Richard Ludwig Heinrich Avenarius (19 November 1843 – 18 August 1896) was a German-Swiss philosopher. He formulated the radical positivist doctrine of "empirical criticism" or empirio-criticism.
Avenarius attended the Nicolaischule in Leipzig and studied at the University of Zurich, Berlin, and the University of Leipzig. At the University of Leipzig, he received the Doctor of Philosophy in 1868 with his thesis on Baruch Spinoza and his pantheism, obtained the habilitation in 1876 and taught there as Privatdozent. One year later, he became professor at the University of Zurich. He died in Zurich in 1896.
Avenarius believed that scientific philosophy must be concerned with purely descriptive definitions of experience, which must be free of both metaphysics and materialism. His opposition to the materialist assertions of Karl Vogt resulted in an attack upon empirio-criticism by Vladimir Lenin in the latter's Materialism and Empirio-criticism.
Avenarius' principal works are the famously difficult Kritik der reinen Erfahrung (Critique of Pure Experience, 1888–1890) and Der menschliche Weltbegriff (The Human Concept of the World, 1891) which influenced Ernst Mach, Ber Borochov and, to a lesser extent, William James.
Avenarius was the second son of the German publisher Eduard Avenarius and Cäcilie née Geyer, a daughter of the actor and painter Ludwig Geyer and a (step-)sister of Richard Wagner. However, there are speculations in science that her father was the biological father of Richard Wagner too. Richard's brother, Ferdinand Avenarius, led the cultural organization Dürerbund and belonged to the initiators of a culture reform movement in Germany. Wagner was Avenarius' godfather.