentano’s views on the historical development of philosophical inquiry and the causes determining that development, the present state of philosophy and its views regarding the future, he has set forth in various publications: _Die Geschichte der Philosophie im Mittelalter_ (Möhler’s _Kirchengeschichte_, vol. ii. 1868); _Über die Gründe der Entmutigung auf philosophischem Gebiete_ (Vienna, 1874), delivered as an inaugural address on entering upon his work at Vienna University; _Was für ein Philosoph manchmal Epoche macht_ (Vienna, 1876); _Über die Zukunft der Philosophie_ (Vienna, 1893); and _Die vier Phasen der Philosophie und ihr augenblicklicher Stand_ (Stuttgart, 1895).